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Register a new userDensity driven fluctuations in a two-dimensional superconductor
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In the vicinity of a phase transition, the order parameter starts fluctuating before vanishing at the critical point. The fluctuation regime, i.e. the way the ordered phase disappears, is a characteristics of a transition, and determines the universality class it belongs to. This is valid for thermal transitions, but also for zero temperature Quantum Phase Transitions (QPT). In the case of superconductivity, the order parameter has an amplitude and a phase, which can both fluctuate according to well identified scenarios. The Ginzburg-Landau theory and its extensions describe the fluctuating regime of regular metallic superconductors, and the associated dynamics of the pair amplitude and the phase. When the system is two-dimensional and/or very disordered, phase fluctuations dominate. Here, we address the possibility that a new type of fluctuations occurs in superconductors with an anomalous dynamics. In particular we show that the superconducting to metal QPT that occurs upon changing the gate voltage in two-dimensional electron gases at LAO/STO and LTO/STO interfaces displays anomalous scaling properties, which can be explained by density driven superconducting critical fluctuations. A Finite Size Scaling (FSS) analysis reveals that the product z.nu (nu is the correlation length exponent and z the dynamical critical one) is z.nu = 3/2. We argue that critical superconducting fluctuations acquire an anomalous dynamics with z=3, since they couple to density ones in the vicinity of a spontaneous electronic phase separation, and that nu=1/2 corresponds to the mean-field value. This approach strongly departs from the conventional z=1 scenario in disordered 2D systems based on long-range Coulomb interactions with dominant phase fluctuations. This scenario can explain recent data in LSCO ultra-thin films, and apply to a whole class of two-dimensional superconductors.
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Motivated by recent advances in the fabrication of Josephson junctions in which the weak link is made of a low-dimensional non-superconducting material, we present here a systematic theoretical study of the local density of states (LDOS) in a clean 2D normal metal (N) coupled to two s-wave superconductors (S). To be precise, we employ the quasiclassical theory of superconductivity in the clean limit, based on Eilenbergers equations, to investigate the phase-dependent LDOS as function of factors such as the length or the width of the junction, a finite reflectivity, and a weak magnetic field. We show how the the spectrum of Andeeev bound states that appear inside the gap shape the phase-dependent LDOS in short and long junctions. We discuss the circumstances when a gap appears in the LDOS and when the continuum displays a significant phase-dependence. The presence of a magnetic flux leads to a complex interference behavior, which is also reflected in the supercurrent-phase relation. Our results agree qualitatively with recent experiments on graphene SNS junctions. Finally, we show how the LDOS is connected to the supercurrent that can flow in these superconducting heterostructures and present an analytical relation between these two basic quantities.
We find a series of topological phase transitions in a half-metal/superconductor heterostructure, by tuning the direction of the magnetization of the half-metal film. These include transitions between a topological superconducting phase with a bulk gap and another phase without a bulk gap but has a ubiquitous local gap. At the same time, the edge states change from counter-propagating Majorana edge modes to unidirectional Majorana edge modes. In addition, we find transitions between the second phase and a nodal phase which turns out to be a two-dimensional Weyl superconductor with Fermi line edge states. We identify the topological invariants relevant to each phase and the symmetry that protects the Weyl superconductivity phase.
Here we report the first observation of superconductivity in the AB2X4-type ternary telluride CuIr2Te4, which is synthesized by a solid-state method in an evacuated quartz jacket. It adopts a disordered trigonal structure with space group P3m1 (No. 164), which embodies a two-dimensional IrTe2 layers and intercalated by Cu between the layers. We use a combination of experimental and first principles calculation analysis to look insight into the structural and physical properties. CuIr2Te4 consistently exhibited a bulk superconductivity transition at 2.5 K in electrical resistivity, magnetic susceptibility and specific heat measurements. Resistivity and magnetization measurements suggest a charge density wave transition (TCDW = 250 K) coexists in CuIr2Te4, which further signify the coexistence of superconductivity and CDW in the ternary telluride chalcogenide CuIr2Te4. Our discovery of the new CDW-bearing superconductor CuIr2Te4 opens a door for experimental and theoretical studies of the interplay between CDW and superconductivity quantum state in the condensed matter field.
Magnetically mediated Cooper pairing is generally regarded as a key to establish the unified mechanism of unconventional superconductivity. One crucial evidence is the neutron spin resonance arising in the superconducting state, which is commonly interpreted as a spin-exciton from collective particle-hole excitations confined below the superconducting pair-breaking gap ($2Delta$). Here, on the basis of inelastic neutron scattering measurements on a quasi-two-dimensional iron-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$, we have discovered a two-dimensional spin resonant mode with downward dispersions, a behavior closely resembling the low branch of the hour-glass-type spin resonance in cuprates. The resonant intensity is predominant by two broad incommensurate peaks near $Q=$(0.5, 0.5) with a sharp energy peak at $E_R=16$ meV. The overall energy dispersion of the mode exceeds the measured maximum total gap $Delta_{rm tot}=|Delta_k|+|Delta_{k+Q}|$. These experimental results deeply challenge the conventional understanding of the resonance modes as magnetic excitons regardless of underlining pairing symmetry schemes, and it also points out that when the iron-based superconductivity becomes very quasi-two-dimensional, the electronic behaviors are similar to those in cuprates.
Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate metallic state occupying a substantial region of the phase diagram whose nature is intensely debated. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe$_2$ in inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the intermediate metallic state. We find a new power law scaling with field in this phase, which is consistent with the Bose metal model where metallic behavior arises from strong phase fluctuations caused by the magnetic field.
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