Low-temperature Superconducting Phase Research Articles

First-order quantum phase transition in the hybrid metal-Mott insulator transition metal dichalcogenide 4Hb-TaS2.

Coupling together distinct correlated and topologically nontrivial electronic phases of matter can potentially induce novel electronic orders and phase transitions among them. Transition metal dichalcogenide compounds serve as a bedrock for exploration of such hybrid systems. They host a variety of exotic electronic phases, and their Van der Waals nature enables to admix them, either by exfoliation and stacking or by stoichiometric growth, and thereby induce novel correlated complexes. Here, we investigate the compound 4Hb-TaS2 that interleaves the Mott-insulating state of 1T-TaS2 and the putative spin liquid it hosts together with the metallic state of 2H-TaS2 and the low-temperature superconducting phase it harbors using scanning tunneling spectroscopy. We reveal a thermodynamic phase diagram that hosts a first-order quantum phase transition between a correlated Kondo-like cluster state and a depleted flat band state. We demonstrate that this intrinsic transition can be induced by an electric field and temperature as well as by manipulation of the interlayer coupling with the probe tip, hence allowing to reversibly toggle between the Kondo-like cluster and the depleted flat band states. The phase transition is manifested by a discontinuous change of the complete electronic spectrum accompanied by hysteresis and low-frequency noise. We find that the shape of the transition line in the phase diagram is determined by the local compressibility and the entropy of the two electronic states. Our findings set such heterogeneous structures as an exciting platform for systematic investigation and manipulation of Mott-metal transitions and strongly correlated phases and quantum phase transitions therein.

Light-induced coherent interlayer transport in stripe-ordered La1.6−xNd0.4SrxCuO4

We have investigated the photoexcited transient responses of stripe-ordered phase in a cuprate superconductor, ${\rm La}_{1.6-x}{\rm Nd}_{0.4}{\rm Sr}_{x}{\rm CuO}_{4}~(x = 0.12)$ using optical-pump terahertz (THz)-probe spectroscopy. Upon the near-infrared photoexcitation with the electric field polarized along the $c$-axis, a clear plasma edge appears in the THz reflection spectrum along the $c$-axis with its position nearly coinciding with the Josephson plasma resonance of similarly doped ${\rm La}_{2-x}{\rm Sr}_{x}{\rm CuO}_{4}~(x = 0.125)$ in the low-temperature superconducting phase. The appearance of light-induced plasma edge sustains up to the onset temperature of the charge-stripe order, indicating the inherent interplay between the light-induced phase and the charge-stripe order. The optical conductivity spectrum of the light-induced state is mostly reproduced by the Drude model with a scattering rate as small as a few meV, and its imaginary part does not exhibit $1/{\omega}$-divergence behavior in any temporal region after the photoexcitation. We discuss the possible origin of the observed coherent interlayer transport behavior as manifested by the narrow Drude response in the THz reflectivity along the $c$-axis.

Structural evolution and molecular dissociation of H2S under high pressures

Solid H2S as the precursor for H3S with incredible superconducting properties under high pressure, has recently attracted extensive attention. Here in this work, we propose two new phases of H2S with P42/n and I41/a lattice symmetries in a pressure range of 0 GPa–30 GPa through first-principles structural searches, which complement the phase transition sequence. Further an ab initio molecular dynamics simulation confirms that the molecular phase P2/c of H2S is gradually dissociated with the pressure increasing and reconstructs into a new P21/m structure at 160 GPa, exhibiting the superconductivity with T c of 82.5 K. Our results may provide a guidance for the theoretical study of low-temperature superconducting phase of H2S.

Superconductivity and Phase Stability of Potassium-Intercalated p-Quaterphenyl.

To explore more novel superconductors, we have synthesized the potassium-doped p-quaterphenyl by an annealing or just a pestling process. The Meissner effect with critical temperatures ranging from 3.5 to 120 K is found by the magnetic susceptibility measurements in doped samples. The primary superconducting phase with a critical temperature of 7.2 K can be duplicated in the annealed and pestled samples. The charge transfer from metal to molecule is confirmed from the Raman scattering measurements. The X-ray diffraction analysis suggests that the low-temperature superconducting phase is due to the two-electron doping, whereas the high-temperature one corresponds to the high doping content. The occurrence of superconductivity in potassium-doped p-quaterphenyl supports the chain link organic molecules as promising candidates for high-temperature superconductors. This work also provides a simple method for synthesizing organic superconductors by pestling without annealing.

A Tale of Two Domes

Iron selenide films peppered with potassium atoms exhibit a high-temperature superconducting phase that emerges separately from a low-temperature superconducting phase.

Low temperature superconducting phase of the ternary pnictide SrPd2Sb2 with CaBe2Ge2-type structure

We find that low temperature superconducting phase (LSP) of SrPd2Sb2 with the CaBe2Ge2-type structure at Tc = 1.4 K. Superconducting transition is found at Tc = 1.4 K in electrical resistivity ρ(T). Specific heat C(T) shows a clear anomaly at Tc = 1.2 K, which shows that superconductivity is of bulk nature. From ρ(T) in several magnetic fields, the upper critical field μ0Hc2(0) is estimated to be 36 mT, which is strongly suppressed, and is 16.7 times smaller than that of 1.9 K phase.

Superconductivity in highly disordered dense carbon disulfide

High pressure plays an increasingly important role in both understanding superconductivity and the development of new superconducting materials. New superconductors were found in metallic and metal oxide systems at high pressure. However, because of the filled close-shell configuration, the superconductivity in molecular systems has been limited to charge-transferred salts and metal-doped carbon species with relatively low superconducting transition temperatures. Here, we report the low-temperature superconducting phase observed in diamagnetic carbon disulfide under high pressure. The superconductivity arises from a highly disordered extended state (CS4 phase or phase III[CS4]) at ~6.2 K over a broad pressure range from 50 to 172 GPa. Based on the X-ray scattering data, we suggest that the local structural change from a tetrahedral to an octahedral configuration is responsible for the observed superconductivity.

Fluctuations and order of antiferromagnetism induced by paramagnetic pair breaking in superconducting vortex lattices

Effects of the strong Pauli-paramagnetic pair breaking (PPB) on the vortex lattice in $d$-wave superconductors are theoretically studied by putting emphasis on consequences of the PPB-induced antiferromagnetic (AFM) ordering in the spatial modulation in the vortex lattice. It is shown that the PPB-induced AFM fluctuation in the superconducting state leads to an enhancement of the vortex lattice form factor which is a measure of spatial variations of the internal magnetic field and that the enhancement becomes more remarkable as an AFM instability is approached. It is also demonstrated that the PPB-induced AFM ordering is assisted by the vortex-lattice modulation, and thus, that the resulting AFM order is spatially modulated, while it is not localized in the vortex cores but coexistent with the nonvanishing superconducting order parameter. These results are discussed in connection with two phenomena observed in CeCoIn${}_{5}$, the anomalous field dependence of the vortex lattice form factor and the AFM order appearing inside the high-field and low-temperature superconducting phase.

Superconducting film with randomly magnetized dots: A realization of the two-dimensionalXYmodel with random phase shifts

We consider a thin superconducting film with randomly magnetized dots on top of it. The dots produce a disordered pinning potential for vortices in the film. We show that for dots with permanent and random magnetization normal or parallel to the film surface, our system is an experimental realization of the two-dimensional XY model with random phase shifts. The low-temperature superconducting phase, that exists without magnetic dots, survives in the presence of magnetic dots for sufficiently small disorder.

Microscopic evidence of the FFLO state in CeCoIn 5 probed by NMR

We found an anomalous temperature dependence of 115In-NMR spectrum in the newly discovered high field and low-temperature superconducting phase of CeCoIn 5. The anomalies near the upper limit of H c 2 are attributed to spatially distributed local density of states of quasiparticles. From a simulation model with sinusoidally modulated superconducting order parameter, the present NMR study at a microscopic point of view provides a direct evidence that the Fulde–Ferrell–Larkin–Ovchinikov (FFLO) state is realized at the high-field phase in CeCoIn 5.

Renormalization-group approach to strong-coupled superconductors

We develop an asymptotically exact renormalization group (RG) approach that treats electron-electron and electron-phonon interactions on equal footing. The approach allows an unbiased study of the instabilities of Fermi liquids without the assumption of a broken symmetry. We apply our method to the problem of strongly coupled superconductors and find the temperature T* below which the high-temperature Fermi liquid state becomes unstable towards Cooper pairing. We show that T* is the same as the critical temperature Tc obtained in Eliashberg's strong coupling theory starting from the low-temperature superconducting phase. We also show that Migdal's theorem is implicit in our approach. Finally, our results lead to a novel way to calculate numerically, from microscopic parameters, the transition temperature of superconductors.

Theory of decoupling in the mixed phase of extremely type-II layered superconductors

The mixed phase of extremely type-II layered superconductors in perpendicular magnetic field is studied theoretically via the layered XY model with uniform frustration. A partial duality analysis is carried out in the weak-coupling limit. It consistently accounts for both intralayer (pancake) and interlayer (Josephson) vortex excitations. The main conclusion reached is that dislocations of the two-dimensional (2D) vortex lattices within layers drive a unique second-order melting transition at high perpendicular fields between a low-temperature superconducting phase that displays a Josephson effect and a high-temperature ``normal'' phase that displays no Josephson effect. The former state is best described by weakly coupled 2D vortex lattices, while the latter state is best characterized by a decoupled vortex liquid. It is further argued on the basis of the duality analysis that the second-order melting transition converts itself into a first-order one as the perpendicular field is lowered and approaches the dimensional crossover scale. The resulting critical end point potentially accounts for the same phenomenon that is observed in the mixed phase of clean high-temperature superconductors.

Magnetism and superconductivity of UPt 3 by muon spin techniques

We summarize our recent μSR results on the magnetic and superconducting properties of the heavy-fermion superconductor UPt 3. Whereas zero-field measurements show no signature of the superconducting and magnetic phase transitions, the experiments in high transverse field evidence the presence of two inequivalent magnetic regions for temperatures ranging from 0.2 to 115 K with an anomaly near the magnetic transition. From low transverse field experiments in the low-temperature superconducting phase (B phase), we deduce the temperature dependence of the superfluid densities. We briefly discuss all these results in relation to those obtained for other compounds of this family.

Ginzburg-Landau model of hexagonal superconductors: Application to UPt3.

We consider the so-called E theory of unconventional hexagonal superconductors in which the superconducting order parameter belongs to one of the two-dimensional E representations of the hexagonal point group. We show that in the presence of a symmetry-breaking field (such as antiferromagnetism in ${\mathrm{UPt}}_{3}$) there are two distinct mechanisms for obtaining a split transition in zero magnetic field, one of which has been previously discussed. Coupling of the superconductivity to another symmetry-breaking field associated with charge-density-wave modulations is also considered, thus allowing for an interpretation of the observed pressure-temperature phase diagram of ${\mathrm{UPt}}_{3}$. Also, the anisotropy (or lack of it) of the upper critical field of ${\mathrm{UPt}}_{3}$ is accounted for, and it is shown that the antiferromagnetic order becomes a canted antiferromagnetic order in the low-temperature superconducting phase for one of the two models presented here.

The superconductive energy gap of UPt3 investigated by point-contact spectroscopy

Point-contact spectroscopy, via the mechanism of Andreev reflection between a normal metal and a superconductor, can be employed to obtain information about the superconductive energy gap. In the present work, point-contacts between the heavy-fermion superconductor UPt3 and Pt have been studied down to 36 mK and in magnetic fields up to 6 T. Andreev reflection is observed only in the low-field, low-temperature superconducting phase. The unusual temperature and magneticfield dependence together with the gap anisotropy give direct experimental evidence that the different superconductive phases in UPt3 have distinctly different order parameters.

Optical transition studies of YBa 2Cu 3O 7−δ superconducting films in 1.5–3.0 eV spectral range: Temperature evolution and photoinduced changes of the absorption spectra

The temperature measurements of 1.5–3.0 eV absorption spectra of superconducting YBa 2Cu 3O 7− δ high quality films display two types of bands: dielectric- and metallic-types. Along with the smooth temperature changes in the spectrum at T > T c , a sharp anomaly at T = T c is observed. The low-temperature spectra give conclusive evidence of the existence of an optical gap E g = 1.95 eV. High sensitivity of the optical absorption to the superconducting state and to the presence of several-sorted superconducting phases in the sample was confirmed by SQUID magnetic measurements. It is found that all absorption spectra measured and especially dielectric charge transfer bands at 1.95 and 2.8 eV become more transparent under photoirradiation, i.e. there is the photoinduced effect for low-temperature superconducting phase with oxygen deficiency.

Flux-flow resistance in frustrated Josephson-junction arrays.

We carry out equilibrium and steady-state simulations of a periodic square Josephson-junction array in the presence of a transverse magnetic field giving a uniform frustration of f=2/5. We find a first-order transition to the low-temperature superconducting phase. Linear resistivity above ${\mathit{T}}_{\mathit{c}}$ and nonlinear resistivity below ${\mathit{T}}_{\mathit{c}}$ are interpreted in terms of vortex fluctuations.