Condensed Matter

Superconductivity is often found in a dome around quantum critical points, i.e. 2nd-order quantum phase transitions. Here, we show that an enhancement of superconductivity is avoided at the critical pressure of the charge-density-wave (CDW) state in NbSe 2 . We present comprehensive high-pressure Hall effect and magnetic susceptibility measurements of the CDW and superconducting state in NbSe 2 . Initially, the 2nd-order CDW transition is suppressed smoothly but it drops to zero abruptly at PCDW = 4.4 GPa thus indicating a change to 1st order whilstthe superconducting transition temperature Tc rises continuously up to PCDW but is constant above. The putative 1st-order nature of the CDW transition is suggested as the cause for the absence of a superconducting dome at PCDW. Indeed, we show that the suppression of the superconducting state at low pressures is due to the loss of density of states inside the CDW phase whilst the initial suppression of the CDW state is accounted for by the stiffening of the underlying bare phonon mode.

Microscopic studies on thin film superconductors play an important role for probing non-equilibrium phase transitions and revealing dynamics at the nanoscale. However, magnetic sensors with nanometer scale spatial and picosecond temporal resolution are essential for exploring these. Here, we present an all-optical, microwave-free method, that utilizes the negatively charged nitrogen-vacancy (NV) center in diamond as a non-invasive quantum sensor and enables the spatial detection of the Meissner state in a superconducting thin film. We place an NV implanted diamond membrane on a superconducting LSCO thin film. The strong B-field dependence of the NV photoluminescence (PL) allows us to investigate the Meissner screening in LSCO under an externally applied magnetic field in a non-resonant manner.

It is generally believed that, at a certain temperature below the critical one, magnetic response of a superconductor (SC) is determined solely by its intrinsic properties. Here we show that the mechanical rotation of a SC can easily change the values of the critical fields at which the superconductivity is destroyed (type-1 SC) or the vortices penetrate into (exit from) the material (type-2 SC). This is due to a superposition of the Meissner current induced by the external field, and the spontaneous current on the surface of the SC induced by the mechanical rotation. As a result, the critical fields of a SC can be increased or decreased, depending on the geometrical form of the material and the relative orientation of rotation and the external field.

Despite several theoretical approaches describing multiple Andreev reflections (MAR) effect in superconductor-normal metal-superconductor (SNS) junction are elaborated, the problem of comprehensive and adequate description of MAR is highly actual. In particular, a broadening parameter Γ is still unaccounted at all, whereas a ballistic condition (the mean free path for inelastic scattering l to the barrier width d ratio) is considered only in the framework of Kümmel, Gunsenheimer, and Nikolsky (KGN), as well as Gunsenheimer-Zaikin approaches, for an isotropic case and fully-transparent constriction. Nonetheless, an influence of l/d ratio to the dynamic conductance spectrum ( dI/dV ) features remains disregarded, thus being one of the aims of the current work. Our numerical calculations in the framework of an extended KGN approach develop the l/d variation to determine both the number of the Andreev features and their amplitudes in the dI/dV spectrum. We show, in the spectrum of a diffusive SNS junction ( l/d→1 ) a suppression of the Andreev excess current, dramatic change in the current voltage I(V) -curve slope at low bias, with only the main harmonic at eV=2Δ bias voltage remains well-distinguished in the dI/dV -spectrum. Additionally, we attempt to make a first-ever comparison between experimental data for the high-transparency SNS junctions (more than 85 % ) and theoretical predictions. As a result, we calculate the temperature dependences of amplitudes and areas of Andreev features within the extended KGN approach, which qualitatively agrees with our experimental data obtained using a "break-junction" technique.

Under disturbances, superconductors may experience sudden, most undesirable phase transitions (quench) from superconducting to normal conducting state. Quench may lead to damage or even to catastrophic conductor failure. A superconductor is stable if it does not quench. Exact determination of superconductor transient, resistive states (flux flow, Ohmic) thus is mandatory to safely avoid quench. This request sharp comparison of local, transient conductor temperature and current transport density with local values of critical superconductor temperature, TCrit, and critical current density, JCrit, and the latter is a strong function of temperature. Numerical, Finite Element simulations reported previously and in this paper have provided the requested, transient temperature distribution; under disturbances, this distribution may strongly be non-uniform. But what happens if the other variable, TCrit, might not uniquely be defined? A multi-physics model (fractional parentage, Pauli selection rule, time of flight-concept with a mediating Boson, the Yukawa model and the uncertainty principle) provides relaxation time at which TCrit, as the thermodynamic, equilibrium state, finally would be obtained. But relaxation time diverges the closer the electron system approaches the phase transition, which questions existence of uniquely defined TCrit within finite process time. In this paper, focus is on multiple internal heat transfer (solid conduction and, in thin films, radiation), a suggested operator method to solve the incompleteness problem of radiative transfer, and time dependence of the order parameter obtained from the quantum-mechanical model. Traditional stability models cannot provide this information.

We present a complete analysis of the Nernst signal due to superconducting fluctuations in a large variety of superconductors from conventional to unconventional ones. A closed analytical expression of the fluctuation contribution to the Nernst signal is obtained in a large range of temperature and magnetic field. We apply this expression directly to experimental measurements of the Nernst signal in Nb x Si 1−x thin films and a URu 2 Si 2 superconductors. Both magnetic field and temperature dependence of the available data are fitted with very good accuracy using only two fitting parameters, the superconducting temperature T c0 and the upper critical field H c2 . The obtained values agree very well with experimentally obtained values. We also extract the ghost lines (maximum of the Nernst signal for constant temperature or magnetic field) from the complete expression and also compare it to several experimentally obtained curves. Our approach predicts a linear temperature dependence for the ghost critical field well above T c0 . Within the errors of the experimental data, this linearity is indeed observed in many superconductors far from T c0 .

Electron-phonon superconductors at high pressures have displayed the highest values of critical superconducting temperature T c on record, now rapidly approaching room temperature. Despite the importance of high- P superconductivity in the quest for room-temperature superconductors, a mechanistic understanding of the effect of pressure and its complex interplay with phonon anharmonicity and superconductivity is missing, as numerical simulations can only bring system-specific details clouding out key players controlling the physics. Here we develop a minimal model of electron-phonon superconductivity under an applied pressure which takes into account the anharmonic decoherence of the optical phonons. We find that T c behaves non-monotonically as a function of the ratio Γ/ ω 0 , where Γ is the optical phonon damping and ω 0 the optical phonon energy at zero pressure and momentum. Optimal pairing occurs for a critical ratio Γ/ ω 0 when the phonons are on the verge of decoherence ("diffuson-like" limit). Our framework gives insights into recent experimental observations of T c as a function of pressure in the complex BCS material TlInTe 2 .

We present a systematic study of electrical resistivity, Hall coefficient, magneto-optical imaging, magnetization, and STEM analyses of KCa 2 Fe 4 As 4 F 2 single crystals. Sharp diamagnetic transition and magneto-optical imaging reveal homogeneity of single crystal and prominent Bean-like penetrations of vortices. Large anisotropy of electrical resistivity, with ρ c / ρ ab > 100, and semiconductor-like ρ c suggest that the electronic state is quasi two-dimensional. Hall effect measurements indicate that KCa 2 Fe 4 As 4 F 2 is a multiband system with holes as main carriers. Magnetization measurements reveal significantly larger J c compared with that in other iron-based superconductors with different values of J c depending on the direction of magnetic field. Origin of these J c characteristics is discussed based on microstructural observations using STEM. In addition, further enhancement of J c in KCa 2 Fe 4 As 4 F 2 for future application is demonstrated in terms of heavy-ion irradiation.

High critical temperature superconductivity often occurs in systems where an antiferromagnetic order is brought near T=0K by slightly modifying pressure or doping. CaKFe 4 As 4 is a superconducting, stoichiometric iron pnictide compound showing optimal superconducting critical temperature with T c as large as 38 K. Doping with Ni induces a decrease in T c and the onset of spin-vortex antiferromagnetic order, which consists of spins pointing inwards to or outwards from alternating As sites on the diagonals of the in-plane square Fe lattice. Here we study the band structure of CaK(Fe 0.95 Ni 0.05 ) 4 As 4 (T c = 10 K, T N = 50 K) using quasiparticle interference with a Scanning Tunneling Microscope (STM) and show that the spin-vortex order induces a Fermi surface reconstruction and a fourfold superconducting gap anisotropy.

The anisotropic London equations taking into account the normal currents are derived and applied to the problem of the surface impedance in the Meisner state of anisotropic materials. It is shown that the complex susceptibility of anisotropic slab depends on the orientation of the applied microwave field relative to the crystal axes. In particular, the anisotropic sample in the microwave field is subject to a torque, unless the field is directed along with one of the crystal principle axes.