The crystal structure and electrical resistance of the structurally-layered EuFe2As2 have been studied up to 70 GPa and down to temperature of 10 K, using a synchrotron x-ray source and the designer diamond anvils. The room-temperature compression of the tetragonal phase of EuFe2As2 (I4/mmm) results in an increase in the a-axis and a rapid decrease in c-axis with increasing pressure. This anomalous compression reaches a maximum at 8 GPa and the tetragonal lattice behaves normal above 10 GPa with a nearly constant c/a axial ratio. The rapid rise in superconducting transition temperature (Tc) to 41 K with increasing pressure is correlated to this anomalous compression and a decrease in Tc is observed above 10 GPa. We present P-V data or equation of state of EuFe2As2 in both the ambient tetragonal phase and the high pressure collapsed tetragonal phase to 70 GPa.
We have studied the structural and superconductivity properties of the compound LaFeAsO0.9F0.1 under pressures up to 32GPa using synchrotron radiation and diamond anvil cells. We obtain an ambient pressure bulk modulus K_0 = 78(2)GPa, compressibility comparable to some cuprates. At high pressures, the sample is in the overdoped region, with a linear decrease with pressure variation of the superconducting transition temperature.
Dome-shape superconductivity phase diagram can commonly be observed in cuprate and iron-based systems via tuning parameters such as charge carrier doping, pressure, bond angle, and etc. We report doping electrons from transition-metal elements (TM = Co, Ni) substitution can induce high-Tc superconductivity around 35 K in Ca0.94La0.06Fe2As2, which emerges abruptly before the total suppression of the innate spin-density-wave/anti-ferromagnetism (SDW/AFM) state. Unexpectedly, the onset critical temperature for the high-Tc superconductivity stays constant for a wide range of TM doping. Possible ex…
Considerable excitement has been caused recently by the discovery that the binary boride system with stoichiometry MgB2 is superconducting at the remarkably high temperature of 39 K (1). This potentially opens the way to even higher Tc values in a new family of superconductors with unexpectedly simple composition and structure. The simplicity in the electronic and crystal structures could allow the understanding of the physics of high-Tc superconductivity without the presence of the multitude of complicated features, associated with the cuprates. Synchrotron X-ray diffraction was used to measu…
The compressibility of an extended $d-p$ Hubbard model is investigated by the Roth's two-pole approximation. Using the factorization procedure proposed by Beenen and Edwards, superconductivity with singlet $d_{x^2-y^2}$-wave pairing is also considered. Within this framework, the effects of $d-p$ hybridization and Coulomb interaction $U$ on the compressibility are studied carefully. It has been found that the compressibility diverges and then it becomes negative near the half-filling. Within Roth's method, it has been verified that an important contribution for the negative compressibility come…
Two-dimensional transition metal dichalcogenides entwine interaction, spin-orbit coupling, and topology. Hole-doped systems lack spin degeneracy: states are indexed with spin and valley specificity. This unique structure offers new possibilities for correlated phases and phenomena. We realize an unconventional superconducting pairing phase which is an equal mixture of a spin singlet and the $m = 0$ spin triplet. It is stable against large in-plane magnetic fields, and its topology allows quasiparticle excitations of net nonzero Berry curvature via pair-breaking circularly polarized light.
Recent evidence for a charge-Kondo effect in superconducting samples of Pb$_{1-x}$Tl$_x$Te [1] has brought renewed attention to the possibility of negative U superconductivity in this material, associated with valence fluctuations on the Tl impurity sites [2]. Here, we use indium as an electron-donor to counterdope Pb$_{.99}$Tl$_{.01}$Te and study the effect of the changing chemical potential on the Kondo-like physics and on the superconducting critical temperature, $T_c$. We find that, as the chemical potential moves away from the value where superconductivity, Kondo-like physics, and chemica…
Anomalous acoustoelectric signatures of chiral superconductivity
Technion – Israel Institute of Technology（以色列理工学院）；Guangdong Technion – Israel Institute of Technology（广东以色列理工学院）；Rzhanov Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Science（俄罗斯科学院西伯利亚分院列扎诺夫半导体物理研究所）；Novosibirsk State Technical University（新西伯利亚国立技术大学）
Superconducting and normal-state properties of the beta-pyrochlore oxide KOs2O6 are studied by means of thermodynamic and transport measurements. It is shown that the superconductivity is of conventional s-wave type and lies in the extremely strong-coupling regime. Specific heat and resistivity measurements reveal that there are characteristic low-energy phonons that give rise to unusual scattering of carriers due to strong electron-phonon interactions. The entity of the low-energy phonons is ascribed to the heavy rattling of the K ion confined in an oversized cage made of OsO6 octahedra. It i…
Pressure-dependent diffraction response of the superconducting phase separated Cs0.72Fe1.57Se2 has been studied using synchrotron radiation up to the pressure of 19 GPa. The main and secondary phases of Cs0.72Fe1.57Se2 have been observed in the whole pressure range. The main ordered phase has been found to undergo an order-disorder transition in the Fe-sublattice at least at P = 11 GPa with the corresponding kinetics on the order of hours. Contrary to the analogous temperature induced transition, the secondary phase has not been suppressed suggesting that its stability pressure range is higher…
We present a combined experimental and theoretical study of the evolution of the Fermi surface of the anomalous superconductor Pb$_{1-x}$Tl$_x$Te as a function of thallium concentration, drawing on a combination of magnetotransport measurements (Shubnikov de Haas oscillations and Hall coefficient), Angle Resolved Photoemission Spectroscopy (ARPES), and density functional theory (DFT) calculations of the electronic structure. Our results indicate that for Tl concentrations beyond a critical value the Fermi energy coincides with resonant impurity states in Pb$_{1-x}$Tl$_x$Te, and we rule out the…
We discuss the mechanisms of unconventional superconductivity and superfluidity in 3D and 2D fermionic systems with purely repulsive interaction at low densities. We construct phase diagrams of these systems and find the areas of the superconducting state in free space, as well as on the lattice in the framework of the Fermi-gas model with hard-core repulsion, the Hubbard model, the Shubin-Vonsovsky model, and the $t-J$ model. We demonstrate that the critical superconducting temperature can be greatly increased in the spin-polarized case or in a two-band situation already at low densities. The…
There have been continuous efforts in searching for unconventional superconductivity over the past five decades. Compared to the well-established d-wave superconductivity in cuprates, the existence of superconductivity with other high-angular-momentum pairing symmetries is less conclusive. Bi/Ni epitaxial bilayer is a potential unconventional superconductor with broken time reversal symmetry (TRS), for that it demonstrates superconductivity and ferromagnetism simultaneously at low temperatures. We employ a specially designed superconducting quantum interference device (SQUID) to detect, on the…
In several members of the ferro-pnictides, spin density wave (SDW) order coexists with superconductivity over a range of dopings. In this letter we study the anomalous magnetic Zeeman response of this coexistence state and show that it can be used to confirm the extended s-wave gap structure as well as structure of superconducting (SC) gap in coexisting phase. On increasing the field, a strongly anisotropic reduction of SC gap is found. The anisotropy is directly connected to the gap structure of superconducting phase. The signature of this effect in quasiparticle interference measured by STM,…
We report the magnetic field-amplitude and field-angle dependence of the superconducting onset temperature Tc_onset of the organic superconductor (TMTSF)2ClO4 in magnetic fields H accurately aligned to the conductive ab' plane. We revealed that the rapid increase of the onset fields at low temperatures occurs both for H // b' and H // a, irrespective of the carrier confinement. Moreover, in the vicinity of the Pauli limiting field, we report a shift of a principal axis of the in-plane field-angle dependence of Tc_onset away from the b' axis. This feature may be related to an occurrence of Fuld…
We study a tight-binding model on the honeycomb lattice of chiral $d$-wave superconductivity that breaks time-reversal symmetry. Due to its nontrivial sublattice structure, we show that it is possible to construct a gauge-invariant time-reversal-odd bilinear of the pairing potential. The existence of this bilinear reflects the sublattice polarization of the pairing state. We show that it generates persistent loop current correlations around each lattice site and opens a topological mass gap at the Dirac points, resembling Haldane's model of the anomalous quantum Hall effect. In addition to the…
We study the physics of the Josephson effect in nematic superconductors with $E_u$ odd parity in the Ginzburg-Landau approach. Two-component vector superconducting order parameter makes this effect rather unusual. We get that the Meissner kernel has off-diagonal components. We derive current-phase relations for different configurations of the junction, crystallographic axes of the sample, and nematicity direction. We show that an anomalous Josephson Hall effect can be observed in such a system without any magnetization. That is, for definite orientations of the junction and crystal axes, a com…
We present a microscopic study of superconductivity in OsB2 , and discuss the origin and characteristic length scales of the superconducting state. From first-principles we show that OsB2 is characterized by three different Fermi sheets, and we prove that this fermiology complies with recent quantum-oscillation experiments. Using the found microscopic properties, and experimental data from the literature, we employ Ginzburg-Landau relations to reveal that OsB2 is a distinctly type-I superconductor with very low Ginzburg-Landau parameter kappa - a rare property among compound materials. We show…
The Ginzburg-Landau (GL) theory is recast using a Hamiltonian involving the complete kinetic energy density which requires that the surface energy must contain a term \nabla |ψ|^2 to support superconducting (SC) states. The GL equations contain two temperature, t, dependent parameters α(t) and β(t), which are respectively the coefficients of the SC pair density \propto |ψ|^2, and the pair interaction term \propto |ψ|^4 in the free energy density. The sign of these parameters, which define distinct solution classes, and the ratio s(t) = \sqrt{|α|/|β|} are governed by the characteristics of the …
We report the discovery of superconductivity in Nb$_3$Pd$_{x}$Se$_7$ with a $x$-dependent superconducting transition-temperature as high as $T_c \simeq 2.1 $ K for $x \simeq0.7$ (middle point of the resistive transition). Needle-like single crystals display anisotropic upper-critical fields with an anisotropy $γ= H^{b}_{c2}/H^{a}_{c2}$ as large as 6 between fields applied along their needle axis (or $b-$axis) or along the $a-$axis. As for the Fe based superconductors $γ$ is temperature-dependent suggesting that Nb$_3$Pd$_{0.7}$Se$_7$ is a multi-band superconductor. This is supported by band st…
The pressure dependent electrical resistivity of URu$_2$Si$_2$ has been studied at high pressure across the first order phase boundary of $P_x$ where the ground state switches under pressure from "hidden order" (HO) to large moment antiferromagnetic (LAFM) states. The electrical transport in URu$_2$Si$_2$ at low temperatures shows a strong sample dependence. We have measured an ultra-clean single crystal whose quality is the highest among those used in previous studies. The generalized power law $ρ{\,}={\,}{ρ_0}+{\,}{A_n}T{^n}$ analysis finds that the electric transport property deviates from …
The anomalies of electromagnetic properties of carbon condensates are discussed. Basic attention has been done to electromagnetic properties of thin carbon films produced by means of sputtering of spectral pure graphite in electrical arc (CA films). The room temperature rf-to-dc conversion associated with the ac Josephson effect, the jumps of conductivity stipulated by variation of current and temperature and zero spin density can be pointed among the anomalous properties of CA films. The number of basic hypotheses are presented in trying to explain this anomalies. As the main hypothesis the h…
Cooperation and competition between the antiferromagnetic, d-wave superconducting and Mott-insulating states are explored for the two-dimensional Hubbard model including nearest and next-nearest-neighbor hoppings at zero temperature. Using the variational cluster approach with clusters of different shapes and sizes up to 10 sites, it is found that the doping-driven transition from a phase with microscopic coexistence of antiferromagnetism and superconductivity to a purely superconducting phase is discontinuous for strong interaction and accompanied by phase separation. At half-filling the syst…
We report the anomalous charge density wave (CDW) state evolution and dome-like superconductivity (SC) in CuIr2Te4-xSex series. Room temperature powder X ray-diffraction (PXRD) results indicate that CuIr2Te4-xSex compounds retain the same structure as the host CuIr2Te4 and the unit cell constants a and c manifest a linear decline with increasing Se content. Magnetization, resistivity and heat capacity results suggest that superconducting transition temperature (Tc) exhibits a weak dome-like variation as substituting Te by Se with the maximum Tc = 2.83 K for x = 0.1 followed by suppression in T…
Signatures of unconventional superconductivity near reentrant and fractional quantum anomalous Hall insulators
Tsung-Dao Lee Institute, Shanghai Jiao Tong University（上海交大李政道研究所）；School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学物理与天文学院）；Hong Kong University of Science and Technology（香港科技大学）；University of Tennessee（田纳西大学）；Fudan University（复旦大学）；Southern University of Science and Technology (SUSTech)（南方科技大学）；National Institute for Materials Science（国立材料研究所）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；Hefei National Laboratory（合肥国家实验室）；Min H. Kao Department of Electrical Engineering and Computer Science, University of Tennessee（田纳西大学明H. Kao电气与计算机工程系）
The linear compressibilities and the thermal expansion of an untwinned, nearly optimal doped YBa2Cu3O6.94 single crystal were measured along the three crystallographic axes with a high-resolution dilatometer mounted in a high-pressure cell. The measurements were performed in a temperature range from 50 K to 320 K under hydrostatic gas pressure of maximal 0.65 GPa. At a temperature of 300 K the measured bulk modulus is reduced and shows a very strong enhancement under increasing pressure due to pressure-induced oxygen ordering. This ordering process can also be related to a glass transition see…
The electronic structure of Bi2Sr2-xRxCuOy(R=La, Eu) near the (pi,0) point of the first Brillouin zone was studied by means of angle-resolved photoemission spectroscopy (ARPES). The temperature T* above which the pseudogap structure in the ARPES spectrum disappears was found to have an R dependence that is opposite to that ofthe superconducting transition temperature Tc. This indicates that the pseudogap state is competing with high-Tc superconductivity, and the large Tc suppression observed with increasing the out-of-plane disorder is due to the stabilization of the pseudogap state.
We show that Berezinskii's classification of the symmetries of Cooper pair amplitudes holds for driven systems even in the absence of translation invariance. We then consider a model Hamiltonian for a superconductor coupled to an external driving potential and, treating the driving potential as a perturbation, we investigate the corrections to the anomalous Green's function, density of states, and spectral function. We find that in the presence of an external drive the anomalous Green's function develops terms that are odd in frequency and that the same mechanism responsible for these odd-freq…
We use scanning tunneling microscopy to investigate Bi2Sr2Ca2Cu3O10+δ trilayer cuprates from the optimally doped to overdoped regime. We find that the two distinct superconducting gaps from the inner and outer CuO2 planes both decrease rapidly with doping, in sharp contrast to the nearly constant Tc. Spectroscopic imaging reveals the absence of quasiparticle interference in the antinodal region of overdoped samples, showing an opposite trend to that in single- and double-layer compounds. We propose that the existence of two types of inequivalent CuO2 planes and the intricate interaction betwee…
Using micro fabrication techniques, we extracted individual channels of 3-Kelvin (3-K) phase superconductivity in Sr2RuO4-Ru eutectic systems and confirmed odd-parity superconductivity in the 3-K phase, similar to pure Sr2RuO4. Unusual hysteresis in the differential resistance-current and voltage-current characteristics observed below 2 K indicates the internal degrees of freedom of the superconducting state. A possible origin of the hysteresis is current-induced chiral-domain-wall motion due to the chiral p-wave state.
Doped Hubbard model is a simple model for the high-Tc cuprate superconductors, while its ground state remains a challenge. Here, by performing state-of-the-art variational Monte Carlo calculations for the strong-coupling Hubbard model, we find evidences that the d-wave superconducting phase emerges always near the phase separation region and the superconducting order has one-to-one correspondence with the enhancement of charge compressibility. The order as well as the phase separation are vulnerable to realistic intersite Coulomb interaction while the superexchange interaction enhances both. A…
In order to clarify that the Cooper pair in $β$-pyrochlore oxides is mediated by anharmonic oscillation of guest atom, i.e., rattling, we propose an experiment to detect anomalous isotope effect. In the formula of $T_{\rm c} \propto M^{-η}$, where $T_{\rm c}$ is superconducting transition temperature and $M$ denotes mass of the oscillator, it is found that the exponent $η$ is increased with the increase of anharmonicity of a potential for the guest atom. We predict that $η$ becomes larger than 1/2 in rattling-induced superconductor, in sharp contrast to $η=1/2$ for weak-coupling superconductiv…
Making use of generalized Eliashberg equations, we describe the Altshuler-Aronov (AA) effect and superconductivity on equal footing. We derive explicit expressions for the Coulomb pseudopotential in 3D, taking into account also the anomalous diffusion. We present a full numerical solution for two normal-state and two anomalous self-energies. In the normal state, we amend the known results for the purely electronic AA effect; with electron-phonon coupling turned on, we find additional anomalies in the density of states close to the phonon energy. We study how the critical temperature and densit…
We present a microscopic theory of the superconducting proximity effect in a semiconducting thin film with spin-orbit interaction ($N_{SO}$) in an external magnetic field. We demonstrate that an effective 1D Hamiltonian which describes induced superconductivity in $N_{SO}$ in contact with a usual $s$-wave superconductor possesses not only spin-singlet induced superconducting order parameter term, as commonly adopted, but spin triplet order parameter term also. Using this new effective Hamiltonian we confirm previous results for a normal current across contacts of $N_{SO}$ with a normal metal a…
We show that Floquet chiral topological superconductivity arises naturally in Josephson junctions made of magnetic topological insulator-superconductor sandwich structures. The Josephson phase modulation associated with an applied bias voltage across the junction drives the system into the anomalous Floquet chiral topological superconductor hosting chiral Majorana edge modes in the quasienergy spectrum, with the bulk Floquet bands carrying zero Chern numbers. The bias voltage acts as a tuning parameter enabling novel dynamical topological quantum phase transitions driving the system into a myr…
Recent emergence of two-dimensional (2D) crystalline superconductors has provided a promising platform to investigate novel quantum physics and potential applications. To reveal essential quantum phenomena therein, ultralow temperature transport investigation on high quality ultrathin superconducting films is critically required, although it has been quite challenging experimentally. Here we report a systematic transport study on the ultrathin crystalline PdTe2 films grown by molecular beam epitaxy (MBE). Interestingly, a new type of Ising superconductivity in 2D centrosymmetric materials is r…
The electronic structure of the high-Tc cuprates is studied in terms of "large-U" and "small-U" orbitals. A striped structure and three types of quasiparticles are obtained, polaron-like "stripons" carrying charge, "svivons" carrying spin, and "quasielectrons" carrying both. The anomalous properties are explained, and specifically the behavior of the resistivity, Hall constant, and thermoelectric power. High-temperature superconductivity results from transitions between pair states of quasielectrons and stripons.
Tunneling measurements of the temperature dependence of the electronic density of states (DOS) of ultrathin bilayers of Pb and Ag reveal that their superconducting energy gap, $Δ(T)$, evolves similarly to BCS predictions despite the presence of a large anomalous DOS at the Fermi energy that persists as $T\to 0$. The gap ratio, ${2Δ(0)}\over {k_BT_c}$, systematically decreases more than 20% below the BCS universal value of 3.52 in bilayers with the lowest superconducting transition temperatures, $T_c$; a behavior we deem super weakly coupled. A semi-quantitative model suggests that the reduced …
In the iron-based superconductors, understanding the relation between superconductivity and electronic structure upon doping is crucial for exploring the pairing mechanism. Recently it was found that in iron selenide (FeSe), enhanced superconductivity (Tc over 40K) can be achieved via electron doping, with the Fermi surface only comprising M-centered electron pockets. Here by utilizing surface potassium dosing, scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoemission spectroscopy (ARPES), we studied the electronic structure and superconductivity of (Li0.8Fe0.2OH)Fe…
A search for superconductivity has been carried out on the hexagonal polymorph of Laves-phase CaLi2, a compound for which Feng, Ashcroft, and Hoffmann predict highly anomalous behavior under pressure. No superconductivity is observed above 1.10 K at ambient pressure. However, high-pressure ac susceptibility and electrical resistivity studies to 81 GPa reveal bulk superconductivity in CaLi2 at temperatures as high as 13 K. The normal-state resistivity shows a dramatic increase with pressure.
In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron doped cuprates, the low upper critical field allows one to study the putative QCP at low temperature and to understand its connection to the long standing problem of the origin of the high Tc superconductivity. Here we present measurements of the low temperature normal state thermopower (S) of the electron-doped cuprate superconductor La2-xCexCuO4 (LCCO) from x=0.11 to 0.19. We observe quantum critical S divided by T versus ln(1/T) behavior over an unexpectedly …
When the electron density of highly crystalline thin films is tuned by chemical doping or ionic liq- uid gating, interesting effects appear including unconventional superconductivity, sizeable spin-orbit coupling, competition with charge-density waves, and a debated low-temperature metallic state that seems to avoid the superconducting or insulating fate of standard two-dimensional electron systems. Some experiments also find a marked tendency to a negative electronic compressibility. We suggest that this indicates an inclination for electronic phase separation resulting in a nanoscopic inhomo…
The BiS2-based superconductors were discovered recently. The superconductivity has been proved by many other groups. Since the previous experiments were all done on polycrystalline samples, therefore there remains a concern whether the superconductivity is really derived from the materials intrinsically or from some secondary phases. Experiments on single crystals are highly desired. In this paper, we report the successful growth of the NdO1-xFxBi1-yS2 single crystals. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at about 5 K, while an unexpected g…
Superconductivity in LaRu$_3$Si$_2$ with the honeycomb structure of Ru atoms has been investigated. It is found that the normal state specific heat C/T exhibits a deviation from the Debye model down to the lowest temperature. A relation $C/T = γ_n+βT^2-ATlnT$ which concerns the electron correlations can fit the data very well. The suppression to the superconductivity by the magnetic field is not the mean-field like, which is associated well with the observation of strong superconducting fluctuations. The field dependence of the induced quasiparticle density of states measured by the low temper…
Proximity to a Mott insulating phase is likely to be an important physical ingredient of a theory that aims to describe high-temperature superconductivity in the cuprates. Quantum cluster methods are well suited to describe the Mott phase. Hence, as a step towards a quantitative theory of the competition between antiferromagnetism (AFM) and d-wave superconductivity (SC) in the cuprates, we use Cellular Dynamical Mean Field Theory to compute zero temperature properties of the two-dimensional square lattice Hubbard model. The d-wave order parameter is found to scale like the superexchange coupli…
The interplay between superconductivity, spin-orbit coupling, and Zeeman or exchange field, is studied theoretically in two different setups: a single wire in which all these fields coexist, and a double wire system in which superconducting pairing and the spin-dependent fields are spatially separated. We first explore a magnetoelectric effect, namely the appearance of anomalous charge supercurrents. We determine the conditions under which such currents are allowed by symmetry and express them in terms of the SU(2) electric and magnetic fields. In leading order in the strength of the fields we…
Superconductivity and charge density wave (CDW) appear in the phase diagram of a variety of materials including the high - $T$$_c$ cuprate family and many transition metal dichalcogenides (TMDs). Their interplay may give rise to exotic quantum phenomena. Here, we show that superconducting arrays can spontaneously form in TiSe$_2$ - a TMD with coexisting superconductivity and CDW - after lithium ion intercalation. We induce a superconducting dome in the phase diagram of Li$_x$TiSe$_2$ by using the ionic solid-state gating technique. Around optimal doping, we observe magnetoresistance oscillatio…
Multiply connected superconductors smaller than the coherence length show destructive superconductivity, characterized by reentrant quantum phase transitions driven by magnetic flux. We investigate the dependence of destructive superconductivity on flux, transverse magnetic field, temperature, and current in InAs nanowires with a surrounding epitaxial Al shell, finding excellent agreement with mean-field theory across multiple reentrant transitions. Near the crossover between destructive and nondestructive regimes, an anomalous metal phase is observed with temperature-independent resistance, c…
We analyze how the coherence peaks observed in Scanning Tunneling Spectroscopy (STS) of cuprate high temperature superconductors are transferred from the cuprate layer to the oxide layers adjacent to the STS microscope tip. For this purpose, we have carried out a realistic multiband calculation for the superconducting state of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) assuming a short range d-wave pairing interaction confined to the nearest-neighbor Cu $d_{x^2-y^2}$ orbitals. The resulting anomalous matrix elements of the Green's function allow us to monitor how pairing is then induced not only w…
