Superconducting Phase Research Articles

Superconductivity and Charge-Density-Wave-Like Transition in Th2Cu4As5.

We report the synthesis, crystal structure, and physical properties of a novel ternary compound, Th2Cu4As5. The material crystallizes in a tetragonal structure with lattice parameters a = 4.0639(3) Å and c = 24.8221(17) Å. Its structure can be described as an alternating stacking of fluorite-type Th2As2 layers with antifluorite-type double-layered Cu4As3 slabs. The measurement of electrical resistivity, magnetic susceptibility, and specific heat reveals that Th2Cu4As5 undergoes bulk superconducting transition at 4.2 K. Additionally, all these physical quantities exhibit anomalies at 48 K, accompanied by a sign change in the Hall coefficient, suggesting a charge-density-wave-like (CDW) phase transition. Drawing from both experimental data and band calculations, we propose that the superconducting and CDW-like phase transitions are, respectively, associated with the Cu4As3 slabs and the As plane in the Th2As2 layers.

Absence of floating phase in superconductors with time-reversal symmetry breaking on any lattice

Absence of floating phase in superconductors with time-reversal symmetry breaking on any lattice

Pressure‐Induced Superconductivity and Structure Phase Transition in SnAs‐Based Zintl Compound SrSn2As2

AbstractLayered SnAs‐based Zintl compounds exhibit a distinctive electronic structure, igniting extensive research efforts in areas of superconductivity, topological insulators, and quantum magnetism. In this paper, the crystal structures and electronic properties of the Zintl compound SrSn2As2 upon compression are systematically investigated. Pressure‐induced superconductivity is observed in SrSn2As2 with a nonmonotonic evolution of superconducting transition temperature Tc. Theoretical calculations together with high‐pressure synchrotron X‐ray diffraction and Raman spectroscopy have identified that SrSn2As2 undergoes a structural transformation from a rhombohedral Rm phase to the monoclinic C2/m phase. Beyond 28.3 GPa, Tc is suppressed due to a reduction of the density of state (DOS) at the Fermi level. The discovery of pressure‐induced superconductivity, accompanied by structural transitions in SrSn2As2, greatly expands the physical properties of layered SnAs‐based compounds and provides new ground states upon compression.

Behavior of localized states in double twisted ABC trilayer graphene

We show that a double-twisted ABC trilayer graphene, formed by stacking two ABC trilayers with a rotation angle between them, is a semiconductor with a gap of about 30 meV. Importantly, flat bands in the electronic structure are observed even for large angles, and the localization of electrons follows the same pattern as in ABC trilayer graphene. Our first-principles calculations show that this behavior holds at least for rotation angles from 7 to 21 degrees. We further study the charge redistribution as a function of the angle. Below two degrees, the charge escapes from the rotated layers and AA zones, moving to the outer layers. This behavior is general for any ABA trilayer with one of the outer layers twisted (ABT). Our findings shed light on some peculiarities of rotated graphene, explaining the absence of a superconductivity phase on these double-trilayer and ABT systems.

Quantum phases of spin-1/2 extended XY model in transverse magnetic field

In this study, a spin-1/2 extended anisotropic XY chain has been introduced in which both time reversal and SU(2) symmetries are broken but Z 2 symmetry is preserved. Magnetic and topological phase diagrams in the parameter space have been drawn in the presence of transverse magnetic field. Entanglement measures like mutual information and quantum discord are also evaluated and it indicates that these transitions are second order in nature. Quantum phase transition is noted at zero magnetic field, as well as magnetic long range order is found to withstand magnetic field of any strength. Exact analytic results for spin-spin correlation functions have been obtained in terms of Jordan-Wigner fermionization. Existence of long range magnetic order has been investigated numerically by finding correlation functions as well as the Binder cumulant in the ground state. Dispersion relation, ground state energy, and energy gap are obtained analytically. In order to find the topologically nontrivial phase, sign of Pfaffian invariant and value of winding number have been evaluated. Both magnetic and topological phases are robust against the magnetic field and found to move coercively in the parameter space with the variation of its strength. Long range orders along two orthogonal directions and two different topological phases are found and their one-to-one correspondence has been found. Finally casting the spinless fermions onto Majorana fermions, properties of zero energy edge states are studied. Three different kinds of Majorana pairings are noted. In the trivial phase, next-nearest-neighbor Majorana pairing is found, whereas two different types of nearest-neighbor Majorana pairings are identified in the topological superconducting phase.

Emergent Superconductivity and Competing Charge Orders in Hole-Doped Square-Lattice t-J Model.

The square-lattice Hubbard and closely related t-J models are considered as basic paradigms for understanding strong correlation effects and unconventional superconductivity (SC). Recent large-scale density matrix renormalization group simulations on the extended t-J model have identified d-wave SC on the electron-doped side (with the next-nearest-neighbor hopping t_{2}>0) but a dominant charge density wave (CDW) order on the hole-doped side (t_{2}<0), which is inconsistent with the SC of hole-doped cuprate compounds. We re-examine the ground-state phase diagram of the extended t-J model by employing the state-of-the-art density matrix renormalization group calculations with much enhanced bond dimensions, allowing more accurate determination of the ground state. On six-leg cylinders, while different CDW phases are identified on the hole-doped side for the doping range δ=1/16-1/8, a SC phase emerges at a lower doping regime, with algebraically decaying pairing correlations and d-wave symmetry. On the wider eight-leg systems, the d-wave SC also emerges on the hole-doped side at the optimal 1/8 doping, demonstrating the winning of SC over CDW by increasing the system width. Our results not only suggest a new path to SC in general t-J model through weakening the competing charge orders, but also provide a unified understanding on the SC of both hole- and electron-doped cuprate superconductors.

Magnetic Eigenmodes in Chains of Coupled φ0-Josephson Junctions with Ferromagnetic Weak Links

A coupled chain of superconductor/ferromagnet/superconductor (S/F/S) with anomalous ground phase shift \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${{\\varphi }_{0}}$$\\end{document} represents a system realizing long-range interaction between magnetic moments of the weak links. The interaction is of magnetoelectric origin and is mediated by the condensate phase of superconductors. The system is a paradigmic platform for investigation of collective magnetic states governed by the superconducting phase. Here we study the magnetic eigenmodes of such a system and demonstrate that the eigenfrequencies are determined by the magnetic configuration of the whole system and are controlled by the superconducting phase. Depending on the orientation of the magnetic easy axis the eigenmodes can be very different ranging from individual oscillations of different magnets to highly-cooperative behavior.

Physical properties of the (Tl, Hg)-1223 superconductor phase substituted by lanthanum and samarium fluorides

Physical properties of the (Tl, Hg)-1223 superconductor phase substituted by lanthanum and samarium fluorides

Observing dynamical phases of BCS superconductors in a cavity QED simulator.

In conventional Bardeen-Cooper-Schrieffer superconductors1, electrons with opposite momenta bind into Cooper pairs due to an attractive interaction mediated by phonons in the material. Although superconductivity naturally emerges at thermal equilibrium, it can also emerge out of equilibrium when the system parameters are abruptly changed2-8. The resulting out-of-equilibrium phases are predicted to occur in real materials and ultracold fermionic atoms, but not all have yetbeen directly observed. Here we realize an alternative way to generate the proposed dynamical phases using cavity quantum electrodynamics (QED). Our system encodes the presence or absence of a Cooper pair in a long-lived electronic transition in 88Sr atoms coupled to an optical cavity and represents interactions between electrons as photon-mediated interactions through the cavity9,10. To fully explore the phase diagram, we manipulate the ratio between the single-particle dispersion and the interactions after a quench and perform real-time tracking of the subsequent dynamics of the superconducting order parameter using nondestructive measurements. We observe regimes in which the order parameter decays to zero (phase I)3,4, assumes a non-equilibrium steady-state value (phase II)2,3 or exhibits persistent oscillations (phase III)2,3. This opens up exciting prospects for quantum simulation, including the potential to engineer unconventional superconductors and to probe beyond mean-field effects like the spectral form factor11,12, and for increasing the coherence time for quantum sensing.

Properties and applications of two-dimensional quantum materials beyond carbon

Two-dimensional quantum materials are currently a hot research area in the field of materials. These unique materials allow electrons to move freely in only two dimensions and the other dimension is limited within the atom scale. The isolation of Graphene in 2004 showed the advantages of two-dimensional (2D) materials and led a rapid development in this field. Meanwhile, it stimulates the synthesis and research of the materials beyond carbon. 2D materials beyond carbon have different components and structures, showing a broader range of remarkable properties and applications than traditional graphene. This article will pay attention to the phenomenon and mechanism of these exceptional properties, including superconductivity, ferromagnetism, antiferromagnetism, and quantum spin liquid phase. Additionally, potential applications and future prospects of 2D materials beyond carbon will be explored. With the progress of technology, 2D materials beyond carbon are expected to have exciting developments in various fields, leading to significant changes in human life and production

Pressure-driven superconductivity in layered isostructural germanium phosphides

The discovery of superconductivity and its modulation are long-standing cutting-edge research topics in condensed matter physics. As a powerful tool, the high-pressure technique can be used to achieve novel superconductors and tune their physical properties. One typical example is binary germanium phosphides with different stoichiometries, which exhibit abundant physical properties with layered lattice structures similar to blue phosphorus. The detailed phase diagrams of the Ge–P systems are important for understanding the influence of stoichiometry on pressure-driven superconductivity, but it remains unexplored. Here, we measured and compared the detailed superconducting phase diagrams of the Ge–P systems of layered isostructural germanium phosphides GeP3 and GeP5 under pressure. Even though these two binary phosphides exhibit obviously different atomic occupations in the crystal structure due to their distinct stoichiometric ratios, the onset superconducting transition temperatures Tc of GeP3 and GeP5 both show dramatic enhancements from ∼2.5 K at 12.0 GPa to the maximum values of ∼9.0 K at 28.0 GPa, which are higher than those of other binary metal phosphides. Such pressure-enhanced superconductivity therein is accompanied by significant pressure-induced phonon mode softening, which is confirmed via in situ high-pressure Raman measurements. Our observations deepen the physical understanding of pressure-driven superconductivity in phosphorous-rich layered compounds and pave the way for potential applications in microsuperconducting devices.

Pair density wave, unconventional superconductivity, and non-Fermi liquid quantum critical phase in a frustrated Kondo lattice

Pair density wave, unconventional superconductivity, and non-Fermi liquid quantum critical phase in a frustrated Kondo lattice

Improvement in electrical properties of Bi-2212 superconducting materials substituted with large-scale nano-sized tin

In the current work, the effect of nano-sized Sn (50 nm)/Sr partial replacement on the superconducting properties such as crystal structure, quality of intra- and intergrain boundary coupling, dc electrical resistivity, and dc magnetization in the Bi-2212 ceramic superconductors were investigated. Ceramic superconductors with nominal composition of Bi2Sr2-x(Sn)xCa1Cu1.75Na0.25Oy where x = 0.25, 0.30, 0.35 and 0.40 were prepared by solid-state reaction method and characterized by powder X-ray diffraction (XRD), dc electrical resistivity, scanning electron microscopy (SEM) and magnetic hysteresis (M–H) measurements. Phase examination of by XRD indicated that the doping of x = 0.25 ratio nano-sized SnO2 to the strontium sites improved the formation of the Bi-2212 high-temperature superconducting phase. SEM micrographs showed that the morphological structure of all samples consisted of plate-like grains, which were separated from each other by grain boundaries, indicating the Bi-2212 superconducting phase. The highest superconductivity transition temperature among the samples was measured as the Tconset = 86 K at x = 0.25 in Bi2Sr2-x(Sn)xCa1Cu1.75Na0.25Oy. M–H loops of the Bi2Sr2-x(Sn)xCa1Cu1.75Na0.25Oy sample at x = 0.25 ratio is larger compared to other examples, indicating improvement intergrain connectivity as well as enhanced flux pinning centers. In addition, the critical current (Jc) values of the samples were calculated from M–H measurement using Bean’s critical current model. The best Jc values were obtained as 570 A/cm2 at 15 K, which is a relatively high value for BSCCO superconductors with polycrystalline structure.

Field-induced compensation of magnetic exchange as the possible origin of reentrant superconductivity in UTe2

The potential spin-triplet heavy-fermion superconductor UTe2 exhibits signatures of multiple distinct superconducting phases. For field aligned along the b axis, a metamagnetic transition occurs at μ0Hm ≈ 35 T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to Hm. Once the field is tilted away from the b towards the c axis, a reentrant superconducting phase emerges just above Hm. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of μ0Hc2 ≈ 73 T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above Hm in the angular range around 30° caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above Hm.

Theoretical study of CDW phases for bulk NbX2 (X = S and Se).

In most two-dimensional transition metal chalcogenides, the superconducting phase coexists with the charge density wave (CDW) phase. There exists at least one case, i.e. bulk 2H-NbS2, that does not conform to this picture. Scientists have shown great interest in trying to experimentally find the CDW phase of bulk NbS2 since 1975. Is there any theoretically more stable thermodynamic state than its higher-temperature metal phase, especially in the case of charge injection? Theoretically more stable CDW bulk configurations (TC for 2H-NbS2 and TTs for 2H-NbSe2) with partial pseudo energy gaps were predicted through the harmonic phonon softening theory and first-principles calculations. The ratios of larger to smaller pseudo gaps around K-H segment in the Brillouin zone for CDW phases are basically equal to those of superconductivity phases for bulk 2H-NbX2 (X = S and Se). The CDW phase should coexist with its superconductor state below the critical temperature rather than the metal phase for bulk 2H-NbS2. The presence of CDW phase should be more easily observed experimentally when the injected charge reaches 0.5e/Nb18S36 for bulk 2H-NbS2. Our calculations of density of state (DOS) indicated that, during Nb atoms contracting to form the CDW phases with symmetry breaking in the in-plane direction, dominant conductive carriers are always of p-type for bulk 2H-NbS2 while the alternation of carrier type from p-type to n-type occurs for bulk 2H-NbSe2. The Fermi level continuously drops and then the M-L segment of the out-of-plane energy band emerges from the Fermi surface, which corresponds to the reversal of p-n type sign. Lifshitz transition of pocket-vanishing types occurs in the out-of-plane direction without symmetry breaking during the geometrical structural phase transition for bulk 2H-NbSe2. Our calculations have theoretically addressed the long-standing coexistence issue of CDW and superconducting phases.

A new pressure-densified orthogonal hard superconducting phase of RhB4.

A new hard superconducting phase of RhB4 with the space group Cmca is predicted, and the phase transition and mechanical and superconducting properties of RhB4 under 300 GPa are studied using first principles. We predict a new high-pressure phase of RhB4 by substituting the most stable Cmca structure of OsB4, known for its excellent mechanical properties. The calculated enthalpy shows that above 112.6 GPa, Cmca is superior to Pmmn as was previously predicted by particle swarm optimization. The stability of the predicted phase is checked using formation enthalpy, elastic constant and phonon dispersion. Additionally, the convex hull of the Rh-B system confirms that the phase is expected to be synthesized experimentally. The Cmca phase is an incompressible hard material with a hardness of 23.75 GPa at 300 GPa attributed to strong intralayer covalent B-B bonds. Furthermore, the phase is a relatively pressure-insensitive superconductor, with a Tc of 8.6 K at 112.6 GPa and a pressure-dependent coefficient of -0.03 K GPa-1. The finding reveals a superconducting hard material that is well-suited for extreme high-pressure environments.

The effect of O2 pressure on phase formation and superconductivity properties of Tl-based HTS

Thallium-based high-temperature superconductors exhibit excellent superconductivity performance (Tc, Jc). Formation of the Tl-based superconducting materials critically depends on the used precursor and synthesis conditions. In the present investigation, we used the in situ polymerization method for synthesizing high-purity and reaction-ability precursors. We find the critical dependence of Tl-based superconductor phase formation on the quantity of applied oxygen pressure. The experimental results showed that the Tl-1223 phase formed at an oxygen gas pressure at 1 to 2 atm, Tl-2223 phase at 3 atm, and Tl-2212 phase at 4 to 8 atm.

Phases of Wilson lines: conformality and screening

We study the rich dynamics resulting from introducing static charged particles (Wilson lines) in 2+1 and 3+1 dimensional gauge theories. Depending on the charges of the external particles, there may be multiple defect fixed points with interesting renormalization group flows connecting them, or an exponentially large screening cloud can develop (defining a new emergent length scale), screening the bare charge entirely or partially. We investigate several examples where the dynamics can be solved in various weak coupling or double scaling limits. Sometimes even the elementary Wilson lines, corresponding to the lowest nontrivial charge, are screened. We consider Wilson lines in 3+1 dimensional gauge theories including massless scalar and fermionic QED4, and also in the N\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{N} $$\\end{document} = 4 supersymmetric Yang-Mills theory. We also consider Wilson lines in 2+1 dimensional conformal gauge theories such as QED3 with bosons or fermions, Chern-Simons-Matter theories, and the effective theory of graphene. Our results in 2+1 dimensions have potential implications for graphene, second-order superconducting phase transitions, etc. Finally, we comment on magnetic line operators in 3+1 dimensions (’t Hooft lines) and argue that our results for the infrared dynamics of electric and magnetic lines are consistent with non-Abelian electric-magnetic duality.

Compact description of quantum phase slip junctions

Quantum circuit theory is a powerful tool to describe superconducting circuits. In its language, quantum phase slips (QPSs) are considered to be the exact dual to the Josephson effect. This duality renders the integration of QPS junctions into a unified theoretical framework challenging. As we argue, different existing formalisms may be inconsistent, and the correct inclusion of time-dependent flux driving requires introducing a large number of auxiliary, nonphysical degrees of freedom. We resolve these issues by describing QPS junctions as inductive rather than capacitive elements, and reducing the Hilbert space to account for a compact superconducting phase. Our treatment provides an approach to circuit quantization exclusively in terms of node-flux-node variables, and eliminates spurious degrees of freedom. Finally, the inductive treatment reveals the possibility of a voltage-dependent renormalization of the QPS amplitude, by accounting for spatial variations of the electric field built up across the junction.

Identifying Chern numbers of superconductors from local measurements

Fascination in topological materials originates from their remarkable response properties and exotic quasiparticles which can be utilized in quantum technologies. In particular, large-scale efforts are currently focused on realizing topological superconductors and their Majorana excitations. However, determining the topological nature of superconductors with current experimental probes is an outstanding challenge. This shortcoming has become increasingly pressing due to rapidly developing designer platforms which are theorized to display very rich topology and are better accessed by local probes rather than transport experiments. We introduce a robust machine learning protocol for classifying the topological states of two-dimensional (2D) chiral superconductors and insulators from local density of states (LDOS) data. Since the LDOS can be measured with standard experimental techniques, our protocol contributes to overcoming the almost three decades standing problem of identifying the topological phase of 2D superconductors with broken time-reversal symmetry.