We study the temperature dependence of the critical current modulation I-c(H) for two types of planar Josephson junctions: a low-T-c Nb/CuNi/Nb and a high-T-c YBa2Cu3O7-delta bicrystal grain-boundary junction. At low T both junctions exhibit a conventional behavior, described by the local sine-Gordon equation. However, at elevated T the behavior becomes qualitatively different: the I-c(H) modulation field Delta H becomes almost T independent and neither Delta H nor the critical field for the penetration of Josephson vortices vanish at T-c. Such an unusual behavior is in good agreement with theoretical predictions for junctions with nonlocal electrodynamics. We extract absolute values of the London penetration depth lambda from our data and show that a crossover from local to nonlocal electrodynamics occurs with increasing T when lambda(T) becomes larger than the electrode thickness.

We report on specific heat measurements on clean overdoped BaFe2(As1-xPx)(2) single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature gamma(r) = C/T backslash(T -> 0) is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band s-wave alpha model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of Delta(0) similar to 5.3 meV, corresponding to Delta(0)/k(B)T(c) similar to 2.2. Increasing the phosphorus concentration x, the main gap reduces till a value of Delta(0) similar to 1.9 meV for x = 0.55 and a second weaker gap becomes evident. From the magnetic field effect on gamma(r), all samples however show similar behavior [gamma(r)(H) -gamma(r)(H = 0) proportional to H-n, with n between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

Gallium (Ga) displays several metastable phases. Superconductivity is strongly enhanced in the metastable beta-Ga with a critical temperature T-c = 6.04(5) K, while stable alpha-Ga has a much lower T-c < 1.2 K. Here we use a membrane-based nanocalorimeter to initiate the transition from alpha-Ga to beta-Ga on demand, as well as study the specific heat of the two phases on one and the same sample. The in situ transformation is initiated by bringing the temperature to about 10 K above the melting temperature of alpha-Ga. After such treatment, the liquid supercools down to 232 K, where beta-Ga solidifies. We find that beta-Ga is a strong-coupling type-I superconductor with Delta(0)/k(B)T(c) = 2.00(5) and a Sommerfeld coefficient gamma(n) = 1.53(4) mJ/molK(2), 2.55 times higher than that in the alpha phase. The results allow a detailed comparison of fundamental thermodynamic properties between the two phases.

We report the synthesis and characterization of phase pure Ta₃Sb, a material predicted to be topological with eightfold degenerate fermionic states [Bradlyn et al., Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Derunova et al., Sci. Adv. 5, eaav8575 (2019)]. We observe superconductivity in Ta₃Sb with T_c∼0.67 K in both electrical resistivity ρ(T) and specific heat c(T) measurements. Field-dependent measurements yield the superconducting phase diagram with an upper critical field of H_c2(0)∼ 0.95 T, corresponding to a superconducting coherence length of ξ≈18.6nm. The gap ratio deduced from specific heat anomaly 2Δ₀/k_BT_c is 3.46, a value close to the Bardeen-Cooper-Schrieffer value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of κ≈90 is obtained, identifying Ta₃Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta₃Sb as an appealing platform to further explore exotic quantum states in multifold degenerate systems.

We present specific heat measurements on a series of BaFe2(As1-xPx)(2) single crystals with phosphorous doping ranging from x = 0.3 to x = 0.55. Our results reveal that BaFe2(As1-xPx)(2) follows the scaling Delta C/T-c approximate to T-c(2) remarkably well. The clean-limit nature of this material imposes additional restraints on theories aimed at explaining the scaling. Furthermore, we find that the Ginzburg-Landau parameter decreases significantly with doping whereas the superconducting anisotropy is Gamma approximate to 2.6, independent of doping.

The effect of Eu doping in the Tsai quasicrystal (QC) GdCd7.88 and its periodic 1/1 approximant crystal (AC) GdCd6 are investigated. This represents the first synthesis of Eu-containing stable QC samples, where three samples with the final composition Gd1-xEux Cd-7.6 +/-alpha at Eu doping concentrations x = 0.06, 0.13, and 0.19 are obtained (alpha similar to 0.2). They are compared to two 1/1 ACs with compositions Gd1-xEuxCd6 (x = 0.12, 0.16). In addition, a new type of 1/1 AC, differing only by the inclusion of extra Cd sites unique to the Eu4Cd25 1/1 AC, has been discovered and synthesized for the concentrations Gd1-xEuxCd6+delta (x = 0.25, 0.33, 0.45, 0.69, 0.73, and 0 < delta <= 0.085). Due to the preferred cube morphology of its single grains, we refer to them as c-type 1/1 ACs and to the conventional standard ones as s-type. In both QCs and s-type ACs, the Eu content appears to saturate at a concentration of similar to 20%. On the other hand, any Gd| Eu ratio is allowed in the c-type ACs, varying continuously between GdCd6 and Eu4Cd25. We describe and contrast the changes in composition, atomic structure, specific heat, and magnetic properties induced by Eu doping in the quasicrystalline phase and the s-type and c-type 1/1 ACs. By comparing our results to the literature data, we propose that the occupancy of the extra Cd sites can be used to predict the stability of Tsai-type quasicrystalline phases.

We investigate the electronic specific heat of superoptimally substituted BaFe2(As1-x P-x(x))(2) single crystals in the superconducting state using high-resolution nanocalorimetry. From the measurements, we extract the substitution dependence of the condensation energy, superconducting gap Delta, and related microscopic parameters. We find that the anomalous scaling of the specific heat jump Delta C proportional to T-c(3) , found in many iron-based superconductors, in this system originates from a T-c-dependent ratio Delta/k(B)T(c) in combination with a substitution-dependent density of states N(epsilon(F)). A clear enhancement is seen in the effective mass m* as the composition approaches the value that has been associated with a quantum critical point at optimum substitution. However, a simultaneous increase in the superconducting carrier concentration n(s) yields a penetration depth lambda that decreases with increasing T-c without sharp divergence at the quantum critical point. Uemura scaling indicates that T-c is governed by the Fermi temperature T-F for this multiband system.

Many thermal measurements in high magnetic fields—including heat capacity, thermal conductivity, thermopower, magnetocaloric, and thermal-Hall-effect measurements—require thermometers that are sensitive over a wide temperature range, are low mass, have a rapid thermal response, and have a minimal easily correctable magnetoresistance. Here, we report the development of a granular-metal oxide ceramic composite (cermet) for this purpose formed by cosputtering of the metallic alloy nichrome (Ni_0.8Cr_0.2) and the insulator silicon dioxide (Si⁢O₂). We find that cosputtering of Ni-Cr alloys with Si⁢O₂ in a reactive-oxygen plus inert-argon-gas mixture produces resistive thin-film thermometers sensitive enough to be used in calorimetry and related measurements from room temperature down to below 100 mK in magnetic fields up to at least 41 T.

Investigations of reaction mixtures RE_x(Au_0.79Si_0.21)_100–x (RE = Y and Gd) yielded the compounds REAu₃Si which adopt a new structure type, referred to as GdAu₃Si structure (tP80, P4₂/mnm, Z = 16, a = 12.8244(6)/12.7702(2) Å, and c = 9.0883(8)/9.0456(2) Å for GdAu₃Si/YAu₃Si, respectively). REAu₃Si was afforded as millimeter-sized faceted crystal specimens from solution growth employing melts with composition RE₁₈(Au_0.79Si_0.21)₈₂. In the GdAu₃Si structure, the Au and Si atoms are strictly ordered and form a framework built of corner-connected, Si-centered, trigonal prismatic units SiAu₆. RE atoms distribute on 3 crystallographically different sites and each attain a 16-atom coordination by 12 Au and 4 Si atoms. These 16-atom polyhedra commonly fill the space of the unit cell. The physical properties of REAu₃Si were investigated by heat capacity, electrical resistivity, and magnetometry techniques and are discussed in the light of theoretical predictions. YAu₃Si exhibits superconductivity around 1 K, whereas GdAu₃Si shows a complex magnetic ordering, likely related to frustrated antiferromagnets exhibiting chiral spin textures. GdAu₃Si-type phases with interesting magnetic and transport properties may exist in an extended range of ternary RE–Au–Si systems, similar to the compositionally adjacent cubic 1/1 approximants RE(Au,Si)_∼6.

In cluster-based quasicrystals, tetrahedra located in conventional Tsai clusters may be replaced by single rare-earth (R) ions at the cluster centers (pseudo-Tsai clusters). In this study, we investigate the effect of the pseudo-Tsai cluster incorporation on the magnetic structures of two approximants, the Tsai-type Tb-Au-Si [denoted TAS(0)] and Ho-Au-Si [denoted HAS(52)] with partial replacement of conventional Tsai clusters by pseudo-Tsai clusters, up to 52%. The mixture of Tsai and pseudo-Tsai clusters can be considered a different source of randomness/disorder other than the conventional chemical mix sites (Au/Si). The effect of the latter has been previously discussed regarding the origin/cause of spin-glass-like ordering and Anderson localization of electronic states in quasicrystals and approximant crystals. Single crystal neutron diffraction experiments at 2 K were performed and bulk physical properties (magnetization and specific heat) were investigated. In addition, earlier collected powder neutron diffraction data of TAS(14) with 14% replacement was reanalyzed in light of the results on TAS(0) and HAS(52). We find that the arrangement of ordered magnetic spins in the icosahedral shells of these phases is similar, while the cluster-center R magnetic states are different. In the case of TAS(14), the cluster-center Tb magnetic moments seem to affect the arrangement of surrounding icosahedral magnetic moments, and the magnetic structure of the icosahedral shell deviates from that of TAS(0). In the case of HAS(52), however, the icosahedral R magnetic moments are less affected by the cluster-center R, while the averaged cluster-center R magnetic moments are significantly diminished. We discuss these results considering the magnetic ordering effect on the bulk physical properties.

Exploration of the Au-Ge rich part of the pseudo-binary section Y-x(Au0.70Ge0.30)(100-x) (3 < x < 18) in the Y-Au-Ge system afforded the intermetallic compounds Y-3(Au0.7054Ge0.2946)(19-x) (x 0.38) and Y-14(Au0.8313Ge0.1687)(51), which are new representatives of the cubic 1/1 Tsai-type approximant crystal (YbCd6 parent type, space group Im (3) over bar) and the hexagonal Gd14Ag51 structure type (space group P6/m), respectively. Y-3(Au0.7054Ge0.2946)(19-x) decomposes peritectically into Y-14(Au, Ge)(51) phase and melt at around 700 degrees C. In this respect the Y-Au-Ge system deviates from the Y-Au-Si system which firstly contains a second type of 1/1 approximant crystal phase with higher Y content, Y-3.25(Au,Si)(18-x) (x approximate to 0.11) and, secondly, adjacent at higher Y composition there is a stoichiometric and ordered tetragonal phase YAu3Si. Y-3(Au,Ge)(19-x) and Y-14(Au,Ge)(51) exhibit bulk superconductivity of conventional type-II BCS type albeit with very low transition temperatures (below 1 K), which is very similar to the corresponding compounds in the Y-Au-Si system.

We probe a quantum mechanical phase rotation induced by a single Abrikosov vortex in a superconducting lead, using a Josephson junction, made at the edge of the lead, as a phase-sensitive detector. We observe that the vortex induces a Josephson phase shift equal to the polar angle of the vortex within the junction length. When the vortex is close to the junction it induces a π step in the Josephson phase difference, leading to a controllable and reversible switching of the junction into the 0-π state. This in turn results in an unusual Φ₀/2 quantization of the flux in the junction. The vortex may hence act as a tunable “phase battery” for quantum electronics.

We study Hall effect in sputtered Ni_xPt_1-x thin films with different Ni concentrations. Temperature, magnetic field andangular dependencies are analyzed and the phase diagram of NiPt thin films is obtained. It is found that films with sub-critical Ni concentration exhibit cluster-glass behavior at low temperatures with a perpendicular magnetic anisotropy below the freezing temperature. Films with super-critical Ni concentration are ferromagnetic with parallel anisotropy. At the critical concentration the state of the film is strongly frustrated. Such films demonstrate canted magnetization with the easy axis rotating as a function of temperature. The magnetism appears via consecutive paramagnetic - cluster glass - ferromagnetic transitions, rather than a single second-order phase transition. But most remarkably, the extraordinary Hall effect changes sign at the critical concentration. We suggest that this is associated with a reconstruction of the electronic structure of the alloy at the normal metal - ferromagnet quantum phase transition.

We study phase shifts in a Josephson junction induced by vortices in superconducting mesoscopic electrodes. The position of the vortices are controlled by suitable geometry of a nano-scale Nb–Pt_1−xNi_x–Nb junction of the overlap type made by Focused Ion Beam (FIB) sculpturing. The vortex is kept outside the junction, parallel to the junction plane. From the measured Fraunhofer characteristics the entrance and exit of vortices are detected. By changing the bias current through the junction at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states which are characterized by different critical currents of the junction. A mesoscopic superconductor thus acts as a non-volatile memory cell in which the junction is used both for reading and writing information (vortex). Furthermore, we observe that the critical current density of Nb–Pt_1−xNi_x–Nb junctions decreases non-monotonously with increasing Ni concentration. It exhibits a minimum at 40 at.% Ni, which is an indication of switching into the π state.

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa2Cu3O7-x/LaMnO3+delta/YBa2Cu3O7-x) tunnel junctions. At a large bias voltage V similar to 1 V we observe a steplike onset of excess current that occurs below the superconducting transition temperature T < T-c and is easily suppressed by a magnetic field. The phenomenon is attributed to a different type of the superconducting proximity effect of nonequilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly nonequilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.

We study the Hall effect in NixPt1-x thin films. It is observed that the ordinary Hall coefficient is always negative (electron-like). The anomalous Hall coefficient is also negative, except in the vicinity of the ferromagnetic quantum phase transition, where it exhibits a sign reversal and turns positive (hole-like). This leads to an anti-ordinary Hall effect with opposite signs of ordinary and anomalous contributions. It clearly shows that the anomalous Hall effect does not reflect the overall topology of the Fermi surface (which remains unchanged), but originates from singular hot spots. We attribute the anti-ordinary contribution to the intrinsic (Berry-phase) origin and propose a spectroscopic explanation of its tunability as a function of temperature and composition.

The specific heat and thermodynamics of Fe2P single crystals around the first-order paramagnetic to ferromagnetic (FM) phase transition at T-C similar or equal to 217 K are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H-T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the c axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields the phase transition is split into two sections, with an intermediate FM phase where there is no spontaneous magnetization along the c axis. The zero-field transition displays a textbook example of a first-order transition with different phase stability limits on heating and cooling. The results have special significance since Fe2P is the parent material to a family of compounds with outstanding magnetocaloric properties.

We experimentally study intrinsic tunneling and high magnetic field (up to 65 T) transport characteristics of the single-layer cuprate Bi2+xSr2-yCuO6+delta, with a very low superconducting critical temperature T-c less than or similar to 4 K. It is observed that the superconducting gap, the collective bosonic mode energy, the upper critical field, and the fluctuation temperature range are scaling down with T-c, while the corresponding pseudogap characteristics remain the same as in high-T-c cuprates with 20 to 30 times higher T-c. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.

Exploration of the gold-rich part of the ternary Gd–Au–Al system afforded the intermetallic compound GdAu_6.75−xAl_0.5+x (x ≈ 0.54) which was structurally characterized by single crystal X-ray diffraction (Pnma, a = 18.7847(4) Å, b = 23.8208(5) Å, c = 5.3010(1) Å). GdAu_6.75−xAl_0.5+x crystallizes in a previously unknown structure type featuring layers of Gd₂(Au, Al)₂₉ and Gd₂(Au, Al)₂₈ clusters which are arranged as in a close-packing parallel to the ac plane. The Gd substructure corresponds to slightly corrugated 3⁶ nets (d_Gd–Gd = 5.30–5.41 Å) which are stacked on top of each other along the b direction with alternating short (5.4, 5.6 Å, within layers) and long distances (6.4 Å, between layers). The title compound has been discussed with respect to a quasicrystal approximant (1/1 AC) GdAu_5.3Al in the same system. The magnetic properties of GdAu_6.75−xAl_0.5+x were found to be reminiscent to those of some ternary ACs, with sharp peaks in the temperature dependent magnetization, and metamagnetic-like transitions. The material becomes antiferromagnetic below 25 K; magnetometry results suggest that the antiferromagnetic state is composed of ferromagnetic ac planes, coupled antiferromagnetically along the b direction.

We present a detailed study of the temperature (T) and magnetic field (H) dependence of the electronic density of states (DOS) at the Fermi level, as deduced from specific heat and Knight shift measurements in underdoped YBa2Cu3Oy. We find that the DOS becomes field independent above a characteristic field H-DOS, and that the H-DOS (T) line displays an unusual inflection near the onset of the long-range 3D charge-density wave order. The unusual S shape of H-DOS (T) is suggestive of two mutually exclusive orders that eventually establish a form of cooperation in order to coexist at low T. On theoretical grounds, such a collaboration could result from the stabilization of a pair-density wave state, which calls for further investigation in this region of the phase diagram.

The Bi₂Sr₂CaCu₂O_8+x high-temperature superconductor represents a natural layered metamaterial composed of metallic CuO bilayers sandwiched between ionic BiO planes. Each pair of CuO bilayers forms an atomic-scale Josephson junction. Here we employ the intrinsic Josephson effect for in situ generation and self-detection of electromagnetic waves in Bi₂Sr₂CaCu₂O_8+x single crystals. We observe that electromagnetic waves form polaritons with several transverse optical phonons. This indicates the presence of unscreened polar response in cuprates, which may lead to unusually strong electron-phonon interaction. Our technique can provide intense local sources of coherent, monochromatic phonon-polaritons with kW/cm² power densities.

We perform a detailed study of temperature, bias, and doping dependence of interlayer transport in the layered high temperature superconductor Bi2Sr2CaCu2O8+delta. We observe that the shape of interlayer characteristics in underdoped crystals exhibits a remarkable crossover at the superconducting transition temperature: from thermal activation-type above Tc to almost T-independent quantum tunneling-type below Tc. Our data provide insight into the nature of interlayer transport and indicate that its mechanism changes with doping: from the conventional single quasiparticle tunneling in overdoped to a progressively increasing Cooper pair contribution in underdoped crystals.

Resonant phenomena are important for the use of intrinsic Josephson junction as THz-oscillators, due to the decreased linewidth of emitted radiation when biasing the junctions near a resonance. We perform a detailed study of flux-flow characteristics and phonon resonances in small Bi(Pb)₂Sr₂CaCu₂O_8+x mesa structures. Magnetic field dependence of flux-flow characteristics up to 17 T and temperature and magnetic field dependence of phonon resonances at temperatures from 2 K to 80 K and in fields up to 15 T are analyzed. A shift of the phonon resonances in the presence of external magnetic fields and an interaction between flux-flow and phonon resonances are observed.

We study Fiske steps in small Bi₂Sr₂CaCu₂O_8+x mesa structures, containing only a few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high-quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the terahertz frequency range.

The quasiparticle density of states in correlated and quantum-critical metals directly probes the effect of electronic correlations on the Fermi surface. Measurements of the nuclear spin-lattice relaxation rate provide one such experimental probe of quasiparticle mass through the electronic density of states. By far the most common way of accessing the spin-lattice relaxation rate is via nuclear magnetic resonance and nuclear quadrupole resonance experiments, which require resonant excitation of nuclear spin transitions. Here we report nonresonant access to spin-lattice relaxation dynamics in AC-calorimetric measurements. The nuclear spin-lattice relaxation rate is inferred in our measurements from its effect on the frequency dispersion of the thermal response of the calorimeter-sample assembly. We use fast, lithographically defined nanocalorimeters to access the nuclear spin-lattice relaxation times in metallic indium from 0.3 to 7 K and in magnetic fields up to 35 T.

Understanding the pairing mechanism that gives rise to high-temperature superconductivity is one of the longest-standing problems of condensed-matter physics. Almost three decades after its discovery, even the question of whether or not phonons are involved remains a point of contention to some. Here we describe a technique for determining the spectra of bosons generated during the formation of Cooper pairs on recombination of hot electrons as they tunnel between the layers of a cuprate superconductor. The results obtained indicate that the bosons that mediate pairing decay over micrometre-scale distances and picosecond timescales, implying that they propagate at a speed of around 10⁶ m s⁻¹. This value is more than two orders of magnitude greater than the phonon propagation speed but close to Fermi velocity for electrons, suggesting that the pairing mechanism is mediated by unconventional repulsive electron–electron, rather than attractive electron–phonon, interactions. 

We perform a detailed comparison of magnetotunneling in conventional low-T_c Nb/Al-AlO_x/Nb junctions with that in slightly overdoped Bi_2−yPb_ySr₂CaCu₂O_8+δ [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calculations. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign change of tunneling magnetoresistance from the negative at the subgap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field H_c2. For Nb an extended region of surface superconductivity at H_c2<H<H_c3 is observed. The parameters of Bi(Pb)-2212 were obtained from self-consistent analysis of magnetotunneling data at different levels of bias, dissipation powers, and for different mesa sizes, which precludes the influence of self-heating. It is found that H_c2(0) for Bi(Pb)-2212 is ≃T→T_c T and decreases significantly at T→T_c. The amplitude of subgap magnetoresistance is suppressed exponentially at T>T_c/2, but remains negative, although very small, above T_c. This may indicate the existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at T_c.

Surprisingly, magnetoquantum oscillations (MQOs) characteristic of a metal with a Fermi surface have been observed in measurements of the topological Kondo insulator SmB₆. As these MQO have only been observed in measurements of magnetic torque (dHvA) and not in measurements of magnetoresistance (SdH), a debate has arisen as to whether the MQO are an extrinsic effect arising from rare-earth impurities, defects, and/or aluminum inclusions or an intrinsic effect revealing the existence of charge-neutral excitations. We report here the first observation of MQO in the low-temperature specific heat of SmB6. The observed frequencies and their angular dependence for these flux-grown samples are consistent with previous results based on magnetic torque for SmB₆ but the inferred effective masses are significantly larger than previously reported. Such oscillations can only be observed if the MQO are of bulk thermodynamic origin; the measured magnetic-field dependent oscillation amplitude and effective mass allow us to rule out suggestions of an extrinsic, aluminum inclusion-based origin for the MQO.

Depending on the Ginzburg-Landau parameter kappa, superconductors can either be fully diamagnetic if kappa < 1/root 2 (type I superconductors) or allow magnetic flux to penetrate through Abrikosov vortices if kappa > 1/root 2 (type II superconductors; refs 1,2). At the Bogomolny critical point, kappa = kappa(c) = 1/root 2, a state that is infinitely degenerate with respect to vortex spatial configurations arises(3,4). Despite in-depth investigations of conventional type I and type II superconductors, a thorough understanding of the magnetic behaviour in the near-Bogomolny critical regime at kappa similar to kappa(c) remains lacking. Here we report that in confined systems the critical regime expands over a finite interval of kappa forming a critical superconducting state. We show that in this state, in a sample with dimensions comparable to the vortex core size, vortices merge into a multi-quanta droplet, which undergoes Rayleigh instability(5) on increasing kappa and decays by emitting single vortices. Superconducting vortices realize Nielsen-Olesen singular solutions of the Abelian Higgs model, which is pervasive in phenomena ranging from quantum electrodynamics to cosmology(6-9). Our study of the transient dynamics of Abrikosov-Nielsen-Olesen vortices in systems with boundaries promises access to non-trivial effects in quantum field theory by means of bench-top laboratory experiments.

Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x mesa structures leads to persistent doping of the mesas. Here, we employ this effect for analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are able to controllably and reversibly change the doping state of the same single crystal from underdoped to overdoped state, without changing its chemical composition. It is observed that such physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping, the critical temperature and the superconducting gap decrease; with underdoping, the c-axis critical current rapidly decreases due to progressively more incoherent interlayer tunneling and the pseudogap rapidly increases, indicative for the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even-in-voltage contribution, attributed to a charge transfer effect, and (ii) odd-in-voltage contribution, attributed to reordering of oxygen vacancies.

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi₂Sr₂CaCu₂O_8+x and Bi_1.75Pb_0.25Sr₂CaCu₂O_8+xsingle crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on λ_J(J_c) and occur at about 0.4 and 1.3 fluxons per λ_J, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

Bi₂Sr₂CaCu₂O_8+x single crystals represent natural stacks of atomic scale intrinsic Josephson junctions, formed between metallic CuO₂–Ca–CuO₂ and ionic insulating SrO–2BiO–SrO layers. Electrostriction effect in the insulating layers leads to excitation of c-axis phonons by the ac-Josephson effect. Here we study experimentally the interplay between and velocity matching (Eck) electromagnetic resonances in the flux-flow state of small mesa structures with c-axis optical phonons. A very strong interaction is reported, which leads to formation of phonon-polaritons with infrared and Raman-active transverse optical phonons. A special focus in this work is made on analysis of the angular dependence of the resonances. We describe an accurate sample alignment procedure that prevents intrusion of Abrikosov vortices in fields up to 17 T, which is essential for achieving high-quality resonances at record high frequencies up to 13 THz.

Many thermal measurements in high magnetic fields—including heat capacity, thermal conductivity, thermopower, magnetocaloric and thermal Hall effect measurements—require thermometers that are sensitive over a wide temperature range, are low mass, have a rapid thermal response and have a minimal, easily correctable magnetoresistance. We recently reported the development of a new granular-metal oxide ceramic composite (cermet) for this purpose formed by co-sputtering of the metallic alloy nichrome (Ni0.8Cr0.2) and the insulator silicon dioxide (SiO2). In this earlier work, we found that co-sputtering of NiCr alloy and SiO2 in a reactive oxygen and inert argon gas mixture can produce resistive thin-film thermometers sensitive enough to be used in calorimetry and related measurements from room temperature down to below 100 mK in magnetic fields up to at least 35 T. In this work, we present results for thin cermet films grown with Cu0.55Ni0.45 and Ti0.05Cr0.95. Growth of CuNi-based thin-film cermets generally requires more oxygen in the working gas compared to NiCr and TiCr and yields thermometers that are much less sensitive than comparable NiCr-based thermometers. TiCr-based cermet thin-film thermometers have somewhat higher resistivity for similar sensitivities compared to NiCr-based cermet thin-film thermometers.

We study the controllable manipulation of vortices in a mesoscopic, superconducting "island" of Nb, using an integrated Josephson junction as a field-sensitive vortex detector. The island, divided by a single Josephson junction and suspended by Nb microbridges, was fabricated from a Nb/P_11-xNi_x/Nb tri-layer using a focused ion beam. We find that the system at select magnetic fields behaves as a vortex memory cell, where current pulses can be used to switch the vortex configuration between metastable states of distinctly different junction critical currents. Non-destructive read-out of a state is then easily done with an intermediate current. Furthermore, we show that the Josephson junction displays a strong magnetoresistive effect at current bias well above the junction critical current but below the onset of flux flow. This enables the junction to be used as a quantitative probe of magnetic field with better than single flux quantum resolution.

Experimental research on mesoscopic systems puts high demands on the measurement infrastructure, including measurement system with associated sample preparation, experimental design, measurement electronics, and data collection. Successful experiments require both the ability to manufacture small samples and to successfully and accurately study their novel properties. Here, we discuss some aspects and recent advancements of general measurement techniques that should benefit several characterization methods such as thermodynamic, magnetic, and transport studies of mesoscopic superconductors.

We report the effect of magnetic dilution on the physical properties of (Gd_1−xY_x)Cd₆ approximant crystals (ACs), close siblings of their corresponding quasicrystal (QC). Compared to the pure system GdCd₆, we observe remarkable changes in the thermodynamic and magnetic bulk properties near and below the static-ordering temperatures from diluting the magnetic Gd atoms with nonmagnetic Y atoms by only 1–3% (x=0.01–0.03). On the other hand, the corresponding QC system exhibits a monotonic change in its spin-glass behavior upon the magnetic dilution. We discuss the origin of the magnetic-dilution behavior in the present AC system in terms of possible magnetic frustration and short-range magnetic correlation that can be linked to its peculiar structure.

We investigate the critical behavior and magnetocaloric effects of the Gd-Au-Si (GAS) ferromagnetic quasicrystal approximants, Gd_13.7 Au_72.7 Si_13.6 [referred to as GAS(0)] and Gd_15.4 Au_68.6 Si_16.0 [GAS(100)]. The former is a conventional Tsai-type 1/1 approximant crystal, while the latter has a slightly different atomic decoration from the Tsai type (thus referred to as “pseudo-Tsai” type). Their critical exponents at the ferromagnetic transitions are close to those of the mean-field theory. Both GAS systems exhibit an interesting magnetic-field dependence of the specific heat, which is reflected in the behavior of their magnetocaloric effect (MCE). The MCE is characterized by an adiabatic cooling (heating) effect over a relatively broad temperature range below ∼30 K, which stems from a broad feature in the specific heat.

We report the structural and physical properties of two Y-Au-Si (YAS) compounds, Y(14.1)AU(69.2)Si(16.7) and Y15.4Au68.6Si16.1, which are 1/1 approximant crystals of a Tsai-type quasicrystal without intrinsic magnetic moments. The compounds differ by the presence of either a tetrahedron (Au,Si)(4) or a single Y atom at the center of their characteristic structural building unit consisting of concentric polyhedral shells. Both compounds exhibit bulk superconductivity, which seems to be of a conventional type-II BCS type. The compound with Y atoms at the cluster center has a slightly higher transition temperature with a sharper step in the specific heat than the compound with tetrahedral units. We discuss the occurrence of this superconducting state in the light of the specific structural and physical properties of these quasicrystal approximants.

We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe(4)As(4 )using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu moments of T-m = 15 K and a superconducting transition temperature of T-c = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of Gamma = 1.7. Ginzburg-Landau parameters of K-c similar to 67 and K-ab similar to 108 indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu magnetism is quasi-two dimensional (2D), as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and <0.04 K. A consequence of the quasi-2D nature of the Eu magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a kinklike anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 and 4 kG.

A differential, membrane-based nanocalorimeter for general specific heat studies of very small samples, ranging from 0.5 mg to sub-mu g in mass, is described. The calorimeter operates over the temperature range from above room temperature down to 0.5 K. It consists of a pair of cells, each of which is a stack of heaters and thermometer in the center of a silicon nitride membrane, in total giving a background heat capacity less than 100 nJ/K at 300 K, decreasing to 10 pJ/K at 1 K. The device has several distinctive features: (i) The resistive thermometer, made of a Ge1-xAux alloy, displays a high dimensionless sensitivity |dlnR/dlnT| greater than or similar to 1 over the entire temperature range. (ii) The sample is placed in direct contact with the thermometer, which is allowed to self-heat. The thermometer can thus be operated at high dc current to increase the resolution. (iii) Data are acquired with a set of eight synchronized lock-in amplifiers measuring dc, 1st and 2nd harmonic signals of heaters and thermometer. This gives high resolution and allows continuous output adjustments without additional noise. (iv) Absolute accuracy is achieved via a variable-frequency-fixed-phase technique in which the measurement frequency is automatically adjusted during the measurements to account for the temperature variation of the sample heat capacity and the device thermal conductance. The performance of the calorimeter is illustrated by studying the heat capacity of a small Au sample and the specific heat of a 2.6 mu g piece of superconducting Pb in various magnetic fields.

We present time-dependent numerical simulations of the thermal and electrical response of a membrane-based nanocalorimeter designed for general studies of heat capacity and latent heat of milligram to sub-microgram samples. The investigated device is based on freestanding, 150 nm thick silicon nitride membranes onto which thin film heaters and temperature sensors are fabricated. This design makes the thermal link small enough to allow both relaxation and ac steady-state methods to be used interchangeably. We compare simulations of the two-dimensional thermal behavior of the nanocalorimeter with the results of experiments. The simulations take current distribution, heat generation and heat flow into consideration, and shed light on the frequency dependent contribution of the membrane heat capacity in ac steady-state experiments. The simulations also illustrate where energy is stored, thus assisting further improvement of the device design.

To achieve accurate results in nanocalorimetry a detailed analysis and understanding of the behavior of the calorimetric system is required. There are especially two system-related aspects that should be taken in consideration: the properties of the empty cell and the effect of the thermal link between sample and cell. Here we study these two aspects for a membrane-based system where heater and thermometer are both in good contact with each other and the center of the membrane. Practical, analytical expressions for describing the frequency dependence of heat capacity, thermal conductance, and temperature oscillation of the system are formulated and compared with measurements and numerical simulations. We finally discuss the experimental conditions for an optimal working frequency, where high resolution and good absolute accuracy are combined.

Heat capacity measurements using the ac steady state method are often considered difficult to provide absolute accuracy. By adjusting the working frequency to maintain a constant phase and using the phase information to obtain the heat capacity, we have found that it is possible to achieve good absolute accuracy. Here we present a thermodynamic study of a similar to 2.6 mu g Pb superconducting crystal to demonstrate the newly opened capabilities. The sample is measured using a differential membrane-based calorimeter. The custom-made calorimetric cell is a pile of thin film Ti heater, insulation layer and Ge1-xAux thermometer fabricated in the center of two Si3N4 membranes. It has a background heat capacity < 100 nJ/K at 300 K, decreasing to 9 pJ/K at 1 K. The sample is characterized at temperatures down to 0.5 K. The zero field transition at T-c = 7.21 K has a width approximate to 20 mK and displays no upturn in C. From the heat capacity jump at T-c and the extrapolated Sommerfeld term we find Delta C/gamma T-c = 2.68. The latent heat curve obtained from the zero field heat capacity measurement, and the deviations of the thermodynamic critical field from the empirical expression H-c = H-c(0) [1 - (T/T-c)(2)] are discussed. Both analyses give results in good agreement with literature.

We have developed a membrane-based microcalorimeter for general measurements of heat capacity and latent heat using a combination of ac steady-state and relaxation methods. The differential calorimeter is designed for sub-microgram samples studied over a wide range of temperatures and magnetic fields. The device is based on free-standing silicon nitride membranes of 150 nm thickness onto which thin film heaters and temperature sensors are fabricated. While production-line fabrication may benefit from back-etching as a final step, it is often easier to start with pre-etched membranes in a research laboratory. With selected nano-fabrication methods this is possible. Due to a robust heater and very low thermal conductance of the final calorimeter, the sample can be heated to more than 100 K above base temperature. This enables instantaneous calibration, and makes the device capable of being used for ultra-fast temperature control, relaxation studies, and measurements that combine good absolute accuracy, high resolution, and information on latent heat.

We report on combined photoconductivity and annealing experiments in whisker-like crystals of the Bi-Sr-Ca-Cu-O (BSCCO) high-T-c superconductor. Both single-phase Bi2Sr2CaCu2O8+delta (Bi-2212) samples and crystals of the mixed phases Bi2Sr2Ca2Cu3O10+x (Bi-2223)/Bi-2212 have been subjected to annealing treatments at 90 degrees C in air in a few hours steps, up to a maximum total annealing time of 47 h. At every step, samples have been characterized by means of electrical resistance versus temperature (R versus T) and resistance versus time at fixed temperature (R versus t) measurements, both in the dark and under illumination with a UV-Vis halogen arc lamp. A careful comparison of the results from the two techniques has shown that, while for single-phase samples no effect is recorded, for mixed-phase samples an enhancement in the conductivity that increases with increasing annealing time is induced by the light at the nominal temperature T = 100 K, i.e. at an intermediate temperature between the critical temperatures of the two phases. A simple pseudo-1D model based on the Kudinov's scheme (Kudinov et al, 1993 Phys. Rev. B 47 9017-28) has been developed to account for the observed effects, which is based on the existence of Bi-2223 filaments embedded in the Bi-2212 matrix and on the presence of electronically active defects at their interfaces. This model reproduces fairly well the photoconductive experimental results and shows that the length of the Bi-2223 filaments decreases and the number of defects increases with increasing annealing time.

We explore tailored labyrinth-like vortex motion in a niobium film placed on top of periodic arrays of T and I -shaped permalloy elements by imaging the vortex distribution in magnetic fields applied perpendicular to the superconducting layer under different in-plane polarizations of the TI structure. At low temperatures, we observe pronounced meandering of vortex motion around the TI elements. Remarkably, vortices can easily penetrate the sample along the TI columns even though the average vortex pinning in the patterned area is larger than in a bare niobium film. Accordingly, at temperatures close to the superconducting transition temperature, Tc, the voltage-current curves in the patterned area show an earlier departure from the zero-resistivity state at small currents, followed by slower growth with increasing current as compared to the un-patterned film. We present a model based on magnetostatic interactions between magnetic charges at the edges of the polarized TI-elements and single magnetic charges of induced vortices to account for this behavior. We expect that similar magnetic structures imposing labyrinth-like vortex motion could be used for collective entanglement of vortices envisioned in quantum circuit operations.

High-quality crystals of CoTeO₄ were grown by application of chemical vapor transport reactions in closed silica ampoules, starting from polycrystalline material in a temperature gradient 640°C → 580°C with TeCl₄ as transport agent. Crystal structure analysis of CoTeO₄ from single crystal X-ray data revealed a dirutile-type structure with Co^II and Te^VI atoms at crystallographically distinct sites, each with point group symmetry . The statistical significance and accuracy of the previously reported structural model based on powder data with the ordered arrangement of Co and Te cations was noticeably improved. CoTeO₄ does not undergo a structural phase transition upon heating, but decomposes stepwise (Co₂Te₃O₈ as intermediate phase) to Co₃TeO₆ as the only crystalline phase stable above 770°C. Temperature-dependent magnetic susceptibility and dielectric measurements suggest antiferromagnetic ordering at ∼50 K. Optical absorption spectroscopy and computational studies reveal wide-band semiconductive behavior for CoTeO₄. The experimentally determined band gap of ∼2.42 eV is also found for CdS, which is frequently used in photovoltaic systems but is hazardous to the environment. Hence, CoTeO₄ might be a possible candidate to replace CdS in this regard.

We report on the specific-heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO(1-x)F(x). In zero field, we find a clear cusplike anomaly in C/T with Delta C/T(c) = 24 mJ/mol K(2) at T(c) = 49.5 K. In magnetic fields along the c axis, pronounced superconducting fluctuations induce broadening and suppression of the specific-heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of Gamma = 8. The small value of Delta C/T(c) yields a Sommerfeld coefficient gamma similar to 8 mJ/mol K(2), indicating that SmFeAsO(1-x)F(x) is characterized by a modest density of states and strong coupling.

Recent advances in electronics and nanofabrication have enabled membrane-based nanocalorimetry for measurements of the specific heat of microgram-sized samples. We have integrated a nanocalorimeter platform into a 4.5 T split-pair vertical-field magnet to allow for the simultaneous measurement of the specific heat and x-ray scattering in magnetic fields and at temperatures as low as 4 K. This multi-modal approach empowers researchers to directly correlate scattering experiments with insights from thermodynamic properties including structural, electronic, orbital, and magnetic phase transitions. The use of a nanocalorimeter sample platform enables numerous technical advantages: precise measurement and control of the sample temperature, quantification of beam heating effects, fast and precise positioning of the sample in the x-ray beam, and fast acquisition of x-ray scans over a wide temperature range without the need for time-consuming re-centering and re-alignment. Furthermore, on an YBa2Cu3O7-delta crystal and a copper foil, we demonstrate a novel approach to x-ray absorption spectroscopy by monitoring the change in sample temperature as a function of incident photon energy. Finally, we illustrate the new insights that can be gained from in situ structural and thermodynamic measurements by investigating the superheated state occurring at the first-order magneto-elastic phase transition of Fe2P, a material that is of interest for magnetocaloric applications.

Recent theoretical studies [G. Chen et al., Phys. Rev. B 82, 174440 (2010); H. Ishizuka et al., Phys. Rev. B 90, 184422 (2014)] for the magnetic Mott insulator Ba2NaOsO6 have proposed a low-temperature order parameter that breaks lattice rotational symmetry without breaking time reversal symmetry, leading to a nematic phase just above the magnetic ordering temperature. We present high-resolution calorimetric and magnetization data of the same Ba2NaOsO6 single crystal and show evidence for a weakly field-dependent phase transition occurring at a temperature of T-s approximate to 9.5 K, above the magnetic ordering temperature of T-c approximate to 7.5 K. This transition appears as a broadened step in the low-field temperature dependence of the specific heat. The evolution of the phase boundary with applied magnetic field suggests that this phase coincides with the phase of broken local point symmetry seen in NMR experiments at high fields [L. Lu et al., Nat. Commun. 8, 14407 (2017)]. Furthermore, the magnetic field dependence of the specific heat provides clear indications for magnetic correlations persisting at temperatures between T-c and T-s where long-range magnetic order is absent, giving support for the existence of the proposed nematic phase.