Temperature Dependence Of Critical Field Research Articles

Nonlinear optical response of twisted bilayer graphene

Twisted bilayer graphene (TBG) exhibits many fascinating electronic and transport properties. Here we analyze the nonlinear optical response in TBG in the terahertz regime. By considering an effective two-band model with a simplified interlayer tunnel, we write the time-dependent wave function in terms of generalized Floquet states consisting of multiple high-order harmonics. A two-component spinor recursion relation is solved up to terms in the third order of the applied field. The third-order conductivity of TBG with different twist angles is obtained. For small twist angles the Kerr effect dominates over the high harmonic processes, while for large twist angles, the high harmonic processes completely dominate the nonlinear response. We attribute this phenomenon to the band dispersion sensitive transition probability in low- and high-energy regimes. The frequency- and temperature-dependent critical field is also presented. The critical field is insensitive to the temperature, indicating that the nonlinear response persists at room temperature.

Reentrant Superconductivity in UTe2

The reentrant superconductivity is the peculiar phenomenon observed in paramagnetic metal UTe2 in magnetic field parallel to the hard magnetisation axis. It is difficult to explain it in terms of field dependent intensity of magnetic fluctuations like it is done for explanation of the formally similar phenomena in the ferromagnetic uranium superconductors URhGe, UCoGe. Extremely large initial slope of the upper critical field temperature dependence suggests that the phenomenon has quasi-two-dimensional nature. Indeed, according to the recent band structure calculations by Y. Xu et al. (Phys. Rev. Lett. 123, 217002 (2019)) the Fermi surface of UTe2 consists of two types slightly corrugated cylinders. The theory of reentrant superconductivity in UTe2 based on its quasi-two-dimensional structure is presented here.

Upper critical field in ferromagnetic metals with triplet pairing

The theory of triplet superconductivity in ferromagnetic metals based on electron–electron interaction by spin fluctuation exchange is developed. The equations for the upper critical field temperature dependence are derived. In contrast to the similar equations for the superconductivity in two band metals they contain the pairing amplitudes and the Fermi velocities depending on magnetic field. The critical field behaviour near the critical temperature and at T=0 is established analytically.

Pressure tuning of collapse of helimagnetic structure in Au2Mn

We identify the phase boundary between spiral spin and ferromagnetic phases in Au$_2$Mn at a critical pressure of 16.4 kbar, as determined by neutron diffraction, magnetization and magnetoresistance measurements. The temperature-dependent critical field at a given pressure is accompanied by a peak in magnetoresistance and a step in magnetization. The critical field decreases with increasing temperature and pressure. The critical pressure separating the spiral phase and ferromagnetism coincides with the disappearance of the magnetroresistance peak, where the critical field goes to zero. The notable absence of an anomalous Hall effect in the the ferromagnetic phase is attributable to the high conductivity of this material.

Increased upper critical field for nanocrystalline MoN thin films deposited on AlN buffered substrates at ambient temperature

Molybdenum nitride (MoN) thin films have been deposited using reactive DC magnetron sputtering on aluminum nitride buffered oxidized silicon substrates at ambient temperature. GIXRD of aluminum nitride (AlN) deposited under similar conditions has revealed the formation of wurtzite phase AlN. GIXRD characterization of molybdenum thin films deposited on AlN buffered oxidized silicon substrates has indicated the formation of nanocrystalline MoN thin films. The electrical resistivity measurements indicate MoN thin films have a superconducting transition temperature of ~8 K. The minimum transition width of the MoN thin film is 0.05 K at 0 T. The inferred upper critical field Bc2(0) for these nanocrystalline MoN thin films obtained by fitting the temperature dependence of critical field with Werthamer, Helfand and Hohenberg theory lies in the range of 17–18 T which is the highest reported in literature for MoN thin films.

Potential analysis in holographic Schwinger effect

We analyze electrostatic potentials in the holographic Schwinger effect. The potential barrier for the pair production is estimated by a static potential consisting of static mass energies, an electric potential from an external electric-field, and the Coulomb potential between a particle and an antiparticle. Given that the Coulomb potential is supposed to be evaluated by the minimal surface attaching on the conformal boundary as usual, the critical field, where the potential barrier vanishes, exhibits a deviation of 30% from the one obtained from the Dirac-Born-Infeld (DBI) action. We reconsider this issue by reexamining the Coulomb-potential part, which is evaluated by the classical action of a string solution attaching on a probe D3-brane sitting at an intermediate position in the bulk AdS. Then the resulting critical-field completely agrees with the DBI result. This agreement gives rise to a strong support for the holographic scenario. We also discuss the finite-temperature case and the temperature-dependent critical-field also agrees with the DBI result.

Evidence of multiband behavior in the superconducting alloy Zr0.96V0.04B2

ZrB${}_{2}$ is a nonsuperconducting Pauli paramagnetic crystallizing in the AlB${}_{2}$ structure. V substitution for Zr in Zr${}_{1\ensuremath{-}x}$V${}_{x}$B${}_{2}$ (0.01 $\ensuremath{\le}$ $x$ $\ensuremath{\le}$ 0.1) at the few percentage level induces superconductivity with critical temperature reaching a maximum of 8.7 K for Zr${}_{0.96}$V${}_{0.04}$B${}_{2}$ near the solid solubility limit. Specific-heat and lower critical field temperature dependence results suggest the possibility of unconventional superconductivity possibly arising from multiband effects.

Magnetodielectric response in the charge ordered oxyborate Fe2OBO3

We present a detailed investigation on the magnetodielectric properties of Fe2OBO3, where we have found a magnetodielectric effect around 150% at room temperature and ν=2 kHz, that cannot solely arise from magnetoresistive extrinsic contributions. In addition, we find a crossover of the magnetodielectric response around TN, that we attribute to changes in the electronic microstructure of this compound that take place at that temperature. For T<TN the dielectric constant experiences a jump at a temperature dependent critical field that we relate to a partial melting of the charge ordering state.

Paramagnetic limit in ferromagnetic superconductors with triplet pairing

The spin susceptibility in the uranium ferromagnet superconductors is calculated. There is shown that the absence of superconductivity paramagnetic limitation for the field directions perpendicular to the direction of the spontaneous magnetization is explained by the itinerant ferromagnet band splitting rather than by a rotation of magnetization toward the external field direction. The qualitative description of the upper critical field temperature dependence is given.

Magnetic and magnetocaloric results of magnetic field-induced transitions in La 1−xCe xMn 2Si 2 ( x=0.35 and 0.45) compounds

The La 1− x Ce x Mn 2Si 2 compounds ( x=0.35 and 0.45) exhibit an antiferromagnetic–ferromagnetic transition caused by the changes in distance between Mn atoms due to temperature changes. A field-induced transition from antiferromagnetic state to ferromagnetic state at a critical field, which decreases with increase in temperature, can also be induced by applying a magnetic field. In this paper our aim is to study the magnetization and magnetocaloric effect, close to transition temperatures. Our subsidiary aim is to examine the temperature dependence of critical field and ferromagnetic fraction of compounds. The variation of magnetocaloric effect with temperature is correlated with the ferromagnetic–antiferromagnetic phase coexistence. Our final aim is to examine the harmony between magnetocaloric effect values calculated both by the Maxwell theory and by the Landau theory.

Spontaneous and Field-Induced Magnetic Phase Transitions in Tb<sub>1-x</sub>R<sub>x</sub>Mn<sub>6</sub>Sn<sub>6</sub> (R = Gd, Y) Compounds

Magnetization curves and magnetic susceptibility of Tb1 xRxMn6Sn6 (R = Gd, Y) compounds have been measured for aligned powder samples. Concentration dependences of the spin-reorientation transition temperature and temperature dependences of critical fields of the field-induced magnetic transitions have been determined. For the compound Tb0.1Y0.9Mn6Sn6 with the spiral magnetic structure, we found that application of hydrostatic pressure facilitates the ferrimagnetic ordering. This unusual feature is supported by the observation of a large negative spontaneous volume magnetostriction for TbMn6Sn6. The obtained results are discussed in a model considering three different interlayer Mn-Mn exchange interactions in these compounds.

Effect of screening current induced pair breaking on magnetization of superconducting lead nanoparticles

Magnetization measurements on lead nanoparticles in the size range 35–45 nm are presented. It is shown that the critical fields in these nanoparticles are enhanced significantly above their bulk values with temperature dependence also distinct from that of bulk. The observed “type II” like shape of the magnetization curves is explained on the basis of the Ginzburg–Landau phenomenology by invoking the pair breaking effect of the London screening currents, which makes the effective penetration depth an increasing function of the field. The temperature dependence of critical field is found to be consistent with our explanation.

Magnetic-field-induced quantum superconductor-insulator transition inNb0.15Si0.85

A study of magnetic-field tuned superconductor-insulator transitions in amorphous ${\mathrm{Nb}}_{0.15}{\mathrm{Si}}_{0.85}$ thin films shows that quantum superconductor-insulator transitions are characterized by an unambiguous signature---a kink in the temperature profile of the critical magnetic field. Using this criterion, we show that the nature of the magnetic-field tuned superconductor-insulator transition depends on the orientation of the field with respect to the film. For perpendicular magnetic field, the transition is controlled by quantum fluctuations with indications for the existence of a Bose insulator; while for parallel magnetic field, the transition is classical, driven by the breaking of Cooper pairs at the temperature dependent critical field ${H}_{c2}$.

Temperature Dependence of the Upper Critical Field of Anisotropic Singlet Superconductivity in a Square Lattice Tight-Binding Model in Parallel Magnetic Fields

Upper critical field parallel to the conducting layer is studied in anisotropic type-II superconductors on square lattices. We assume enough separation of the adjacent layers, for which the orbital pair-breaking effect is suppressed for exactly aligned parallel magnetic field. In particular, we examine the temperature dependence of the critical field H_c(T) of the superconductivity including the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state, in which the Cooper pairs have non-zero center-of-mass momentum q. In the system with the cylindrically symmetric Fermi-surface, it is known that H_c(T) of the d-wave FFLO state exhibits a kink at a low temperature due to a change of the direction of q in contrast to observations in organic superconductors. It is shown that the kink disappears when the Fermi-surface is anisotropic to some extent, since the direction of q is locked in an optimum direction independent of the temperature.

Vortex structure and dynamics in κ-(ET) 2Cu(NCS) 2

We use Josephson plasma resonance (JPR) and DC torque magnetization measurements to investigate the vortex structure and dynamics in the layered organic superconductor κ-(ET) 2Cu(NCS) 2. JPR studies probe the AC response of this extreme type-II superconductor for currents driven along the low conductivity axis. An observed crossover in the magnetic field and temperature dependence of the JPR frequency, in close proximity to the irreversibility line, is attributed either to a melting or a depinning transition in the quasi-two-dimensional (Q2D) vortex structure. Angle and temperature dependent magnetization studies reveal the rich dynamical process by which magnetic flux enters this highly anisotropic superconductor, as well as information on the structure of the vortices once inside. At extremely low temperatures ( T<100 mK), the sudden cessation of avalanching flux jumps above an angle- and temperature-dependent critical field B *(θ, T) , is indicative of Q2D vortex lattice melting. Thus, using these two very different techniques, we are able to probe the vortex phase diagram over a wide temperature and magnetic field range.

Fermi Liquid Effect on Tricritical Superconducting Transitions in Thin TiN Films under the Spin Paramagnetic Limitation

Reentrant first order and second order transitions are observed in thin TiN films under the paramagnetic limitation of superconductivity, where the temperature gradients of critical fields are positive. Then, the superconducting condensate has to have higher entropy than the normal state immediately below the critical fields. The observations of reentrant transition in thin TiN films above the tricritical point reveal that the renormalized Fermi liquid effect plays an important role for the superconducting transition. The antisymmetric Fermi liquid parameters obtained from the temperature dependence of critical field have positive definite values of 0.23. Spin orbit scattering parameters almost inversely increase with decreasing film thickness. Orbital depairing parameters are almost linear to the square of film thickness.

Magnetic phase diagram of a partially frustrated triangular antiferromagnet: The row model.

Phase diagram of a planar antiferromagnet on a stacked triangular lattice subject to a uniform orthorhombic distortion is studied within the Landau approach. This spin system with partially released frustration exhibits a variety of ordered phases in magnetic field parallel to the basal plane including elliptically and linearly polarized incommensurate structures. Four magnetically ordered phases coexist at a multicritical point. The theory is applied to explain the experimental phase diagram of ${\mathrm{RbMnBr}}_{3}$ and satisfactory agreement for temperature dependences of critical fields is found. \textcopyright{} 1996 The American Physical Society.

Transition temperature variation in layered high temperature superconductors

We describe a mathematical technique for studying T c variation with the number of layers in HTSCs. The T c in a uniform magnetic field is shown to be bounded. The upper critical field temperature dependence and the effect of introducing a single inequivalent layer on T c variation are studied.

Bipolaron theory of metalooxide superconductors in the strong magnetic field

The strong electron-phonon coupling in narrow-band crystals leads to the formation of localized pairs, (small bipolarons). It is shown that the small bipolarons system is equivalent to the system of bosons and fermions with a strong pair interaction. The upper critical field temperature dependence Hc2 for anisotropic lattice and arbitrary particle concentration was found. Some peculiar properties of this dependence, (a non-linear form near critical point Tc and strong upward curvature of Hc2(T)) are discussed. The obtained critical Hc2 (T) curve is in reasonable agreement with the experimental data.

MAGNETIC STRUCTURE AND MAGNETOSTRICTION OF EPITAXIAL Er FILMS

MAGNETIC STRUCTURE AND MAGNETOSTRICTION OF EPITAXIAL Er FILMS