Logarithmic Temperature Dependence Research Articles

Damping in high-frequency metallic nanomechanical resonators

We have studied damping in polycrystalline Al nanomechanical resonators by measuring the temperature dependence of their resonance frequency and quality factor over a temperature range of 0.1--4 K. Two regimes are clearly distinguished with a crossover temperature of 1 K. Below 1 K we observe a logarithmic temperature dependence of the frequency and linear dependence of damping that cannot be explained by the existing standard models. We attribute these phenomena to the effect of the two-level systems characterized by the unexpectedly long (at least two orders of magnitude longer) relaxation times and discuss possible microscopic models for such systems. We conclude that the dynamics of the two-level systems is dominated by their interaction with one-dimensional phonon modes of the resonators.

From quantum criticality to enhanced thermopower in strongly correlated layered cobalt oxide

We report on susceptibility measurements in the strongly correlated layered cobalt oxide [BiBa0.66K0.36O2]CoO2, which demonstrate the existence of a magnetic quantum critical point (QCP) governing the electronic properties. The investigated low frequency susceptibility displays a scaling behavior with both the temperature T and the magnetic field B ranging from the high-T non-Fermi liquid down to the low-T Fermi liquid. Whereas the inferred scaling form can be discussed within the standard framework of the quantum critical phenomena, the determined critical exponents suggest an unconventional magnetic QCP of a potentially generic type. Accordingly, these quantum critical fluctuations account for the anomalous logarithmic temperature dependence of the thermopower. This result allows us to conjecture that quantum criticality can be an efficient source of enhanced thermopower.

Crossover from Fermi Liquid to Multichannel Luttinger Liquid in High-Mobility Quantum Wires

We investigate the electrical conductance of long, high-mobility quantum wires formed by the split-gate technique, which allows for adjustment of the wire width and the number of one-dimensional electron subbands, n. In wires with 3<or=n<or=8, a logarithmic temperature dependence of the conductance is observed for 1<T<10 K, which reaches as much as 30% of the Drude conductance. In even narrower wires, the logarithmic dependence changes to a power-law variation. Our observations are shown to be in good agreement with recent theoretical studies, which attribute the logarithmic term to interaction effects in a weakly disordered quasi-one-dimensional conductor. This interaction correction is associated with the emergence of a crossover from a quasi-one-dimensional weakly disordered Fermi liquid to a multichannel Luttinger liquid.

Transport properties of ferromagnetic granular Co–Al–O films

We investigate the transport properties of ferromagnetic granular Co–Al–O films. The electrical resistivity of these films has a T2 dependence at low temperatures, which can be explained in terms of scattering of electrons by spin waves. The resistivity of these films was found to have a logarithmic temperature dependence. The negative magnetoresistivity has long tails in high field ranges.

Zooming on the quantum critical point in Nd-LSCO

Recent studies of the high-Tc superconductor La1.6-xNd0.4SrxCuO4 (Nd-LSCO) have found a linear-T in-plane resistivity ρab and a logarithmic temperature dependence of the thermopower S/T at a hole doping p=0.24 and a Fermi-surface reconstruction just below p=0.24[1,2]. These are typical signatures of a quantum critical point (QCP). Here we report data on the c-axis resistivity ρc(T) of Nd-LSCO measured as a function of temperature near this QCP, in a magnetic field large enough to entirely suppress superconductivity. Like ρab,ρc shows an upturn at low temperature, a signature of Fermi surface reconstruction caused by stripe order. Tracking the height of the upturn as it decreases with doping enables us to pin down the precise location of the QCP where stripe order ends, at p∗=0.235±0.005. We propose that the temperature Tρ below which the upturn begins marks the onset of the pseudogap phase, found to be roughly twice as high as the stripe-ordering temperature in this material.

Crossover from weakly-disordered Fermi liquid to Luttinger liquid in quasi-1D nanostructures

We investigated the temperature–dependent conductance of a long and narrow high-mobility channel, which is formed from an AlGaAs/GaAs heterostructure using the split-gate technique. When the channel is wider than 100 nm, in the range of 1-10 K we observe logarithmic temperature dependence, which is a key signature of the interaction correction in the quasi-one-dimensional weakly-disordered Fermi liquid. For the narrowest channels with width smaller than 100 nm, we observe power-law dependences, which are typical for the Luttinger liquid. The crossover from the weakly-disordered Fermi liquid to the Luttinger liquid takes place when the channel width corresponds to 2-3 one-dimensional subbands.

Magnetoresistance in LaNi1−xCoxO3 (0.3≤x≤0.6)

We report on a detailed study of the electrical resistivity and the magnetoresistance of the metallic members of the LaNi1−xCoxO3 series with 0.3≤x≤0.6. The low-temperature resistivity of the compounds with 0.3≤x≤0.5 exhibits a logarithmic temperature dependence that is characteristic of systems with enhanced spin disorder. We attribute the decreasing contribution of the −ln T term to resistivity with increasing magnetic field to the suppression of spin disorder in an electronically phase separated matrix by the application of magnetic field, and suggest that this mechanism is primarily responsible for the magnetoresistive behavior of these compounds. We propose two models that are based on inter-cluster scattering which could explain the observed effects.

Kondo Effect in the Helical Edge Liquid of the Quantum Spin Hall State

Following the recent observation of the quantum spin Hall (QSH) effect in HgTe quantum wells, an important issue is to understand the effect of impurities on transport in the QSH regime. Using linear response and renormalization group methods, we calculate the edge conductance of a QSH insulator as a function of temperature in the presence of a magnetic impurity. At high temperatures, Kondo and/or two-particle scattering give rise to a logarithmic temperature dependence. At low temperatures, for weak Coulomb interactions in the edge liquid, the conductance is restored to unitarity with unusual power laws characteristic of a "local helical liquid," while for strong interactions, transport proceeds by weak tunneling through the impurity where only half an electron charge is transferred in each tunneling event.

Thickness dependence of superconductivity for In/Mo thin films

We report on the superconducting characteristics of the Indium thin films on molybdenum under-layer as a function of the In film thickness. Our molybdenum under-layer with thickness of 50 Å does not cause the occurrence of superconductivity until 1.5 K and the sheet resistance has logarithmic temperature dependence observed in the present investigation. As thickness of In increased, the oscillation phenomenon of T C was observed at early stage of deposition and the value of T C is higher than the that for bulk of In. Furthermore, it is found that with increase of the In thickness, there are large differences of the strengths of the upper critical magnetic field H C2( T), resistivity and T C between films with thickness below and above 100 Å. On the other hand, the T C decreases monotonously as sheet resistance increases, when the T C is plotted against sheet resistance. To clarify the relation of superconducting characteristics and the surface structure of the films with different thickness, we have performed surface observation by atomic force microscope. As a result, we have found that the surface changes from homogeneous structure to inhomogeneous (or percolative) structure, when the thickness of in films pass through about 100 Å. Superconductivity of In/Mo films with relatively thick-inhomogeneous films cannot be explained in terms of the simple percolation theory. Therefore, we analysis the experimental data of H C2( T) near T C, using a extended Landau–Ginzburg model. It is found out that our In/Mo films must consider some factors; such as, grain size, the distance of grain space, and the strength of couplings between grains.

Low-temperature transport in highly boron-doped nanocrystalline diamond

We studied the transport properties of highly boron-doped nanocrystalline diamond thin films at temperatures down to 50 mK. The system undergoes a doping-induced metal-insulator transition with an interplay between intergranular conductance $g$ and intragranular conductance ${g}_{0}$, as expected for a granular system. The conduction mechanism in the case of the low-conductivity films close to the metal-insulator transition has a temperature dependence similar to Efros-Shklovskii type of hopping. On the metallic side of the transition, in the normal state, a logarithmic temperature dependence of the conductivity is observed, as expected for a metallic granular system. Metallic samples far away from the transition show similarities to heavily boron-doped single-crystal diamond. Close to the transition, the behavior is richer. Global phase coherence leads in both cases to superconductivity (also checked by ac susceptibility), but a peak in the low-temperature magnetoresistance measurements occurs for samples close to the transition. Corrections to the conductance according to superconducting fluctuations account for this negative magnetoresistance.

Spin-wave-mediated quantum corrections to the conductivity of thin ferromagnetic films of gadolinium

We present a study of quantum corrections to the conductivity of thin ferromagnetic gadolinium films. In situ magneto-transport measurements were performed on a series of thin films with thickness d < 135A. For sheet resistances R0 < 4011 Ohm and temperatures T < 30K, we observe a linear temperature dependence of the conductivity in addition to the logarithmic temperature dependence expected from well known quantum corrections in two dimensions. We show that such a linear T-dependence can arise from a spin-wave mediated Altshuler-Aronov type correction.

Low temperature electrical transport properties of carbon matrix containing iron nanoparticles

We present a comparative study of the low temperature electrical transport properties of the carbon matrix containing iron nanoparticles and the films. The conductivity of the nanoparticles located just below the metal-insulator transition exhibits metallic behavior with a logarithmic temperature dependence over a large temperature interval. The zero-field conductivity and the negative magnetoresistance, showing a characteristic upturn at liquid helium temperature, are consistently explained by incorporating the Kondo relation and the two dimensional electron-electron interaction. The films, in contrast, exhibit a crossover of the conductivity from power-law dependence at high temperatures to an activated hopping law dependence in the low temperature region. The transition is attributed to changes in the energy dependence of the density of states near the Fermi level. The observed magnetoresistance is discussed in terms of quantum interference effect on a three-dimensional variable range hopping mechanism.

Evidence of universality in the dynamical response of micromechanical diamond resonators at millikelvin temperatures

We report kelvin- to millikelvin-temperature measurements of dissipation and frequency shift in megahertz-range resonators fabricated from ultrananocrystalline diamond. Frequency shift $\ensuremath{\delta}f/{f}_{0}$ and dissipation ${Q}^{\ensuremath{-}1}$ demonstrate temperature dependence in the millikelvin range similar to that predicted by the glass model of tunneling two-level systems. The logarithmic temperature dependence of $\ensuremath{\delta}f/{f}_{0}$ is in good agreement with such models, which include phonon relaxation and phonon resonant absorption. Dissipation shows a weak power law, ${Q}^{\ensuremath{-}1}\ensuremath{\propto}{T}^{1/3}$, followed by saturation at low temperature. A comparison of both the scaled frequency shift and dissipation in equivalent micromechanical structures made of single-crystal silicon and gallium arsenide indicates universality in the dynamical response.

Magnetic susceptibility of PrMg3 at ultra low temperatures

We report susceptibility measurements of PrMg3 with a non-magnetic T3 crystalline-electric-field ground state down to 0.3 mK. The susceptibility shows the Van-Vleck-like behavior with an additional logarithmic temperature dependence below 10 K. Furthermore, the susceptibility exhibits a small peak around 0.1 K, which is attributed to the quenching of the multipole degrees of freedom. Below 30 mK, the susceptibility is almost constant down to the lowest temperature without showing the contribution of Pr nuclear spins. The disappearance of nuclear spin contribution suggests that the quenching of the multipole degrees of freedom strongly affects the nuclear spin system.

Magnetic field induced superconductor-insulator transitions for ultra-thin Bi films on the different underlayers

This work shows the experimental results of the superconductor-insulator (S-I) transition for ultra-thin Bi films in magnetic fields. The quench-condensed (q-c) Bi film onto insulating underlayers have been interpreted to be homogeneous. In contrast, the Bi film without underlayers has been regarded as a granular film. The electrical transport properties of ultra-thin metal films near the S-I transition depend on the structure of the film. In order to confirm the effect of the underlayer to the homogeneity of the superconducting films, we investigate the characteristics of S-I transitions of q-c nominally homogeneous Bi films on underlayers of two insulating materials, SiO, and Sb. Under almost the same deposition condition except for the material of underlayer, we prepared the Bi films by repeating the additional deposition and performed in-situ electrical measurement. It is found that the transport properties near the S-I transitions show the remarkable difference between two films on different underlayers. As for Bi films on SiO, it turned out that the temperature dependence of resistance per square Rsq(T) of the field-tuned transition and the thickness-tuned transition shows similar behavior; it was a thermally activated form. On the other hand, the Rsq(T) of Bi films on Sb for thickness-tuned S-I transition showed logarithmic temperature dependence, but that for field-tuned S-I transition showed a thermally activated form.

Crystal structure and physical properties of a novel Kondo antiferromagnet:U3Ru4Al12

A novel ternary compound U3Ru4Al12 has been identified in the U–Ru–Al ternary diagram. Single-crystal x-ray diffraction indicates a hexagonalGd3Ru4Al12-type structure for this uranium-based intermetallic. While this structure type usuallyinduces geometrically a spin-glass behaviour, an antiferromagnetic ordering is observed atTN = 8.4 K in the present case. The reduced effective magnetic moment of U atoms(μeff = 2.6 µB) can be explained by Kondo-like interactions and crystal field effects that have beenidentified by a logarithmic temperature dependence of the electrical resistivity, negativevalues of the magnetoresistivity and particular shape of the Seebeck coefficient.

Simultaneous Occurrence of an Antiferroquadrupolar and a Ferromagnetic Transitions in Rare-Earth Palladium Bronze CePd3S4

Neutron powder diffraction experiments have been performed on CePd 3 S 4 as well as measurements of magnetic and transport properties on a single-crystalline sample. Magnetic susceptibility increases markedly below T C = 6.3 K and a spontaneous magnetization appears below T C . The logarithmic temperature dependence of electrical resistivity implies that CePd 3 S 4 is classified into a Kondo-lattice compound whose Kondo temperature is comparable to T C . Magnetic susceptibility is analyzed satisfactorily with a crystalline electric field model that the 4 f ground state of Ce 3+ is the Γ 8 quartet with the excited Γ 7 doublet located at 630 K. The magnetic structure of CePd 3 S 4 was determined to be a canted structure with a canting angle of 51°. Such a canted structure is a result of the ferromagnetic (FM) ordering and simultaneous [100]-type antiferroquadrupolar (AFQ) ordering at T C . The smaller magnetization values and an unexpectedly high T C can be explained by the simultaneous occurrence of AFQ an...

Electronic conduction in (Bi,Pb)–Sr–Ca–Cu–O granular superconductors

Granular superconductors are very interesting materials owing to their untypical electrical properties caused by the presence of Coulomb effects, electron tunnelling, Josephson coupling between granules and various aspects of disorder. The electrical properties of (Bi,Pb)–Sr–Ca–Cu–O granular superconductors obtained by the solid state crystallization method have been studied in this paper. The materials may be considered as a system of granules of a high-temperature suprconductor embedded in the insulating matrix. The main parameter which determines the properties of granular materials is the tunnelling conductivity between the neighboring grains. Composed of small weakly coupled granules they are characterized by hopping conductivity exponential dependence. Samples containing relatively large granules of the 2212 phase (above 30nm), with larger inter-grain conductivity, above the transition temperature, exhibit a logarithmic temperature dependence of resistivity.

Logarithmic Fermi-Liquid Breakdown inNbFe2

The d-electron low temperature magnet NbFe2 is poised near the threshold of magnetism at ambient pressure, and can be tuned across the associated quantum critical point by adjusting the precise stoichiometry within the Nb1-yFe2+y homogeneity range. In a nearly critical single crystal (y= -0.01), we observe a T3/2 power-law dependence of the resistivity rho on temperature T and a logarithmic temperature dependence of the Sommerfeld coefficient gamma=C/T of the specific heat capacity C over nearly 2 orders of magnitude in temperature, extending down to 0.1 K.

Disorder and temperature dependence of the anomalous Hall effect in thin ferromagnetic films: Microscopic model

We consider the anomalous Hall (AH) effect in thin disordered ferromagnetic films. Using a microscopic model of electrons in a random potential of identical impurities including spin-orbit coupling, we develop a general formulation for strong, finite range impurity scattering. Explicit calculations are done within a short range but strong impurity scattering to obtain AH conductivities for both the skew scattering and side-jump mechanisms. We also evaluate quantum corrections due to interactions and weak localization effects. We show that for arbitrary strength of the impurity scattering, the electron-electron interaction correction to the AH conductivity vanishes exactly due to general symmetry reasons. On the other hand, we find that our explicit evaluation of the weak localization corrections within the strong, short-range impurity scattering model can explain the experimentally observed logarithmic temperature dependences in disordered ferromagnetic Fe films.