Charge density wave transport in NbSe<Subscript>3</Subscript> at low temperatures under high magnetic field

Charge density wave (CDW) depinning and sliding regimes have been studied in NbSe₃ at low temperatures down to 1.5 K under magnetic field of 19 T oriented along the c-axis. We found that the threshold field for CDW depinning becomes temperature independent below T ₀ ≈ 15 K. Also CDW current to frequency ratio characterizing CDW sliding regime increases by factor 1.7 below this temperature. The results are discussed as a crossover from thermal fluctuation to tunneling CDW depinning at T < T ₀. Besides, we found that CDW sliding strongly suppresses the amplitude of Shubnikov-de Haas oscillations of magnetoresistance.     

Tunneling in single-layer Bi2Sr2CuO6+δ single crystals in high magnetic field

In tunneling experiments with high-quality single crystals of a single-layer cuprate superconductor Bi₂Sr₂CuO_6+δ using the break junction and point-contact techniques at T<T _c, the coexistence of the superconducting-state gap and the normal-state gap was observed. The values of the superconducting energy gap 2Δ_p?p are in the range from 13.4 to 15 meV (Δ_p?p=6.7–7.5 meV). The values of 2Δ_p?p are similar for two samples with T _c=4 K and for two samples with T _c=9–10 K and are independent of the carrier concentration. The normal-state gap, with the magnitude approximately equal to 50 meV, persists at T<T _c and in the magnetic field H?H _c2 up to 28 T. After the transition of the sample to the normal state, the intensity of the tunneling conductance rapidly decreases with increasing magnetic field strength and temperature. The observed large broadening of the tunneling spectra and large zero-bias conductances can be caused by a strong angular dependence of the superconducting gap. The tunneling results are in full agreement with the data of the angle-resolved photoemission spectroscopy measurements.     

Interaction of both charge density waves in NbSe3 from interlayer tunneling experiments

An interlayer tunneling technique has been used for spectroscopy of charge density wave (CDW) energy gaps (Δ_1,2) in NbSe₃ subsequently opened at the Fermi surface on decreasing temperature at T _p1 = 145 K (CDW1) and at T _p2 = 60 K (CDW2). We found that the CDW2 formation is accompanied by an increase of the CDW1 gap below T _p2. The maximum enhancement of Δ₁, δΔ₁ is about 10%. The effect observed has been predicted theoretically as resulting from the joint phase locking of both CDWs with the underlying crystalline lattice below T _p2. The text was submitted by the authors in English.     

The origin of the pseudogap phase: precursor superconductivity versus a competing energy gap scenario

In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature T_c is different from the energy gap onset temperature T^∗. In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter Δ_sc is to be distinguished from the excitation gap Δ; in this way, pseudogap effects persist below T_c. It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper, we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T^∗ and T_c on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self-consistently determined chemical potential in the presence of arbitrary attractive coupling g. We have applied this mean field theory with some success to tunneling, transport, thermodynamics, and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter.     

Tunneling measurements on a BiSrCuO single crystal up to the critical magnetic field

The tunneling to superconducting BiSrCuO (critical temperatureT _c=10K) has been studied in an applied magnetic field up to 10 T. At 0.7T a pronounced change has been observed in the tunneling conductance: the gap-like structure of the Nb counter electrode disappears, while the bias-voltage position of the second one diminishes by an amount of the Nb gap. At higher fields the gap-like peak of BiSrCuO is smeared out and shifts to lower biasvoltages linearly with the pair-breaking field.     

Discrete temperatures in high-temperature superconductors

The low-amplitude AC susceptibility on intact and deformed Bi2223/Ag tapes has been measured as a function of temperature, frequency, AC amplitude and DC magnetic field. The deformation resulted in the splitting of the χ″(T) peak into three peaks situated near 30, 58 and 90 K. In zero magnetic field, these temperatures were identified as the Kosterlitz–Thouless transition temperatures of low number stacks of superconducting layers. An external magnetic field redistributed the dissipation among the peaks, and moved them to lower temperatures (and suppressed the highest temperature peak). In a finite field, each peak corresponds to the stack melting temperature T_m. The melting temperature in each stack was found to be a field-dependent parameter, with a minimum value=T_KT of a stack of thickness that is less by one layer. The T_m decreases exponentially with the field, and the rate of decrease depends on the interstack Josephson and magnetic interactions. With a universal set of T_KT, the vortex melting line of a tape is a linear combination of the T_m(H) for the low-number stacks.     

Hysteresis in the electrical properties of orthorhombic tantalum trisulphide: Evidence for an incommensurate-commensurate charge-density wave transition?

A study of electrical conduction in orthorhombic TaS₃ has revealed the existence of thermal hysteresis throughout the temperature range 55 K < T < 205 K. This is attributed to variability in the wavevector q of the charge-density wave (CDW) which develops below T_p = 215 K, and confirms the recent finding, from electron diffraction, that at temperatures not too far below T_p the CDW is incommensurate with the underlying lattice. Evidence that q becomes commensurate, at least along the chain direction at 55 K is provided by the vanishing of hysteresis at that temperature, and also by a rise in the threshold field for continuous motion of the CDW.From its dependence on temperature it is concluded that between T_p and 55 K the conduction in the linear regime is better described as that of a Peierls semi-metal, rather than that of a Peierls intrinsic semiconductor. At most temperatures within that range electrical hysteresis also is observed, and a detailed study of this leads to the tentative conclusion that translation of the CDW conveys negative charge, carried presumably by negatively-charged discommensurations. The mechanisms of conduction below 55 K remain uncertain.     

Interlayer tunneling spectroscopy of graphite at high magnetic field oriented parallel to the layers

Interlayer tunneling in graphite mesa-type structures is studied at a strong in-plane magnetic field H up to 55 T and low temperature T = 1.4 K. The tunneling spectrum dI/dV vs. V has a pronounced peak at a finite voltage V ₀. The peak position V ₀ increases linearly with H. To explain the experiment, we develop a theoretical model of graphite in the crossed electric E and magnetic H fields. When the fields satisfy the resonant condition E = vH, where V is the velocity of the two-dimensional Dirac electrons in graphene, the wave functions delocalize and give rise to the peak in the tunneling spectrum observed in the experiment.     

Phase transition in anyon superconductivity at finite temperature

The magnetic response of the charged anyon fluid at temperatures higher than the fermion energy gap (T ⪢ ω_c) is investigated in the self-consistent field approximation. In this temperature region a new phase, characterized by an inhomogeneous magnetic penetration, is found. The inhomogeneity is linked to the existence of an imaginary magnetic mass which increases with the temperature. It is explicitly proved that the electromagnetic field magnetic masses and rest-energies are different in the (T ⪢ ω_c)-phase.     

Oscillations of echo amplitude in glasses in a magnetic field induced by nuclear dipole-dipole interaction

The influence of a magnetic field on the dipole echo amplitude in glasses (at temperatures of about 10 mK) induced by the dipole-dipole interaction of nuclear spins has been theoretically studied. It has been shown that a change in the mutual position of nuclear spins at tunneling and the Zeeman energy E _H of their interaction with the external magnetic field lead to a nonmonotonic magnetic-field dependence of the dipole echo amplitude. The approximation that the nuclear dipole-dipole interaction energy E _d is much smaller than the Zeeman energy has been found to be valid in the experimentally important cases. It has been shown that the dipole echo amplitude in this approximation may be described by a simple universal analytic function independent of the microscopic structure of the two-level systems. An excellent agreement of the theory with the experimental data has been obtained without fitting parameters (except for the unknown echo amplitude).     

Interlayer tunneling spectroscopy of the field induced CDW state in graphite

Using interlayer tunneling spectroscopy we studied anomalous magnetoresistance state in graphite in pulsed magnetic fields up to 55 T. At low temperatures we found the opening of a pseudogap on tunneling spectra at fields above 17 T. The gap value is saturated above 30 T to 2Δ=70 mV. The gap feature is gradually smearing out with temperature but is still observed up to temperatures of ∼250 K. We discuss possible origin of the pseudogap as being related with the field induced charge density wave (CDW) state in analogy with that recently observed in NbSe₃ above Peierls transition temperature.     

Charge density wave transition in Na2Ti2Sb2O probed by 23Na NMR

Herein we investigated the electronic properties of layered transition-metal oxides Na2Ti2Sb2O by23Na nuclear magnetic resonance(NMR)measurement.The resistivity,susceptibility and specific heat measurements show a phase transition at approximately 114 K(TA).No splitting or broadening in the central line of23Na NMR spectra is observed below and above the transition temperature indicating no internal field being detected.The spin-lattice relaxation rate divided by T(1/T1T)shows a sharp drop at about 110 K which suggests a gap opening behavior.Below the phase transition temperature zone,1/T1T shows Fermi liquid behavior but with much smaller value indicating the loss of large part of electronic density of states(DOS)because of the gap.No signature of the enhancement of spin fluctuations or magnetic order is found with the decreasing temperature.These results suggest a commensurate charge-density-wave(CDW)phase transition occurring.     

Mixing experiments in NbSe3

The response of NbSe₃ to combined a.c. and d.c. fields has been characterized at T = 125 K below the first charge-density wave (CDW) transition. The a.c. and d.c. conductances were measured, along with the rectification and harmonic mixing at megahertz frequencies due to the CDW non linearity. These experimental results are shown to be consistent with a revised tunneling theory of CDW depinning.     

Cyclotron resonance study of conduction electrons in n-type indium antimonide under a strong magnetic field—I: Thermal equilibrium case

The far IR cyclotron resonance of conduction electrons is investigated in n-type indium antimonide in the quantum regimes, ?ω_ck_BT and ?ω_c?k_BT. The resonance peak position, half width, and the degree asymmetry in the line shape are studied as a function of temperature. In analyzing the experimental data, the three band model has been employed together with modern theoretical results of electron scattering by ionized impurities in the presence of a strong magnetic field. It is found that, for example for an experiment at 84 μm, the Une width depends very little on temperature between 4.2 and 45 K where the ionized impurity scattering is dominant, and increases rapidly with temperature above 45 K where the onset of phonon scattering is expected. Further details of the ionized impurity scattering were investigated by using three different laser wavelengths 84, 119 and 172μm. The line width at the phonon-limited temperature region depends very little on magnetic field and sample. The temperature dependence of the band gap was also determined by analysis of the resonance peak shift.     

Deformation of a charge density wave in the vicinity of a point contact with a normal metal

The current-voltage characteristics of Cu-K_0.3MoO₃ point contacts between a metal and a semiconductor with a charge density wave (CDW) are studied for various diameters of the contacts in a wide range of temperatures T and voltages V. In the interval 80 K ? T ? 150 K, the current-voltage characteristics are correctly described in the framework of a semiconductor model: screening of an external electric field causes CDW deformation, shifts the chemical potential of quasiparticles, and changes the point contact resistance. It is shown that the chemical potential is above the middle of the Peierls gap in equilibrium and approaches the middle upon an increase in temperature. The current-voltage characteristics of point contacts with a diameter d ? 100 Å exhibit a sharp decrease in resistance for |V| > V _t , which is associated with the beginning of local CDW sliding within the contact region. The V _t (d, T) dependence can be explained by the size effect in the CDW phase slip.     

Intercluster conduction in lightly doped La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La _{1 ? x} Ca _x MnO₃ polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature T _C. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ T _C is fitted by the relation logρ ～ T ^?1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.     

Zero-bias tunneling anomaly in a two-dimensional electron system with disorder

The temperature dependence of a zero-bias anomaly in the tunneling conductance of an Al/δ-GaAs tunneling structure with a two-dimensional electron density in the δ-layer of 3.5 × 10¹² cm^?2 has been investigated. It has been shown that the respective drop Δρ(?, T) in the tunneling density of states ρ near the Fermi level E _F of the two-dimensional electron system depends logarithmically on the energy ? within the range of 2.7kT < |?| < ?/τ, where ? is measured with respect to E _F and τ is the momentum relaxation time of two-dimensional electrons. It has been found that the drop depth Δρ(0, T)/ρ is also proportional to ln(kT/?₀) in the temperature range T = 0.1–20 K and saturates below 0.1 K.     

Heat capacity and entropy of two electrons quantum dot in a magnetic field with parabolic interaction

The thermodynamic properties of two electrons in two dimensional parabolic GaAs quantum dot are studied where both the magnetic field and the e–e interaction are fully considered. The e–e interaction has been treated by a model potential which makes the Hamiltonian exactly solvable. The energy spectrum is used to calculate the canonical partition function, and then we obtain the thermodynamic properties; mean energy, heat capacity and entropy as a function of temperature (T) and magnetic field (B).A steep transition from zero to 4k_B is observed in the heat capacity as a function of temperature for small values of magnetic field and saturates within a small temperature range, also the heat capacity has a peak-like structure at low temperature, while for high magnetic field heat capacity develops a shoulder at 2k_B then it approaches the saturation value with further increase in temperature. The entropy increases with increasing temperature, but at higher temperature, it remains almost independent of the magnetic field. It is shown that, at low magnetic field values, the effect of magnetic field on heat capacity is tangible and it attains a constant value with further increase in magnetic field. Entropy is almost linearly proportional with increasing magnetic field strength.     

Electrical transport and magnetic properties of nanostructured La0.67Ca0.33MnO3

We report fabrication of nanostructured La_0.67Ca_0.33MnO₃ (NS-LCMO) by pulsed-laser deposition on the surface of porous Al₂O₃. The resistance peak temperature (T_p) of the NS-LCMO increases with increasing average thickness of the films, while their Curie temperatures (T_c) remain unchanged. The coercive field of the samples increases with decreasing film thickness and its temperature dependence can be well described by H_c(T)=H_c(0)[1-(T/T_B)^1/2]. A large magnetoresistance and strong memory effect were observed for the NS-LCMO. The results are discussed in terms of the size effect, Coulomb blockade and magnetic tunneling effect. This work also demonstrates a new way to get nanostructured manganites. PACS 75.70.Ak; 75.75.+a; 72.80.Ga