Superconducting Phase Research Articles

Field Induced Multiple Superconducting Phases in UTe_{2} along Hard Magnetic Axis.

The superconducting (SC) phase diagram in uranium ditelluride is explored under magnetic fields (H) along the hard magnetic b axis using a high-quality single crystal with T_{c}=2.1 K. Simultaneous electrical resistivity and ac magnetic susceptibility measurements discern low- and high-field SC (LFSC and HFSC, respectively) phases with contrasting field-angular dependence. Crystal quality increases the upper critical field of the LFSC phase, but the H^{*} of ∼15 T, at which the HFSC phase appears, is always the same through the various crystals. A phase boundary signature is also observed inside the LFSC phase near H^{*}, indicating an intermediate SC phase characterized by small flux pinning forces.

Customizing topological phases in the twisted bilayer superconductors with even-parity pairings

We investigate the topological properties of twisted bilayer superconductors with different even-parity pairings in each layer. In the presence of spin–orbit coupling, the Hamiltonian is mapped into an effective odd-parity superconductor. Based on this, we deduce the topological properties by examining the relative configuration between Fermi surface and Dirac pairing node. We show that mixed Rashba and Dresselhaus spin–orbit coupling and anisotropic hopping terms, which break the C 4 symmetry of the Fermi surface, can induce first-order topological superconductors with non-zero bulk Chern number. This provides a versatile way to control the topological phases of bilayer superconductors by adjusting the twisted angle and chemical potential. We demonstrate our results using a typical twisted angle of 53.13°, at which the translation symmetry is restored and the Chern number and edge state are calculated using the Moiré momentum.

Twisted chiral superconductivity in photodoped frustrated Mott insulators

Recent advances in ultrafast pump-probe spectroscopy provide access to hidden phases of correlated matter, including light-induced superconducting states, but the theoretical understanding of these nonequilibrium phases remains limited. Here we report how a new type of chiral superconducting phase can be stabilized in photodoped frustrated Mott insulators. The metastable phase features a spatially varying order parameter with a $120^\circ$ phase twist which breaks both time-reversal and inversion symmetry. Under an external electric pulse, the $120^\circ$ chiral superconducting state can exhibit a second-order supercurrent perpendicular to the field in addition to a first-order parallel response, similar to a nonlinear anomalous Hall effect. This phase can be tuned by artificial gauge fields when the system is dressed by high-frequency periodic driving. The mechanism revealed in this study applies to Mott insulators on various frustrated lattices and the hidden superconducting phase can be realized in both cold-atom quantum simulators and correlated solids.

Landau-Ginzburg theory of charge density wave formation accompanying lattice and electronic long-range ordering

We propose an analytical Landau-Ginzburg theory of the charge density waves coupled with lattice and electronic long-range order parameters. Examples of long-range order include electronic wave function of superconducting Cooper pairs, structural distortions, electric polarization, and magnetization. We formulate the Landau-Ginzburg free energy density as power expansion with respect to the charge density and other long-range order parameters, as well as their spatial gradients, and biquadratic coupling terms. We introduced a biquadratic coupling between the charge density gradient and long-range order parameters, as well as nonlinear higher gradients of the long-range order parameters. The biquadratic gradient coupling is critical to the appearance of different spatially-modulated phases in charge-ordered ferroics and high-temperature superconductors. We derived the thermodynamic conditions for the stability of the spatially-modulated phases, which are the intertwined spatial waves of charge density and lattice/electronic long-range order. The analytical expressions for the energies of different phases, corresponding order parameters, charge density waves amplitudes and modulation periods, obtained in this work, can be employed to guide the comprehensive physical explanation, deconvolution and Bayesian analysis of experimental data on quantum materials ranging from charge-ordered ferroics to high-temperature superconductors.

Majorana vortex modes in spin-singlet chiral superconductors with noncollinear spin ordering: Local density of states study

In the present study topologically nontrivial edge and vortex bound states are described in the coexistence phase of chiral spin-singlet superconductivity and noncollinear spin ordering on a triangular lattice in the presence of few (up to four) vortices. We consider the topological phase transition induced by the magnetic order between the phase hosting Majorana modes and the initial phase of the chiral d-wave superconductivity supporting non-Majorana modes which is also topologically nontrivial. The change of the excitation spectrum at the critical point is obtained in both cases of open and periodic boundary conditions in the presence of vortices. It is proved that zero energy Majorana modes localized at vortex cores are caused by noncollinear long-range magnetic ordering. Even though nearby excitation energies of subgap states including the edge-localized and vortex-localized states are very close to each other, the energy difference between different vortex bound states is an order of magnitude higher. This difference determines the energy gap for Majorana vortex modes separating them from other vortex bound states. It is found that even in the presence of noncollinear spin ordering its value can be estimated from the excitation energy of vortex bound states in the pure chiral d-wave state for the nonmagnetic case. By studying local density of states near the vortex cores the possibility to experimentally detect the described Majorana vortex modes by scanning tunneling microscopy is discussed. It is demonstrated that Majorana vortex modes and Majorana antivortex modes induced by noncollinear magnetism have different features in energy and spatially resolved density of states due to the chiral symmetry on the superconducting order parameter.

Vortex Glass-Vortex Liquid Transition in BaFe2(As1-xPx)2 and CaKFe4As4 Superconductors from Multi-Harmonic AC Magnetic Susceptibility Studies.

For practical applications of superconductors, understanding the vortex matter and dynamics is of paramount importance. An important issue in this context is the transition of the vortex glass, which is a true superconducting phase, into a vortex liquid phase having a linear dissipation. By using multi-harmonic susceptibility studies, we have investigated the vortex glass-vortex liquid phase transitions in CaKFe4As4 and BaFe2(As0.68P0.32)2 single crystals. The principle of our method relates the on-set of the third-harmonic susceptibility response with the appearance of a vortex-glass phase in which the dissipation is non-linear. Similar to the high-critical temperature cuprate superconductors, we have shown that even in these iron-based superconductors with significant lower critical temperatures, such phase transition can be treated as a melting in the sense of Lindemann's approach, considering an anisotropic Ginzburg-Landau model. The experimental data are consistent with a temperature-dependent London penetration depth given by a 3D XY fluctuations model. The fitting parameters allowed us to extrapolate the vortex melting lines down to the temperature of liquid hydrogen, and such extrapolation showed that CaKFe4As4 is a very promising superconducting material for high field applications in liquid hydrogen, with a melting field at 20 K of the order of 100 T.

The mixed-state entanglement in holographic p-wave superconductor model

In this paper, we investigate the mixed-state entanglement in a model of p-wave superconductivity phase transition using holographic methods. We calculate several entanglement measures, including holographic entanglement entropy (HEE), mutual information (MI), and entanglement wedge cross-section (EWCS). Our results show that these measures display critical behavior at the phase transition points, with the EWCS exhibiting opposite temperature behavior compared to the HEE. Furthermore, we explore the behavior of thermodynamics and holographic quantum information at the zeroth-order phase transition point and find that it is opposite to that observed in the first-order phase transition. Additionally, we find that the critical exponents of all entanglement measures are twice those of the condensate. Our findings also suggest that the EWCS is a more sensitive indicator of the critical behavior of phase transitions than the HEE. Lastly, we uncover a universal inequality in the growth rates of EWCS and MI near critical points in thermal phase transitions, such as p-wave and s-wave superconductivity, suggesting that MI captures more information than EWCS when a phase transition first occurs.

Comparative Study of Structural, Electrical, and Mechanical Properties of (Tl, Hg)-1223 High Temperature Superconducting Phase Substituted by Lead Oxide and Lead Dioxide

Comparative Study of Structural, Electrical, and Mechanical Properties of (Tl, Hg)-1223 High Temperature Superconducting Phase Substituted by Lead Oxide and Lead Dioxide

Parity Transition of Spin-Singlet Superconductivity Using Sublattice Degrees of Freedom.

Recently, a superconducting (SC) transition from low-field (LF) to high-field (HF) SC states was reported in CeRh_{2}As_{2}, indicating the existence of multiple SC states. It has been theoretically noted that the existence of two Ce sites in the unit cell, the so-called sublattice degrees of freedom owing to the local inversion symmetry breaking at the Ce sites, can lead to the appearance of multiple SC phases even under an interaction inducing spin-singlet superconductivity. CeRh_{2}As_{2} is considered as the first example of multiple SC phases owing to this sublattice degree of freedom. However, microscopic information about the SC states has not yet been reported. In this study, we measured the SC spin susceptibility at two crystallographically inequivalent As sites using nuclear magnetic resonance for various magnetic fields. Our experimental results strongly indicate a spin-singlet state in both SC phases. In addition, the antiferromagnetic phase, which appears within the SC phase, only coexists with the LF SC phase; there is no sign of magnetic ordering in the HF SC phase. The present Letter reveals unique SC properties originating from the locally noncentrosymmetric characteristics.

First-principles study of moderate phonon-mediated pairing in high-pressure monoclinic phase of BiS2-based superconductors

Isotope effect on superconductive transition temperature (T c) is an essential indicator to examine whether the mechanism of superconductors is conventional. Unconventional isotope effect of BiS2-based superconductors has been previously reported in ambient-pressure tetragonal phase. However, to comprehensively ascertain the nature of superconductivity, the investigation of BiS2-based system in high-pressure structure is highly desirable. In this work, we carried out the first-principles calculations of phonon spectra and superconductivity in high-pressure monoclinic phase of LaO0.5F0.5BiS2 with 32S and 34S, and observed that the corresponding isotope coefficient is 0.13 ≤ α ≤ 0.20. This value is much greater than that of BiS2-based superconductors in ambient-pressure phase, but slightly smaller than that of conventional MgB2. Taking into account the calculated T c lower than experimental results, we finally conclude that the moderate phonon-mediated pairing plays a significant role in forming superconductivity of BiS2-based system in high-pressure phase, moreover, the cooperative multiple paring interactions should also be considered.

Effect of hydrostatic pressure on the unconventional charge density wave and superconducting properties in two distinct phases of doped kagome superconductors CsV3−xTixSb5

Recent studies on the kagome metal ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ have revealed an intricate interplay between unconventional charge density waves (CDWs) and superconductivity (SC). Substitutions of Ti for V can perturb the CDW and produce two distinct SC phases with different pairing symmetries in the series of $\mathrm{Cs}{\mathrm{V}}_{3\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{Sb}}_{5}$. Here, we performed a comprehensive high-pressure study on Ti-doped $\mathrm{Cs}{\mathrm{V}}_{3\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{Sb}}_{5}$ ($x=0.04,0.15$) samples to elucidate the combined effects of physical pressure and chemical doping on the coexistence of CDWs and SC. Under high pressures, the M-shaped double superconducting dome observed in the parent ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ is retained but very weakened for $x=0.04$, and it is replaced by a single broad superconducting dome for $x=0.15$. Accordingly, the optimal superconducting ${T}_{\mathrm{c}}$ under high pressures decreases gradually with increasing Ti-doping level. On the other hand, the upper critical field first exhibits an obvious rise with enhanced flux pinning effects by disorders for $x=0.04$ and then is suppressed upon further increasing Ti doping. The constructed $T\text{\ensuremath{-}}P$ phase diagrams of $\mathrm{Cs}{\mathrm{V}}_{3\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{Sb}}_{5}$ reveal the competition and coexistence of CDWs and SC. Our results provide more insight into the intertwined competing electronic orders in the Ti-doped kagome superconductors $\mathrm{Cs}{\mathrm{V}}_{3\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{Sb}}_{5}$.

Nematic single-component superconductivity and loop-current order from pair-density wave instability

We investigate the nematic and loop-current type orders that may arise as vestigial precursor phases in a model with an underlying pair-density wave (PDW) instability. We discuss how such a vestigial phase gives rise to a highly anisotropic stiffness for a coexisting single-component superconductor with low intrinsic stiffness, as is the case for the underdoped cuprate superconductors. Next, focusing on a regime with a mean-field PDW ground state with loop-current and nematic $xy$ (B$_{2g}$) order, we find a preemptive transition into a low and high-temperature vestigial phase with loop-current and nematic order corresponding to $xy$ (B$_{2g}$) and $x^2-y^2$ (B$_{1g}$) symmetry respectively. Near the transition between the two phases, a state of soft nematic order emerges for which we expect that the nematic director is readily pinned away from the high-symmetry directions in the presence of an external field. Results are discussed in relation to findings in the cuprates, especially to the recently inferred highly anisotropic superconducting fluctuations [W{\aa}rdh {\em et al.}, ``Colossal transverse magnetoresistance due to nematic superconducting phase fluctuations in a copper oxide'', arXiv:2203.06769], giving additional evidence for an underlying ubiquitous PDW instability in these materials.

Topological properties in an Aubry–André–Harper model with p-wave superconducting pairing

Abstract We study the topological properties of the one-dimensional p-wave Aubry–André–Harper (AAH) model with periodic incommensurate potential and transition coupling. The calculation results show that due to co-influence of the incommensurate potential and modulation phase, three topological phases arise in different parameter regions: a topologically trivial phase, Su–Schrieffer–Heeger (SSH)-like topological phase, and Kitaev-like topological superconducting phase with Majorana zero modes. By evaluating the Andreev reflection conductance, we see that in the Kitaev-like phase, the quantized conductance plateau comes into being at the zero-bias limit, due to the occurrence of resonant Andreev reflection. In addition, when the disorder effect is incorporated, the SSH-like topology is modified sensitively and the degenerate topological states split, whereas the Kitaev-like topological phase is robust to weak disorder. Finally, we find that disorder can induce topological phase transition, i.e. from the topologically trivial phase to the topological phase. Based on these results, we believe that our findings have significance for studying the topological phase transition in a one-dimensional topological superconducting system. Also, it provides a feasible scheme for clarifying different topological phases.

Charged compact stars with color–flavor-locked strange quark matter in f(R,T) gravity

Due to the high density of neutron stars, there is still an open question on the complete structure of the inner core of compact object. It is a great challenge to propose a model describing the highly-dense matter of neutron stars since there are neither experimentally nor observationally adequate results. In theoretical physics, Quantum Chromodynamics (QCD) predicts that at high densities and low temperatures the inner core of neutron stars could be made of exotic quark matter in the color-flavor-locked (CFL) phase of color superconductivity. Regarding the equation-of-state, we predict the existence of electrically charged strange quark stars in the background of the modified theory of gravity f(R,T). In our discussion, we assume a linear relation between charge density and the fluid energy density, and also utilize the f(R,T)=R+2βT model to numerically compute the generalized Tolman–Oppenheimer–Volkoff (TOV) structure equations. In order to compute the properties of quark stars, we discuss the mass–radius profile, the effect of the charge in the stellar interior, factor of compactness, and mass-central energy density relation for stellar stability. Finally, we provide a set of basic differences between the standard Einstein-Maxwell gravity and the modified f(R,T) gravity theory.

Effect of independent alloying of zirconium with bronze and niobium on superconducting properties of Nb3Sn in a Cu(Sn)/Nb system

The present work focuses on the effect of internal (niobium) and external (bronze) alloying of Zr on the superconducting properties of Nb3Sn in a Cu(Sn)/Nb system through the diffusion couple method. Alloying of Zr with either Cu(5.5 at.% Sn) or Nb enhances the critical current density (Jc) of the Nb3Sn superconductor phase developed at the interdiffusion zone of Cu(5.5 at.% Sn, 2 at.% Zr)/Nb and Cu(5.5 at.% Sn)/Nb(2 at.% Zr). Enhancement of Jc of Zr-alloyed Nb3Sn has been attributed to an increment in grain boundary fraction for Nb3Sn. Dew Hughes model provided an insight into the pinning mechanisms responsible for enhancement of Jc. Strengthening of pinning forces of Nb3Sn with internal alloying of Zr might have originated because of the nanometer-sized β-(Nb, Zr) second-phase developed within the Nb3Sn phase matrix. No change in critical temperature for Zr-alloyed Nb3Sn makes Zr a suitable alloying element for applications.

Dynamics of photoinduced ferromagnetism in oxides with orbital degeneracy

By using intense coherent electromagnetic radiation, it may be possible to manipulate the properties of quantum materials very quickly, or even induce new and potentially useful phases that are absent in equilibrium. For instance, ultrafast control of magnetic dynamics is crucial for a number of proposed spintronic devices, and it can also shed light on the possible dynamics of correlated phases out of equilibrium. Inspired by recent experiments on spin-orbital ferromagnet ${\mathrm{YTiO}}_{3}$, we consider the nonequilibrium dynamics of a Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model the dynamics of the magnon excitations in this system following an optical pulse that resonantly excites infrared-active phonon modes. As the phonons ring down, they can dynamically couple the orbitals with the low-lying magnons, leading to a dramatically modified effective bath for the magnons. We show that this transient coupling can lead to a dynamical acceleration of the magnetization dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the parameter space more, we find that the magnon dynamics can also even completely reverse, leading to a negative relaxation rate when the pump is blue-detuned with respect to the orbital bath resonance. We therefore show that by using specially targeted optical pulses, one can exert a much greater degree of control over the magnetization dynamics, allowing one to optically steer magnetic order in this system. We conclude by discussing interesting parallels between the magnetization dynamics we find here and recent experiments on photoinduced superconductivity, where it is similarly observed that depending on the initial pump frequency, an apparent metastable superconducting phase emerges.

Electric Control of 2D Van Hove Singularity in Oxide Ultra-Thin Films.

Divergent density of states (DOS) can induce extraordinary phenomena such as significant enhancement of superconductivity and unexpected phase transitions. Moreover, van Hove singularities (VHSs) lead to divergent DOS in 2D systems. Despite recent interest in VHSs, only a few controllable cases have been reported to date. In this work, by utilizing an atomically ultra-thin SrRuO3 film, the electronic structure of a 2D VHS is investigated with angle-resolved photoemission spectroscopy and transport properties are controlled. By applying electric fields with alkali metal deposition and ionic-liquid gating methods, the 2D VHS and the sign of the charge carrier are precisely controlled. Use of a tunable 2D VHS in an atomically flat oxide film could serve as a new strategy to realize infinite DOS near the Fermi level, thereby allowing efficient tuning of electric properties.

Monte Carlo study of cuprate superconductors in a four-band model: role of orbital degrees of freedom

Understanding the various competing phases in cuprate superconductors is a long-standing challenging problem. Recent studies have shown that orbital degrees of freedom, both Cu e g orbitals and O p orbitals, are a key ingredient for a unified understanding of cuprate superconductors, including the material dependence. Here we investigate a four-band model derived from the first-principles calculations with the variational Monte Carlo method, which allows us to elucidate competing phases on an equal footing. The obtained results can consistently explain the doping dependence of superconductivity, antiferromagnetic and stripe phases, phase separation in the underdoped region, and also novel magnetism in the heavily-overdoped region. The presence of p orbitals is critical to the charge-stripe features, which induce two types of stripe phases with s)-wave and d-wave bond stripe. On the other hand, the presence of orbital is indispensable to material dependence of the superconducting transition temperature (), and enhances local magnetic moment as a source of novel magnetism in the heavily-overdoped region as well. These findings beyond one-band description could provide a major step toward a full explanation of unconventional normal state and high in cuprate supercondutors.

Superconducting phase diagram of lanthanum films on the substrate of Si(100)

Lanthanum films were grown on the substrate of Si(100) (La/Si(100)) by the molecular beam epitaxy (MBE) method and their superconducting properties were studied via the magnetic and electrical transport measurements. The superconducting transition temperature (Tc) is around 5.3 K. Besides, the second magnetization peak (SMP) occurs well below the upper critical field Hc2(T) due to the thermomagnetic flux-jump instabilities. In addition, we also calculated the critical current density (Jc) and studied the vortex-pinning behavior. The Jc is higher than 10 MA/cm2 at certain temperatures. The pinning analysis suggests the dominant role is the δl pinning. Moreover, the comprehensive superconducting phase diagram is offered. Our study completes the superconducting properties of lanthanum, which would facilitate the understanding of superconducting mechanism in the type Ⅱ superconductors.

An ultrafast spectroscopy system for studying dynamic properties of superconductors under high pressure and low temperature conditions

An ultrafast pump-probe spectroscopy system combined with a cryogenic diamond anvil cell (DAC) instrument is developed to investigate the photo-excitation dynamic properties of condensed materials under low temperature and high pressure (LTHP) conditions. The ultrafast dynamics study is performed on Bi2Sr2CaCu2O8+δ (Bi-2212) thin film under LTHP conditions. The superconducting (SC) phase transition has been observed by analyzing the ultrafast dynamics of Bi-2212 as a function of pressure and temperature. Our results suggest that the pump-probe spectroscopy system combined with a cryogenic DAC instrument is an effective method to study the physical mechanism of condensed matter physics at extreme conditions, especially for the SC phase transition.