Superconducting Phase Research Articles

Metallic states beyond the Tomonaga-Luttinger liquid in one dimension

In this paper, we propose some new strongly correlated gapless states (or critical states) of spin-1/2 electrons in 1+1-dimensions, such as doped anti-ferromagnetic spin-1/2 Ising chain. We find doped anti-ferromagnetic Ising chain to be a different metallic phase from the doped ferromagnetic Ising chain, despite the two have identical symmetry. The doped anti-ferromagnetic Ising chain has a finite energy gap for all charge-1 fermionic excitations even without pairing caused by attractive interactions, resembling the pseudo-gap phase of underdoped high Tc superconductors. Applying a transverse field to the ferromagnetic and anti-ferromagnetic metallic phases can restore the $Z_2$ symmetry, which gives rise to two distinct critical points despite that the two transitions have exactly the same symmetry breaking pattern. We also propose new chiral metallic states. All those new gapless states are strongly correlated in the sense that they do not belong to the usual Tomonaga-Luttinger phase of fermions, i.e., they cannot be smoothly deformed into the non-interacting fermion systems of the same symmetry. Our non-perturbative results are obtained by noticing that gapless quantum systems have emergent categorical symmetries, i.e., non-invertible gravitational anomalies), which are described by multi-component partition functions that are modular covariant. This allows us to calculate the scaling dimensions and quantum numbers of all the low energy operators for those strongly correlated gapless states. This demonstrates an application of emergent categorical symmetries in determining low energy properties of strongly correlated gapless states, which are hard to obtain otherwise.

Kramers pairs of Majorana corner states in a topological insulator bilayer

We consider a system consisting of two tunnel-coupled two-dimensional topological insulators proximitized by a top and bottom superconductor with a phase difference of $\pi$ between them. We show that this system exhibits a time-reversal invariant second-order topological superconducting phase characterized by the presence of a Kramers pair of Majorana corner states at all four corners of a rectangular sample. We furthermore investigate the effect of a weak time-reversal symmetry breaking perturbation and show that an in-plane Zeeman field leads to an even richer phase diagram exhibiting two nonequivalent phases with two Majorana corner states per corner as well as an intermediate phase with only one Majorana corner state per corner. We derive our results analytically from continuum models describing our system. In addition, we also provide independent numerical confirmation of the resulting phases using discretized lattice representations of the models, which allows us to demonstrate the robustness of the topological phases and the Majorana corner states against parameter variations and potential disorder.

Holographic Lifshitz superconductors with Weyl correction

At the probe approximation, we construct a holographic p-wave conductor/superconductor model in the five-dimensional Lifshitz black hole with the Weyl correction via both numerical and analytical methods, and study the effects of the Lifshitz parameter z as well as the Weyl parameter gamma on the superconductor model. As we take into account one of the two corrections separately, the increasing z (gamma ) inhibits(enhances) the superconductor phase transition. When the two corrections are considered comprehensively, they display the obviously competitive effects on both the critical temperature and the vector condensate. In particular, the promoting effects of the Weyl parameter gamma on the critical temperature are obviously suppressed by the increasing Lifshitz parameter. Meanwhile, in the case of z<2.35(z>2.35), the condensate at lower temperature decreases(increases) with the increasing Weyl parameter gamma . What is more, the difference among the condensate with the fixed Weyl parameter(gamma =-frac{6}{100},0,frac{4}{100}) decreases(increases) with the increasing Lifshitz parameter z in the region z<2.35(z>2.35). Furthermore, the increasing z obviously suppresses the real part of conductivity for all value of the Weyl parameter gamma . In addition, the analytical results agree well with the ones from the numerical method.

Investigations of arsenic substitution on the physical, electrical and magnetic properties of Bi-2212 superconductors

ABSTRACT The effect of Arsenic substitution on the properties of Bi-2212 superconductors was studied. Solid-state reaction technique was used for the synthesis of Bi2−xAsxSr2Ca1.1Cu2.1O8 samples, x = 0, 0.1, 0.2 and 0.4. The X-ray diffraction confirmed the formation of the tetragonal phase of Bi-2212 superconductors. The decrement in the c parameter confirmed that As insertions were introduced into the Bi sites of the system. The grain morphology of the surface was analysed by scanning electron microscopy, where the amount of arsenic increases with As substitution. The superconducting transition temperature (Tc) was determined from the DC-electrical resistivity by the standard four-probe technique. Tc was enhanced up to x = 0.1 above which it was suppressed. For magnetism, half magnetic-hysteresis loops were obtained by SQUID magnetometer at different temperatures (5 and 30 K). The hysteresis loops sizes increased with increasing As-concentrations. The critical current density (Jc) and the flux pinning mechanisms were investigated via different models.

Variational Approach for Many-Body Systems at Finite Temperature.

We introduce an equation for density matrices that ensures a monotonic decrease of the free energy and reaches a fixed point at the Gibbs thermal. We build a variational approach for many-body systems that can be applied to a broad class of states, including all bosonic and fermionic Gaussian, as well as their generalizations obtained by unitary transformations, such as polaron transformations in electron-phonon systems. We apply it to the Holstein model on 20×20 and 50×50 square lattices, and predict phase separation between the superconducting and charge-density wave phases in the strong interaction regime.

Pressure tuning of superconductivity in Mo8Ga41 single crystals

We report on pressure tuning of superconductivity in a strongly coupled superconductor ${\mathrm{Mo}}_{8}{\mathrm{Ga}}_{41}$ with a critical temperature ${T}_{c}\ensuremath{\sim}9.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Resistance measurements show that ${T}_{c}$ is suppressed to 3.6 K at 19 GPa followed by a nonmonotonic variation at higher pressures. Synchrotron x-ray diffraction experiments reveal an amorphization taking place at the critical pressure ${P}_{c}\ensuremath{\sim}19\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. As comparing with $\mathrm{Mg}{\mathrm{B}}_{2}$, the ${\mathrm{Mo}}_{8}{\mathrm{Ga}}_{41}$ shows a relative low decreasing rate of ${T}_{c}$, which might be attributed to its unique vertex-sharing type of cluster architecture. Survival of superconductivity across the crystalline to amorphous transition indicates that, although long-range structural order of endohedral Ga clusters is destroyed, short-range-ordered intracluster structure still exists and allows for local Cooper pairing. Due to presence of disorders in the amorphous phase, establishment of Cooper pair coherence is shifted to lower temperatures, which causes unusual transport behaviors near ${T}_{c}$, e.g., the anomalous resistance peak and dip. The amorphous superconducting phase is able to be retained to ambient pressure and therefore provides an ideal platform to study disorder effects on superconductivity.

η -paired superconducting hidden phase in photodoped Mott insulators

We show that a metastable $\eta$--pairing superconducting phase can be induced by photodoping doublons and holes into a strongly repulsive fermionic Hubbard model. The doublon-hole condensate originates from an intrinsic doublon-hole exchange interaction and does not rely on the symmetry of the half-filled Hubbard model. It extends over a wide range of doublon densities and effective temperatures. Different non-equilibrium protocols to realize this state are proposed and numerically tested. We also study the optical conductivity in the superconducting phase, which exhibits ideal metallic behavior, i.e., a delta function at zero-frequency in the conductivity, in conjunction with a negative conductivity at large frequencies. These characteristic optical properties can provide a fingerprint of the $\eta$-pairing phase in pump-probe experiments.

Normal-state and superconducting properties of the cubic Laves phase ThIr2

The physical properties of the cubic Laves phase superconductor ThIr2 have been studied by resistivity, magnetic susceptibility and thermodynamic measurements. This compound is found to be a weak coupling type-II superconductor with a Tc of 5.58 K and a fully isotropic superconducting gap. The electronic specific heat coefficient, upper critical field and Ginzburg-Laudau parameter are determined to be 13.1 mJ mol−1 K−2, 2.25 T, and 20.4, respectively. In the normal state, the susceptibility follows a Curie-Weiss behavior with a paramagnetic moment of 0.46 μB per f.u.. Furthermore, the low-temperature resistivity exhibits a quadratic temperature dependence, yielding a large Kadowaki-Woods ratio of 7.6×10−5μΩ cm/(mJ mol−1 K−1)2. This, together with a Wilson ratio of ~1.4, suggests the importance of electronic correlation in ThIr2. First-principles calculations show that the valence band states below the Fermi level are dominated by the Ir-5d orbitals, yet significant hybridizations with the Ir-6p, Th-6d, 7p orbits are detected at the Fermi level. The inclusion of spin-orbit coupling has little influence on the Fermi surface shape but lowers the density of states toward the experimental value. A comparison with isostructural CaIr2 and SrIr2 is also presented.

An analytic study on the excited states of holographic superconductors

Based on the Sturm-Liouville eigenvalue problem, we develop a general analytic technique to investigate the excited states of the holographic superconductors. By including more higher order terms in the expansion of the trial function, we observe that the analytic results agree well with the numeric data, which indicates that the Sturm-Liouville method is very powerful to study the holographic superconductors even if we consider the excited states. For both the holographic s-wave and p-wave models, we find that the excited state has a lower critical temperature than the corresponding ground state and the difference of the dimensionless critical chemical potential between the consecutive states is around 5. Moreover, we analytically confirm that the holographic superconductor phase transition with the excited states belongs to the second order, which can be used to back up the numerical findings for both s-wave and p-wave superconductors.

Strong In-Plane Magnetic Field-Induced Reemergent Superconductivity in the van der Waals Heterointerface of NbSe2 and CrCl3.

A magnetic field is generally considered to be incompatible with superconductivity as it tends to spin-polarize electrons and breaks apart the opposite-spin singlet superconducting Cooper pairs. Here, an experimental phenomenon is observed that an intriguing reemergent superconductivity evolves from a conventional superconductivity undergoing a hump-like intermediate phase with a finite electric resistance in the van der Waals heterointerface of layered NbSe2 and CrCl3 flakes. This phenomenon merely occurred when the applied magnetic field is parallel to the sample plane and perpendicular to the electric current direction as compared to the reference sample of a NbSe2 thin flake. The strong anisotropy of the reemergent superconducting phase is pointed to the nature of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state driven by the strong interfacial spin-orbit coupling between NbSe2 and CrCl3 layers. The theoretical picture of FFLO state nodes induced by Josephson vortices collectively pinning is presented for well understanding the experimental observation of the reemergent superconductivity. This finding sheds light on an opportunity to search for the exotic FFLO state in the van der Waals heterostructures with strong interfacial spin-orbit coupling.

Exotic Superconducting States in FeSe-based Materials

High-temperature superconductivity and a wide variety of exotic superconducting states discovered in FeSe-based materials have been at the frontier of research on condensed matter physics over the past decade. Unique properties originating from the multiband electronic structure, strongly orbital-dependent phenomena, extremely small Fermi energy, electronic nematicity, and topological aspects give rise to many distinct and fascinating superconducting states. Here, we provide an overview of our current understanding of the superconductivity of {\it bulk} FeSe-based materials, focusing on FeSe and the isovalent substituted FeSe$_{1-x}$S$_{x}$ and FeSe$_{1-x}$Te$_{x}$. We discuss the highly nontrivial superconducting properties in FeSe, including extremely anisotropic pairing states, crossover phenomena from Bardeen--Cooper--Schrieffer (BCS) to Bose--Einstein condensation (BEC) states, a novel field-induced superconducting phase, and broken time-reversal symmetry. We also discuss the evolution of the superconducting gap function with sulfur and tellurium doping, paying particular attention to the impact of quantum critical nematic fluctuations and the topological superconductivity. FeSe-based materials provide an excellent playground to study various exotic superconducting states.

Towards a Single Chemical Model for Understanding Lanthanide Hexaborides

AbstractLanthanide hexaborides (LnB6) have disparate and often anomalous properties, from structurally homogeneous mixed valency, to superconductivity, spectral anomalies, and unexplained phase transitions. It is unclear how such a diversity of properties may arise in the solids of identical crystal structures and seemingly very similar electronic structures. Building on our previous model for SmB6 (mixed valent, with a peak in specific heat, and pressure induced magnetic phase transitions), we present a unifying dynamic bonding model for LnB6 that explains simultaneously EuB6 (possessing an anomalous peak in specific heat at low T, magnetic phase transitions, and no mixed valency), YbB6 (mixed valent topological insulator), and rather ordinary LaB6. We show that Ln can engage in covalent bonding with boron, and, in some members of the LnB6 family, also easily access alternative bonding states through the electron–phonon coupling. The accessibility, relative energetics, and bonding nature of the states involved dictate the properties.

Majorana-induced DC Shapiro steps in topological Josephson junctions

The demonstration of the non-Abelian properties of Majorana bound states (MBS) is a crucial step toward topological quantum computing. We theoretically investigate how Majorana fusion rules manifest themselves in the current-voltage characteristics of a topological Josephson junction. The junction is built on U-shaped quantum spin Hall edges and hosts a Majorana qubit formed by four MBS. Owing to Majorana fusion rules, inter- and intra-edge couplings among adjacent MBS provide two orthogonal components in the rotation axis of the Majorana qubit. We show that the interplay of the dynamics of the superconductor phase difference and the Majorana qubit governs the Josephson effect. Strikingly, we identify sequential jumps of the voltage across the junction with increasing DC current bias without external AC driving. Its role is replaced by the intrinsic Rabi oscillations of the Majorana qubit. This phenomenon, DC Shapiro steps, is a manifestation of the non-trivial fusion rules of MBS.

Tuning magnetic confinement of spin-triplet superconductivity

Electrical magnetoresistance and tunnel diode oscillator measurements were performed under external magnetic fields up to 41 T applied along the crystallographic b axis (hard axis) of UTe2 as a function of temperature and applied pressures up to 18.8 kbar. In this work, we track the field-induced first-order transition between superconducting and magnetic field-polarized phases as a function of applied pressure, showing suppression of the transition with increasing pressure until the demise of superconductivity near 16 kbar and the appearance of a pressure-induced ferromagnetic-like ground state that is distinct from the field-polarized phase and stable at zero field. Together with evidence for the evolution of a second superconducting phase and its upper critical field with pressure, we examine the confinement of superconductivity by two orthogonal magnetic phases and the implications for understanding the boundaries of triplet superconductivity.

Entanglement Research for the Coupled Superconducting Phase Qubit and a Two-Level System

Entanglement can exist not only in the microscopic system (e.g., atom, photon, and ion trap) but also in macroscopic systems. According to recent research, entanglement can be achieved and controlled in superconducting devices. The quantum dynamics and entanglement mechanism of the coupled superconducting phase qubit and a two-level system (TLS) were demonstrated when the bipartite system was under microwave driving. Besides, the results reveal that when the system was experiencing decoherence, entanglement (concurrence) of the coupled superconducting phase qubit and TLS would oscillate damply with microwave driving time, even exhibiting concurrence sudden death and revival. The coupling effect of the superconducting qubit and TLS system and the resonant microwave together help to achieve entanglement, while concurrence death and concurrence revival are dependent on the decoherence source and mechanism, for example, the resonant microwave driving time acting on the bipartite coupling system. Furthermore, the simulation results show the entanglement of the coupled qubit and TLS system also depends on the purity of the initial states of the system. The article carried out a numerical simulation on the entanglement of different initial states, and the results showed that the entanglement of the coupled system changes with different initial states. For different initial states, entanglement, sudden death, and rejuvenation are still visible.

The retention at ambient of the high-pressure-induced metastable superconducting phases in antimony single crystals

We have investigated, resistively, non-superconducting single-crystalline antimony and detected several structural transitions under pressures up to 63 GPa. Superconductivity associated with the m-HGL phase with a superconducting transition temperature (Tc) of 3.4 K above ~ 9 GPa and the bcc phase with a Tc of 4.0 K above ~ 30 GPa have been successfully retained up to 135 K and 110 K, respectively, upon the complete and rapid removal of pressure at 77 K. The observations offer a possible path to retain at ambient the pressure-induced metastable superconducting phase in HgBa2Ca2Cu3O8+δ with a Tc of 164 K under ~31 GPa and in LaH10 with a Tc of ~260 K under ~200 GPa, which would make their applications practical by removing the serious high-pressure constraint. The same is also true for superconducting topological devices using antimony.

Vestigial Van Hove Singularity and Higher-Temperature Superconducting Phase Induced by Perpendicular Uniaxial Pressures in Quasi-Two-Dimensional Superconductors

We examine quasi-two-dimensional superconductors near half-filling under uniaxial pressures perpendicular to conductive layers (hereafter called perpendicular pressures). It is a natural conjecture that the perpendicular pressure decreases Tc because it increases the interlayer electron hopping energy t_z, which weakens the logarithmic enhancement in the density of states due to the two-dimensional Van Hove singularity. It is shown that, contrary to this conjecture, the perpendicular pressure can significantly enhance Tc in systems off half-filling before it decreases Tc, and the strength of the enhancement significantly depends on the pairing symmetry. When the indices d, d', cz, and sz are defined for the basis functions cos k_x - cos k_y, sin k_x sin k_y, cos k_z, and sin k_z, respectively, it is shown that for s-, d-, cz-, and cz-d-wave pairing, Tc steeply increases with increasing t_z near a cusp at a certain value of t_z. On the other hand, for p-, cz-p-, sz-p-, and d'-wave pairing, Tc is almost unaffected by tz. For sz- and sz-d-wave pairing, Tc exhibits a broad and weak peak. The enhancement in Tc is the largest for this state and the second largest for the d-wave pairing and interlayer singlet (cz-wave) pairing. These results may explain recent observations in Sr2RuO4 under perpendicular pressures. A comparison between the theoretical and experimental results indicates that the p-, cz-p-, and sz-p-wave states, including chiral states, and the d'-wave state are the most likely candidates for the intrinsic 1.5-K phase, and the d-, cz-d-, and cz-wave states are the most likely candidates for the 3-K phase induced by the perpendicular pressure. The cz-p- and sz-p-wave states are interlayer triplet and interlayer singlet p-wave states with horizontal line nodes, respectively.

Unveiling mechanisms of electric field effects on superconductors by a magnetic field response

We demonstrate that superconducting aluminium nano-bridges can be driven into a state with complete suppression of the critical supercurrent via electrostatic gating. Probing both in- and out-of-plane magnetic field responses in the presence of electrostatic gating can unveil the mechanisms that primarily cause the superconducting electric field effects. Remarkably, we find that a magnetic field, independently of its orientation, has only a weak influence on the critical electric field that identifies the transition from the superconducting state to a phase with vanishing critical supercurrent. This observation points to the absence of a direct coupling between the electric field and the amplitude of the superconducting order parameter or $2\pi$-phase slips via vortex generation. The magnetic field effect observed in the presence of electrostatic gating is described within a microscopic model where a spatially uniform inter-band $\pi$-phase is stabilized by the electric field. Such an intrinsic superconducting phase rearrangement can account for the suppression of the supercurrent, as well as for the weak dependence of the critical magnetic fields on the electric field.

Effects of Rashba-spin–orbit coupling on superconducting boron-doped nanocrystalline diamond films: evidence of interfacial triplet superconductivity

Among the many remarkable properties of diamond, the ability to superconduct when heavily doped with boron has attracted much interest in the carbon community. When considering the nanocrystalline boron doped system, the reduced dimensionality and confinement effects have led to several intriguing observations most notably, signatures of a mixed superconducting phase. Here we present ultra-high-resolution transmission electron microscopy imaging of the grain boundary and demonstrate how the complex microstructure leads to enhanced carrier correlations. We observe hallmark features of spin–orbit coupling (SOC) manifested as the weak anti-localization effect. The enhanced SOC is believed to result from a combination of inversion symmetry breaking at the grain boundary interfaces along with antisymmetric confinement potential between grains, inducing a Rashba-type SOC. From a pronounced zero bias peak in the differential conductance, we demonstrate signatures of a triplet component believed to result from spin mixing caused by tunneling of singlet Cooper pairs through such Rashba-SOC grain boundary junctions.

Structural and morphological characterization of YBa2Cu3O7-x films deposited by screen printing from YBa2Cu3O6.962 superconductor in bulk

YBa2Cu3O7-x films were deposited onto flexible copper substrates by screen printing technique. YBa2Cu3O7-x films were prepared from a YBa2Cu3O7-x superconductor powder with ethylene glycol. The mixtures were screen printed and then sintered in air at different temperatures: 373, 473, and 573 K. The structural characterization showed the presence of different phases; the proportion of phases in films depends on sintering temperature. Scanning electron microscope images showed that an annealing temperature increase leads to an increase in grain size due to a coalescence process, which promotes the growth of superconductor phases with higher oxygen content.