Quasiparticle Relaxation Time Research Articles

Abnormally enhanced Hall Lorenz number in the magnetic Weyl semimetal NdAlSi

In Landau’s celebrated Fermi liquid theory, electrons in a metal obey the Wiedemann–Franz law at the lowest temperatures. This law states that electron heat and charge transport are linked by a constant L0, i.e., the Sommerfeld value of the Lorenz number (L). Such relation can be violated at elevated temperatures where the abundant inelastic scattering leads to a reduction of the Lorenz number (L < L0). Here, we report a rare case of remarkably enhanced Lorenz number (L > L0) discovered in the magnetic topological semimetal NdAlSi. Measurements of the transverse electrical and thermal transport coefficients reveal that the Hall Lorenz number Lxy in NdAlSi starts to deviate from the canonical value far above its magnetic ordering temperature. Moreover, Lxy displays strong nonmonotonic temperature and field dependence, reaching its maximum value close to 2L0 in an intermediate parameter range. Further analysis excludes charge-neutral excitations as the origin of enhanced Lxy. Alternatively, we attribute it to the Kondo-type elastic scattering off localized 4f electrons, which creates a peculiar energy distribution of the quasiparticle relaxation time. Our results provide insights into the perplexing transport phenomena caused by the interplay between charge and spin degrees of freedom.

Characterization of quasiparticle relaxation times in microstrips of NbReN for perspective applications for superconducting single-photon detectors

The study of the flux-flow instability in superconducting materials has recently gained renewed attention due to its potential implications for the use of the analyzed materials as micrometer-sized superconducting detectors for single photons. The values of the quasiparticle relaxation time (τE) measured for these detectors are affected by pinning properties. Here, we report electric transport properties of NbReN microstrips of different quality. For the strip characterized by high resistivity, and large critical currents and pinning, we estimate a value of τE that is almost two orders of magnitude larger compared to that of another strip with a smaller value for the critical current, for which we measure τE∼12 ps. This low value is comparable to those reported in the literature for microstrips made of other highly-disordered superconductors. Our results suggest that NbReN microstrips have great potential for the realization of superconducting single-photon detectors, depending on further optimization of their fabrication process and the superconducting properties affected by it.

Thickness dependence of the flux-flow velocity and the vortex instability in nanocrystalline γ-Mo2N thin films

The influence of the thickness on the vortex instability of nanocrystalline γ-Mo2N thin films is analyzed. The samples were grown on Si (100) using reactive sputtering. The quasiparticle relaxation time for films with thickness between 7 and 26 nm is analyzed in the framework of Larkin–Ovchinnikov instability by performing current-voltage curves. Considering self-heating effects due to finite heat removal from the substrate, we determine a fast quasiparticle relaxation time τ ≈ 50 ps for all the samples at low temperatures. On the other hand, close to Tc, the vortex velocity becomes magnetic field-independent, and τ increases from ≈ 40 to 110 ps as the films are made thicker. The results are discussed considering the disorder's contribution and the bridges' geometry on the vortex instability.

Optical spectroscopy and ultrafast pump-probe study of a quasi-one-dimensional charge density wave in CuTe

CuTe is a two-dimensional (2D) layered material; yet it forms a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) along the $a$ axis in the $ab$ plane at high transition temperature ${T}_{\mathrm{CDW}}=335\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. However, the anisotropic properties of CuTe remain to be explored. Here, we performed combined transport, polarized infrared reflectivity, and ultrafast pump-probe spectroscopy studies to investigate the underlying CDW physics of CuTe. Polarized optical measurements clearly revealed that an energy gap gradually forms along the $a$ axis upon cooling, while optical evidence of a gap signature is absent along the $b$ axis, suggesting pronounced electronic anisotropy in this quasi-2D material. Time-resolved optical reflectivity measurements revealed that the amplitude and relaxation time of photoexcited quasiparticles change dramatically across the CDW phase transition. Performing fast Fourier transformation of the oscillation signals arising from collective excitations, we identify the 1.65-THz mode as the CDW amplitude mode, whose energy softens gradually at elevated temperatures. Consequently, we provide further evidence for the formation of completely anisotropic CDW order in CuTe, which is quite rare in quasi-2D materials.

Giant magnetoconductivity in noncentrosymmetric superconductors

We discuss a novel physical mechanism which gives rise to a giant magnetoconductivity in non-centrosymmetric superconducting films. This mechanism is caused by a combination of spin-orbit interaction and inversion symmetry breaking in the system, and arises in the presence of an in-plane magnetic field ${\bf H}_\|$. It produces a contribution to the conductivity, which displays a strong dependence on the angle between the electric field ${\bf E}$ and ${\bf H}_\|$, and is proportional to the inelastic relaxation time of quasiparticles. Since in typical situations the latter is much larger than the elastic one this contribution can be much larger than the conventional conductivity thus leading to giant microwave absorption.

Nanocrystalline superconducting γ-Mo2N ultra-thin films for single photon detectors

We analyze the influence of the surface passivation produced by oxides on the superconducting properties of γ-Mo2N ultra-thin films. The superconducting critical temperature of thin films grown directly on Si (100) with those using a buffer and a capping layer of AlN are compared. The results show that the cover layer avoids the presence of surface oxides, maximizing the superconducting critical temperature for films with thicknesses of a few nanometers. We characterize the flux-flow instability measuring current-voltage curves in a 6.4 nm thick Mo2N film with a superconducting critical temperature of 6.4 K. The data is analyzed using the Larkin and Ovchinnikov model. Considering self-heating effects due to finite heat removal from the substrate, we determine a fast quasiparticle relaxation time ≈ 45 ps. This value is promising for its applications in single-photon detectors.

Flux flow velocity instability and quasiparticle relaxation time in nanocrystalline β-W thin films

We characterized the superconducting properties of a micropatterned 23 nm thick β-W film using electrical transport measurements in magnetic fields. The film is nanocrystalline and displays a smooth surface. The superconducting critical temperature is 4.6 K. The critical current densities display a fast drop with the magnetic field, indicating weak vortex pinning. From the analysis of the instability in the vortex motion at low magnetic fields in the flux-flow state, we observe vortex velocities up 800 m/s. An inelastic lifetime of quasiparticles around 0.6 ns is estimated at low temperatures. Together with high uniformity in thickness and simplicity in the fabrication process, the superconducting properties make β-W films promising for applications in fast single-photon detectors.

Dynamic pair-breaking current, critical superfluid velocity, and nonlinear electromagnetic response of nonequilibrium superconductors

We report numerical calculations of a dynamic pairbreaking current density $J_d$ and a critical superfluid velocity $v_d$ in a nonequilibrium superconductor carrying a uniform, large-amplitude ac current density $J(t)=J_a\sin\Omega t$ with $\Omega$ well below the gap frequency $\Omega\ll \Delta_0/\hbar$. The dependencies $J_d(\Omega,T)$ and $v_d(\Omega,T)$ near the critical temperature $T_c$ were calculated from either the full time-dependent nonequilibrium equations for a dirty s-wave superconductor and the time-dependent Ginzburg-Landau (TDGL) equations for a gapped superconductor, taking into account the GL relaxation time of the order parameter $\tau_{GL}$ and the inelastic electron-phonon relaxation time of quasiparticles $\tau_E$. We show that both approaches give similar frequency dependencies of $J_d(\Omega)$ and $v_d(\Omega)$ which gradually increase from their static pairbreaking GL values $J_c$ and $v_c$ at $\Omega\tau_E\ll 1$ to $\sqrt{2}J_c$ and $\sqrt{2}v_c$ at $\Omega\tau_E\gg 1$. Here $J_d$, $v_d$ and a dynamic superheating field at which the Meissner state becomes unstable were calculated in two different regimes of a fixed ac current and a fixed ac superfluid velocity induced by the applied ac magnetic field $H=H_a\sin\Omega t$ in a thin superconducting filament or a type-II superconductor with a large GL parameter. We also calculated a nonlinear electromagnetic response of a nonequilibrium superconducting state, particularly a dynamic kinetic inductance and a dissipative quasiparticle conductivity, taking into account the oscillatory dynamics of superconducting condensate and the kinetics of quasiparticles driven by a strong ac current. It is shown that an ac current density produces multiple harmonics of the electric field, the amplitudes of the higher-order harmonics diminishing as $\tau_E$ increases.

Drastic enhancement of the thermal Hall angle in a d -wave superconductor

A drastic enhancement of the thermal Hall angle in $d$-wave superconductors was observed experimentally in a cuprate superconductor and in CeCoIn$_5$ at low temperatures and very weak magnetic field [Phys. Rev. Lett. $\bf 86$, 890 (2001); Phys. Rev. B $\bf 72$, 214515 (2005)]. However, to the best of our knowledge, its microscopic calculation has not been performed yet. To study this microscopically, we derive the thermal Hall coefficient in extreme type-II superconductors with an isolated pinned vortex based on the augmented quasiclassical equations of superconductivity with the Lorentz force. Using it, we can confirm that the quasiparticle relaxation time and the thermal Hall angle are enhanced in $d$-wave superconductors without impurities of the resonant scattering because quasiparticles around the gap nodes which become dominant near zero temperature are restricted to the momentum in a specific orientation. This enhancement of the thermal Hall angle may also be observed in other nodal superconductors with large magnetic-penetration depth.

Optical spectroscopy and ultrafast pump-probe study on Bi2Rh3Se2 : Evidence for charge density wave order formation

The parkerite-type ternary chalcogenide Bi$_2$Rh$_3$Se$_2$ was discovered to be a charge density wave (CDW) superconductor. However, there was a debate on whether the observed phase transition at 240 K could be attributed to the formation of CDW order. To address the issue, we performed optical spectroscopy and ultrafast pump-probe measurements on single crystal samples of Bi$_2$Rh$_3$Se$_2$. Our optical conductivity measurement reveals clearly the formation of an energy gap with associated spectral change only at low energies, yielding strong evidence for a CDW phase transition at 240 K. Time resolved pump-probe measurement provides further support for the CDW phase transition. The amplitude and relaxation time of quasiparticles extracted from the photoinduced reflectivity show strong enhancement near transition temperature, yielding further evidence for the CDW energy gap formation. Additionally, a collective mode is identified from the oscillations in the pump-probe time delay at low temperature. This mode, whose frequency decreases gradually at elevated temperature, could be naturally attributed to the amplitude mode of CDW state.

Debye mechanism of giant microwave absorption in superconductors

We discuss a mechanism of microwave absorption in conventional superconductors which is similar to the Debye absorption mechanism in molecular gases. The contribution of this mechanism to the \emph{ac} conductivity is proportional to the inelastic quasiparticle relaxation time $\tau_\mathrm{\mathrm{in}}$ rather than the elastic one $\tau_{\mathrm{el}}$ and therefore it can be much larger than the conventional one. The Debye contribution to the linear conductivity arises only in the presence of a \emph{dc} supercurrent in the system and its magnitude depends strongly on the orientation of the microwave field relative to the supercurrent. The Debye contribution to the nonlinear conductivity exists even in the absence of \emph{dc} supercurrent. Since it is proportional to $\tau_{\mathrm{in}}$ the nonlinear threshold is anomalously low. Microwave absorption measurements may provide direct information about $\tau_\mathrm{in}$ in superconductors.

Giant microwave absorption in [formula omitted]- and [formula omitted]- wave superconductors

We discuss a new mechanism of microwave absorption in s- and d-wave superconductors, which arises in the presence of a dc supercurrent in the system. It produces a contribution to the ac conductivity that is proportional to the inelastic quasiparticle relaxation time. This contribution also determines the supercurrent dependence of the conductivity. It may significantly exceed the conventional contribution because in typical superconductors the inelastic relaxation time is several orders of magnitude longer than the elastic one. We show that the aforementioned contribution to the conductivity may be expressed in terms of the single particle density of states in superconductors in the presence of a dc supercurrent. Our results may enable determination of the inelastic relaxation time in superconductors from microwave absorption measurements.

Quark-flavor dependence of the shear viscosity in a quasiparticle model

We study the temperature dependence of the shear viscosity to entropy density ratio in pure Yang-Mills theory and in QCD with light and strange quarks within kinetic theory in the relaxation time approximation. As effective degrees of freedom in a deconfined phase we consider quasiparticle excitations with quark and gluon quantum numbers and dispersion relations that depend explicitly on the temperature. The quasiparticle relaxation times are obtained by computing the microscopic two-body scattering amplitudes for the elementary scatterings among the quasiparticles. For pure Yang-Mills theory, we show that the shear viscosity to entropy density ratio exhibits a characteristic nonmonotonicity with a minimum at the first-order phase transition. In the presence of dynamical quarks, the ratio smoothens while still exhibiting a minimum near confinement. Furthermore, there is a significant increase of the shear viscosity to entropy density ratio in QCD resulting from the quark contributions. This observation differs from previously reported estimates based on functional methods but is in line with perturbative QCD expectations at higher temperatures.

Solitonic thermal transport in a current-biased long Josephson junction

We investigate the coherent energy and thermal transport in a temperature-biased long Josephson tunnel junction, when a Josephson vortex, i.e., a soliton, steadily drifts driven by an electric bias current. We demonstrate that thermal transport through the junction can be controlled by the bias current, since it determines the steady-state velocity of the drifting soliton. We study the effects on thermal transport of the damping affecting the soliton dynamics. In fact, a soliton locally influences the power flowing through the junction and can cause the variation of the temperature of the device. When the soliton speed increases approaching its limiting value, i.e., the Swihart velocity, we demonstrate that the soliton-induces thermal effects significantly modify. Finally, we discuss how the appropriate material selection of the superconductors forming the junction is essential, since short quasiparticle relaxation times are required to observe fast thermal effects.

Flux-flow instability in 2H-NbSe2 superconducting thin crystals stamped on SiO2/Si substrates

We have investigated the nonlinear flux flow in the mixed state of 2H-NbSe2 superconducting thin crystals stamped on SiO2/Si substrates. The nonlinearity in the flux flow behavior is marked with upward bending and subsequent voltage jump in the current-voltage characteristics. The results were analyzed within the framework of a modified Larkin-Ovchinnikov model, including the estimation of inelastic relaxation time of quasiparticles in the crystal and heat transfer coefficient between the crystal and SiO2/Si substrate. The analysis provides an insight into the quality of acoustic match at the crystal-substrate interface.

Effects of heavy-ion irradiation on the microwave surface impedance of (Ba1−xKx)Fe2As2 single crystals

The electrodynamic response of Ba1−xKxFe2As2 single crystals at the microwave frequencies has been investigated by means of a coplanar resonator technique, at different values of non-magnetic disorder introduced into the samples by heavy-ion irradiation. The surface impedance Zs = Rs + iXs conforms to the classical skin effect above the critical temperature. Below Tc, Rs monotonically decreases while Xs shows a peak, which evolves as a function of the irradiation fluence. The disorder-dependent Zs(T) curves are analyzed within a two-fluid model, suitably modified to account for a finite quasiparticle fraction at T = 0. The analysis gives, for the unirradiated crystal, quasiparticle relaxation times τ that are in good agreement with previous literature. Smaller τ values are deduced for the disordered crystals, both in the normal and in the superconducting states. The limits of application of the model are discussed.

NbRe as candidate material for fast single photon detection

The suitability of NbRe as a promising material for the design of Superconducting Single Photon Detectors is investigated in order to lower both the minimum detectable photon energy and the recovery time of the devices. Both the low values determined for the quasiparticle relaxation time, τE, and its weak temperature dependence are desirable in the design of fast single photon detectors. Both properties can be further improved by coupling NbRe with a ferromagnetic layer, as demonstrated by estimating the characteristic relaxation rates in NbRe/CuNi bilayers.

Observation of pseudogaplike feature aboveTcin LiFeAs by ultrafast optical spectroscopy

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature ${T}_{c}$ (=15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in the SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. The quasiparticle relaxation time due to this gaplike feature is in the range of 1--2 ps. We suggest the gaplike feature in LiFeAs is induced by magnetic fluctuations.

Femtosecond spectroscopy in a nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and Ba(Fe1−xCox)2As2/Fe thin film

Femtosecond spectroscopy has been used to investigate the quasi-particle relaxation times in nearly optimally doped Fe-based superconductors FeSe0.5Te0.5 and optimally doped Ba-122 thin films growth on a Fe buffer layer. Experimental results concerning the temperature dependence of the relaxation time of such pnictides both in the superconducting state are now presented and discussed. Modelling the T-dependence of relaxation times an estimation of both electron-phonon constant and superconducting energy gap in the excitation spectrum of both Fe(Se,Te) and Ba-122 compounds is obtained.

Ultrafast quasiparticle dynamics in spin-density-wave LaOFeAs single crystal

Ultrafast quasiparticle dynamics of single. crystalline LaOFeAs were investigated by pump-probe measurement. The compound experiences structural and spin-density-wave (SDW) phase transitions at 150 K (T-S1) and 130 K (T-S2), respectively. The relaxation time of quasiparticles was somewhat temperature independent at high temperature but exhibited a sharp upturn at T-S1 and reached the maximum at approximately T-S2. The remarkable slowing down of quasiparticle relaxation time is caused by the formation of energy gap. By employing the Rothwarf-Taylor model analysis, we found that there should be already energy gaps opening just below the structural transition. The magnitude of SDW gap was identified to be 72 meV.