Quadratic Temperature Dependence Research Articles

Temperature dependence of quantum lifetime in n-InGaAs/GaAs structures with strongly coupled double quantum wells

Longitudinal ρxx(B) and Hall ρxy(B) magnetoresistances are experimentally investigated as a function of in-plane and transverse magnetic fields in n-InGaAs/GaAs nanostructures with strongly-coupled double quantum wells in the temperature range T = 1.8–70 K and magnetic fields B = 0–9.0 T. Experimental data on the temperature dependence of quantum lifetime in diffusive (kBT/τtr ≪ 1) and ballistic (kBT/τtr ≫ 1) regimes are reported. It has been found that in the ballistic regime in the temperature range where kBT/EF < 0.1, the observed quadratic temperature dependence of quantum lifetime is determined by inelastic electron–electron scattering. However, the temperature dependence of quantum lifetime cannot be quantitatively described by the existing theories in the whole temperature range.

Size effect and the quadratic temperature dependence of the transverse magnetoresistivity in “size-effect” tungsten single crystals

The transverse magnetoresistivity of pure tungsten single crystals with a residual resistivity ratio ρ293K/ρ4.2K of about 75000 was measured from 4.2 to 20 K and in magnetic fields of up to 15 T. The size effect, i.e. the linear dependence of the magnetoconductivity on the inverse cross sample dimensions, was studied in detail at high fields. We show that the size effect can be used for the separation of the contributions from the electron-surface and the electron-phonon scattering mechanisms to the full conductivity. We demonstrate that the electron-phonon scattering leads to the exponential temperature dependence of the conductivity, and the interference between the electron-phonon and the electron-surface processes leads to a new scattering mechanism “electron-phonon-surface” with a quadratic temperature dependence of the magnetoconductivity.

Low temperature thermal and electrical transport properties of ZrZn2 in high magnetic field

The low temperature electrical and thermal transport properties of the itinerant ferromagnet ZrZn2 were investigated in order to explore the nature of the Fermi-liquid breakdown in this material. We have implemented electrical and thermal conductivity measurements down to temperatures of 100 mK and in high magnetic field. In zero field and above 2 K the electrical and effective thermal resistivities take a T5/3 and T-linear form, respectively. These are the signatures of the marginal Fermi-liquid, predicted to occur close to a ferromagnetic quantum critical point by spin fluctuation theory. In contrast, we find that below 2 K and in external magnetic field the electrical resistivity assumes a quadratic temperature dependence, consistent with a return to conventional Fermi-liquid behaviour.

Two-phonon process and hyperfine interaction limiting slow hole-spin relaxation time in InAs/GaAs quantum dots

We study the hole-spin relaxation in p-doped InAs quantum dots. Two relaxation mechanisms are evidenced, at low magnetic field ($0\ensuremath{\le}B\ensuremath{\le}2T$) and low temperature ($2\ensuremath{\le}T\ensuremath{\le}50K$), by using a pump-probe configuration and a recent experimental technique working in the frequency domain. At $T=2K$, the coupling to nuclear spins and the hole wave-function inhomogeneity fix the hole-spin relaxation rate value, ${\ensuremath{\Gamma}}_{1}^{h}\ensuremath{\approx}1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$. It decreases with increasing magnetic field and reaches a plateau at 0.4 $\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$. At $T\ensuremath{\ge}7K$, two-phonon spin-orbit process dominates and leads to a quadratic temperature dependence of ${\ensuremath{\Gamma}}_{1}^{h}$, in good agreement with theory.

Onset of convective instabilities in under-ice melt ponds

The onset of double-diffusive natural convection in under-ice melt ponds is investigated through a linear stability analysis. The three-layer configuration is composed by a fluid layer (melt pond) overlying a saturated porous medium (ice matrix), which in turn overlies another fluid layer (under-ice melt pond). Water density inversion is taken into account by adopting a density profile with a quadratic temperature dependence and a linear concentration dependence. We show that the key parameter affecting stability is the depth of the ice matrix, while the depths of the upper and lower fluid layers play a marginal role. A Hopf bifurcation is observed in the whole range of parameters studied, and the size of the convection cells depends on ice permeability. The influence of the external temperature gradient is investigated by means of the definition of an extra thermal parameter accounting for the relative position of the density maximum. It is shown that convection is favored by larger temperature gradients, which occur during Arctic summer.

Phonon-induced superconductivity and physical properties in intercalated fullerides Rb3C60

The nature of electron pairing mechanism and physical properties leading to superconducting state and normal state resistivity in alkali metal (Rb) intercalated fullerenes are explored. Keeping in mind that free electrons in lowest molecular orbital are coupled with inter-molecular phonons, the coupling with inter-molecular phonon leads to transition temperature (Tc) of about 4.17 K. The electrons also couple with the intra-molecular phonons. Within the framework of strong coupling theory, Tc is estimated at 34 K. The carbon isotope effect exponent, the energy gap ratio, influence of pressure and volume on Tc, and thermodynamical parameters describing the superconducting state confer that Rb3C60 as s-wave superconductor. Estimated contribution to resistivity using scattering with inter- and intra-molecular phonon, when subtracted from single crystal data, infers quadratic temperature dependence over most of the temperature range and is attributed to electron–electron inelastic scattering. Both low frequency intermolecular and high frequency intra-molecular phonons have significant bearing in Rb3C60 superconductor.

The Kondo effect in reduced graphene oxide films

AbstractThe influence of spin in reduced graphene oxide (RGO) is demonstrated through a temperature dependent metal–insulator transition in resistance (at ∼30 K) as well as high field magneto‐resistance (MR) measurements. RGO samples, prepared using an unconventional organic acid reduction method, showed a quadratic temperature dependence of resistance at low temperatures, which changed to a logarithmic dependence at higher temperatures. Analysis of these features in RGO, combined with negative MR which scales with a Kondo characteristic temperature, establishes the interaction between conduction electrons propagating through intact graphene nano‐islands and localized magnetic moments found in disordered regions. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Solar hybrid systems with thermoelectric generators

► We examine four hybrid systems with thermoelectric generators (TEGs). ► We studied effect of radiation concentration on the system’s performance. ► Parameters of TEG based on Bi 2 Te 3 were determined and used for analysis. ► TEGs using advanced nanostructured materials were also considered. ► Cost-efficiency estimations and some recommendations for application are made. The possibility of using of thermoelectric generators in solar hybrid systems has been investigated. Four systems were examined, one working without radiation concentration, of the traditional PV/Thermal geometry, but with TEGs between the solar cells and heat extractor, and three other using concentrators, namely: concentrator – TEG − heat extractor, concentrator − PV cell − TEG − heat extractor, and PV cell – concentrator – TEG – heat extractor. The TEGs based on traditional semiconductor material Bi 2 Te 3 and designed for temperature interval of 50–200 °C were studied experimentally. It was found that the TEG’s efficiency has almost linear dependence on the temperature difference Δ T between its plates, reaching 4% at Δ T = 155 °C (hot plate at 200 °C) with 3 W of power generated over the matched load. The temperature dependencies of current and voltage are also linear; accordingly, the power generated has quadratic temperature dependence. The experimental parameters, as well as parameters of two advanced TEGs taken from the literature, were used for estimation of performance of the hybrid systems. The conclusions are drawn in relation to the efficiency at different modes of operation and the cost of hybrid systems, as well as some recommendations in relation to optimal solar cells for applications in these systems.

Finite element analysis and material sensitivity of Peltier thermoelectric cells coolers

In this work, a finite element simulation of a commercial thermoelectric cell, working as a cooling heat pump, is presented. The specially developed finite element is three-dimensional, non-linear in its formulation (using quadratic temperature-dependence on material properties) and fully coupled, including the Seebeck, Peltier, Thomson and Joule effects. Another special interface finite element is developed to prescribe the electric intensity, taking advantage of repetitions and symmetries. A thorough study of the distributions of voltage, temperature and the corresponding fluxes is presented, and the performance of the cell is compared with that of the manufacturer and with simplified analytical formulations, showing a good agreement. Combining the finite element model with the Monte Carlo technique, a sensitivity analysis is presented to take into account the performance dependence on the material properties, geometrical parameters and prescribed values. This analysis, which can be considered a first step to optimize these devices, concludes that the temperature-dependence of the material properties of electric conductivity and Seebeck coefficient is very relevant on cell performance.

Electron effective mass in the strongly correlated two-dimensional uniform electron fluid from finite-temperature calculations

The very-low temperature thermal effective mass m* of paramagnetic and ferromagnetic electrons in a uniform electron fluid in two dimensions is studied. Analytical and numerical evaluations are used to meaningfully define an m*, even in the the Hartree-Fock approximation. The Hartree-Fock m* decreases linearly with the electron-disk radius r_s. Correlation effects lead to strong cancellations between exchange and correlation. Thus the effective mass is enhanced with increasing r_s for the unpolarized fluid, while m* decreases with the r_s of the polarized fluid. The effective mass is calculated from the coefficient of the quadratic temperature dependence of exchange-correlation free energy F_{xc}. This is calculated in a physically transparent manner using a new formula for the effective mass. This uses the T=0 pair-distribution functions of Gori-Giorgi et al., and the temperature derivative of a quantum analogue of the potential of mean-force well known in the statistical mechanics of classical fluids. The results are compared with recent quantum Monte-Carlo simulations at T=0, as well as with other available experimental and theoretical data for the effective mass.

Oscillations of kinks on dislocation lines in crystals and low-temperature transport anomalies as a “passport” of newly-induced defects

The possible interpretation of experimental data on low-temperature anomalies in weakly deformed metallic crystals prepared form ultra-pure lead, copper, and silver, as well as in crystals of 4He is discussed within the previously proposed theoretical picture of dislocations with dynamical kinks. In the case of pure metals the theoretical predictions give a general picture of interaction of conduction electrons in a sample with newly-introduced dislocations, containing dynamic kinks in the Peierls potential relief. In the field of random stresses appearing due to plastic deformation of a sample, kinks on the dislocation line form a set of one-dimensional oscillators in potential wells of different shapes. In the low temperature region at low enough density of defects pinning kinks the inelastic scattering of electrons on kinks should lead to deviations from the Wiedemann-Franz law. In particular, the inelastic scattering on kinks should result in a quadratic temperature dependence of the thermal conductivity in a metallic sample along preferential directions of dislocation axes. In the plane normal to the dislocation axis the elastic large-angle scattering of electrons is prevalent. The kink pinning by a point defect or by additional dislocations as well as the sample annealing leading to the disappearance of kinks should induce suppression of transport anomalies. Thus, the energy interval for the spectrum of kink oscillations restricted by characteristic amplitude of the Peierls relief is a “passport of deformation history” for each specific sample. For instance, in copper the temperature/energy region of the order of 1 K corresponds to it. It is also planned to discuss in the other publication applicability of mechanism of phonon scattering on mobile dislocation kinks and pinning of kinks by impurities in order to explain anomalies of phonon thermal conductivity of 4He crystals and deformed crystals of pure lead in a superconducting state.

Quadratic temperature dependence of magnetoresistivity of pure tungsten single crystals under static skin effect

Transverse magnetoresistivity of tungsten single crystals with residual resistivity ratio of ρ293K/ρ4.2K ≈75 000 was measured in the temperature range of 2–75 K and in magnetic fields up to 80 kOe. At low temperatures in these “dimensional” crystals a quadratic temperature dependence of magnetoresistivity ρxx is found. It is shown that the observed quadratic dependence ρxx(T) emerges under static skin effect and is a manifestation of the interference scattering mechanism “electron – phonon – surface,” similar to what happens in pure metals without a magnetic field. A qualitative explanation of the observed dependence is proposed here.

Theoretical calculations for line-broadening and pressure-shifting in the ν 1 + ν 2 + ν 4 + ν 5 band of acetylene over a range of temperatures

Semiclassical calculations of self-broadening and self-induced pressure shift coefficients in the ν 1 + ν 2 + ν 4 + ν 5 band of C2H2 have been performed by considering, in addition to the main electrostatic quadrupole–quadrupole interaction, a simple anisotropic dispersion contribution, leading to results in overall satisfactory agreement with recent measurements [C. Povey, A. Predoi-Cross and D.R. Hurtmans, J. Mol. Spectrosc., 268, 177 (2011)]. In these calculations we have used the mean relative velocity and also considered the Maxwell–Boltzmann distribution of relative velocities. From the theoretical results obtained at different temperatures ranging from 200 to 350 K, we have determined temperature exponents of the broadenings using a simple power law, as well as coefficients of empirical linear and quadratic temperature dependences for the line shifts. These theoretical exponents and linear coefficients, derived from averaging over the distribution of velocities and from the mean thermal velocity, are significantly different and they are compared with those obtained from measurements of broadening coefficients and line shifts performed in a comparable temperature range [C. Povey, A. Predoi-Cross and D.R. Hurtmans, J. Mol. Spectrosc., 268, 177 (2011)]. The theoretical variation of the self-shifts with temperature is not linear and can be well fitted by a quadratic polynomial.

Effect of thermally activated dynamics on electron spin–lattice relaxation in glasses

Thermally activated dynamics of double-well systems is reexamined as a possible mechanism of electron spin–lattice relaxation (SLR) of paramagnetic centers in glasses. Using the soft-potential model, we have found that this mechanism can explain a nearly quadratic temperature dependence of the SLR rate in various amorphous materials over the temperature range 1–100 K. This mechanism is expected to be operative over this temperature range, as opposed to the phonon-induced tunneling of two-level systems, which is usually considered as the mechanism responsible for this behavior.

Optical and dc conductivity of the two-dimensional Hubbard model in the pseudogap regime and across the antiferromagnetic quantum critical point including vertex corrections

The conductivity of the two-dimensional Hubbard model is particularly relevant for high-temperature superconductors. Vertex corrections are expected to be important because of strongly momentum dependent self-energies. We use the Two-Particle Self-Consistent approach that satisfies crucial constraints such as the Mermin-Wagner theorem, the Pauli principle and sum rules in order to reach non-perturbative regimes. This approach is reliable from weak to intermediate coupling. A functional derivative approach ensures that vertex corrections are included in a way that satisfies the f sum-rule. The two types of vertex corrections that we find are the antiferromagnetic analogs of the Maki-Thompson and Aslamasov-Larkin contributions of superconducting fluctuations to the conductivity but, contrary to the latter, they include non-perturbative effects. The resulting analytical expressions must be evaluated numerically. The calculations are impossible unless a number of advanced numerical algorithms are used. A maximum entropy approach is specially developed for analytical continuation of our results. The numerical results are for nearest neighbor hoppings. In the pseudogap regime induced by two-dimensional antiferromagnetic fluctuations, the effect of vertex corrections is dramatic. Without vertex corrections the resistivity increases as we enter the pseudogap regime. Adding vertex corrections leads to a drop in resistivity, as observed in some high temperature superconductors. At high temperature, the resistivity saturates at the Ioffe-Regel limit. At the quantum critical point and beyond, the resistivity displays both linear and quadratic temperature dependence and there is a correlation between the linear term and the superconducting transition temperature. A hump is observed in the mid-infrared range of the optical conductivity in the presence of antiferromagnetic fluctuations.

Resistivity and Hall effect of LiFeAs: Evidence for electron-electron scattering

LiFeAs is unique among the broad family of FeAs-based superconductors, because it is superconducting with a rather large $T_c\simeq 18$ K under ambient conditions although it is a stoichiometric compound. We studied the electrical transport on a high-quality single crystal. The resistivity shows quadratic temperature dependence at low temperature giving evidence for strong electron-electron scattering and a tendency towards saturation around room temperature. The Hall constant is negative and changes with temperature, what most probably arises from a van Hove singularity close to the Fermi energy in one of the hole-like bands. Using band structure calculations based on angular resolved photoemission spectra we are able to reproduce all the basic features of both the resistivity as well as the Hall effect data.

Uniaxial Strain Effects on Mott and Superconducting Transitions in κ-(ET)2Cu2(CN)3

We have investigated the anisotropic modulation of a triangular lattice in the spin-liquid Mott insulator κ-(ET) 2 Cu 2 (CN) 3 exhibiting Mott transition accompanied by superconducting transition. Under uniaxial strain along each crystal axis, the superconducting transition temperature T c plotted against the insulator–metal crossover temperature T IM increases. An increase in the superconducting upper critical field confirms the increase in T c under in-plane uniaxial strain. The result is distinct from the abrupt monotonic decrease in T c under hydrostatic pressure, indicating that electron correlation cannot be the primary origin of the enhancement of T c . We discuss uniaxial strain effects on the basis of the quadratic temperature dependence of resistivity and the results of band structure calculation.

Structural and electrical transport properties of hexagonal 4H BaRu 1− xMn xO 3 perovskite

In this paper, structural and electrical transport properties of hexagonal, 4H-type, BaRu 1− x Mn x O 3 ( x = 0 − 0.3 ) , synthesized by the solid-state reaction method at ambient pressure, have been presented. Electrical transport properties are measured over a temperature range of 10–300 K in the magnetic field up to 15 kOe. The low temperature resistivity ( 10 K < T < 50 K ) shows a quadratic temperature dependence ρ ( T ) = ρ 0 + AT 2 for the x=0 sample and a crossover from a T 2 to a T 1.5 temperature dependence, corresponding to the Fermi-liquid phase to a non-Fermi-liquid phase crossover, in the x=0.03 sample. Furthermore, Mn doping drives the system from the metallic state at x=0 to the insulating state x ≥ 0.1 , in which the x=0.3 sample shows a variable range hopping behavior below 100 K. At low temperature region, large magnetoresistance was observed for the samples with x=0.2 and 0.3.

Microwave surface impedance measurements of LiFeAs, LiFe(As,P) and FeSe1−xTex single crystals

We study microwave surface impedance measurements of LiFeAs, LiFe(As,P) and FeSe1−xTex single crystals. The in-plane penetration depths of LiFeAs and LiFe(As,P) depend on temperature exponentially at low temperatures, which is a strong indication that these two materials have the nodeless superconducting gap. The temperature dependence of the superfluid density indicates that LiFeAs and LiFe(As,P) are multi-gap superconductors with at least two isotropic gaps. In FeSe1−xTex, on the other hand, the quadratic temperature dependence of the in-plane penetration depth appears, which indicates the existence of some sort of quasiparticle excitations. In addtion, the real part of the microwave conductivity exhibits a large enhancement below Tc in all three materials, which results from the increase of the quasiparticle relaxation time, τ, below Tc.

Two-dimensional magnetism and spin-size effect in theS = 1 triangularantiferromagnet NiGa2S4

The triangular antiferromagnet is one of the most fundamental systems of geometrically frustrated magnets.NiGa2S4 is alayered chalcogenide compound with an equilateral triangular lattice, and it is a prime candidate foran S = 1 triangular antiferromagnet. Here we focus on low temperature magnetism inNiGa2S4,where quasi-static spins develop a spin-wave-like mode without forming any long-range ordering.We have studied low temperature magnetism of both polycrystalline samples and single crystals ofNi1 − xAxGa2S4 (A = Mn, Fe, Co, and Zn). A scaling law with a single energy scale of the Weisstemperature is found as an impurity effect and a hydrostatic pressure effect,providing evidence that it is in-plane interactions in the two-dimensionalNiS2 plane that drive the critical slowing down to the viscous spin liquid state atT* = 8.5 K and the spin-wave-likeexcitations of NiGa2S4 that emerge below T ∼ 3 K. Furthermore, we find spin-size dependent impurity effects in the temperature dependence of the specific heat ofNi1 − xAxGa2S4. Even with a highimpurity content, Zn2 + (S = 0) andFe2 + (S = 2) substitutedsystems with weak XY anisotropy and integral spins retain the quadratic temperature dependence of the magnetic specific heatlike pure NiGa2S4. A spin glass-like phase, on the other hand, emerges at low temperatureswith the substitution of magnetic impurities with half-odd integer spins: IsingCo2 + (S = 3⁄2) and weak XY Mn2 + S = 5⁄2) spins. This indicates that an integer size of spins is important for stabilizing thetwo-dimensional spin-wave-like behavior, and the unconventional spin state ofNiGa2S4 at low temperatures is distinct from a canonical spin glass.