Quasiparticle density of states in Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} single crystals probed using tunneling spectroscopy at ultra-low temperatures in high magnetic fields

Break-junction tunneling spectroscopy at temperatures 30-50 mK in high magnetic field is used to directly probe the quasiparticle density of states within the energy gap in a single crystal Bi2212 high-$T_c$ superconductor. The measured tunneling conductances $dI/dV(V)$ in the subgap region have a zero flat region with no evidence for a linear increase of the density of states with voltage. A number of tunnel break-junctions exhibited $dI/dV(V)$ curves with a second energy gap structure at the average magnitude 2$\Delta _{p-p}/e=13$ mV. Our data cannot be explained by either a pure $s$ pairing or a pure $d_{x^2-y^2}$ pairing

Metal-to-insulator crossover and pseudogap in single-layer compound Bi2+x_{2+x}Sr2−x_{2-x}Cu1+y_{1+y}O6+δ_{6+\delta} single crystals in high magnetic fields

The in-plane $\rho_{ab}(H)$ and the out-of-plane $\rho_c(H)$ magneto-transport in magnetic fields up to 28 T has been investigated in a series of high quality, single crystal, hole-doped La-free Bi2201 cuprates for a wide doping range and over a wide range of temperatures down to 40 mK. With decreasing hole concentration going from the overdoped (p=0.2) to the underdoped (p=0.12) regimes, a crossover from a metallic to and insulating behavior of $\rho_{ab}(T)$ is observed in the low temperature normal state, resulting in a disorder induced metal insulator transition. In the zero temperature limit, the normal state ratio $\rho_c(H)/\rho_{ab}(H)$ of the heavily underdoped samples in pure Bi2201 shows an anisotropic 3D behavior, in striking contrast with that observed in La-doped Bi2201 and LSCO systems. Our data strongly support that that the negative out-of-plane magnetoresistance is largely governed by interlayer conduction of quasiparticles in the superconducting state, accompanied by a small contribution of normal state transport associated with the field dependent pseudogap. Both in the optimal and overdoped regimes, the semiconducting behavior of $\rho_c(H)$ persists even for magnetic fields above the pseudogap closing field $H_{pg}$. The method suggested by Shibauchi \textit{et al.} (Phys. Rev. Lett. \textbf{86}, 5763, (2001)) for evaluating $H_{pg}$ is unsuccessful for both under- and overdoped Bi2201 samples. Our findings suggest that the normal state pseudogap is not always a precursor of superconductivity.Comment: 11 pages, 8 figures, published in PRB Nov 200

3D-melting features of the irreversibility line in overdoped Bi2_2Sr2_2CuO6_6 at ultra-low temperature and high magnetic field

We have measured the irreversible magnetization of an overdoped Bi$_2$Sr$_2$CuO$_6$ single crystal up to B=28 T and down to T=60 mK, and extracted the irreversibility line $B_{\rm irr}(T)$: the data can be interpreted in the whole temperature range as a 3D-anisotropic vortex lattice melting line with Lindemann number $c_{\rm L}=0.13$. We also briefly discuss the applicability of alternative models such as 2D- and quantum melting, and the connection with magnetoresistance experiments.Comment: M2S-HTSC-VI Conference paper (2 pages, 1 figure), using Elsevier style espcrc2.st

In-plane current-voltage characteristics and oscillatory Josephson-vortex flow resistance in La-free Bi2+x_{2+x}Sr2−x_{2-x}CuO6+δ_{6+\delta} single crystals in high magnetic fields

We have investigated the in-plane $I(V)$ characteristics and the Josephson vortex flow resistance in high-quality La-free Bi$_{2+x}$Sr$_{2-x}$CuO$_{6+\delta}$ (Bi2201) single crystals in parallel and tilted magnetic fields at temperatures down to 40 mK. For parallel magnetic fields below the resistive upper critical field $H^{*}_{c2}$, the $I(V)$ characteristic obey a power-law with a smooth change with increasing magnetic-field of the exponent from above 5 down to 1. In contrast to the double-layer cuprate Bi2212, the observed smooth change suggests that there is no change in the mechanism of dissipation (no Kosterlitz-Thouless transition) over the range of temperatures investigated. At small angles between the applied field and the $ab$-plane, prominent current steps in the $I(V)$ characteristics and periodic oscillations of Josephson-vortex flow resistance are observed. While the current steps are periodic in the voltage at constant fields, the voltage position of the steps, together with the flux-flow voltage, increases nonlinearly with magnetic field. The $ab$-flow resistance oscillates as a function of field with a constant period over a wide range of magnetic fields and temperatures. The current steps in the $I(V)$ characteristics and the flow resistance oscillations can be linked to the motion of Josephson vortices across layers

Heat transport in Bi_{2+x}Sr_{2-x}CuO_{6+\delta}: departure from the Wiedemann-Franz law in the vicinity of the metal-insulator transition

We present a study of heat transport in the cuprate superconductor Bi_{2+x}Sr_{2-x}CuO_{6+\delta} at subkelvin temperatures and in magnetic fields as high as 25T. In several samples with different doping levels close to optimal, the linear-temperature term of thermal conductivity was measured both at zero-field and in presence of a magnetic field strong enough to quench superconductivity. The zero-field data yields a superconducting gap of reasonable magnitude displaying a doping dependence similar to the one reported in other families of cuprate. The normal-state data together with the results of the resistivity measurements allows us to test the Wiedemann-Franz(WF) law, the validity of which was confirmed in an overdoped sample in agreement with previous studies. In contrast, a systematic deviation from the WF law was resolved for samples displaying either a lower doping content or a higher disorder. Thus, in the vicinity of the metal-insulator cross-over, heat conduction in the zero-temperature limit appears to become significantly larger than predicted by the WF law. Possible origins of this observation are discussed.Comment: 9 pages including 7 figures, submitted to Phys. Rev.

Pecularities of Hall effect in GaAs/{\delta}<Mn>/GaAs/In\timesGa1-\timesAs/GaAs (\times {\approx} 0.2) heterostructures with high Mn content

Transport properties of GaAs/{\delta}/GaAs/In\timesGa1-\timesAs/GaAs structures containing InxGa1-xAs (\times {\approx} 0.2) quantum well (QW) and Mn delta layer (DL) with relatively high, about one Mn monolayer (ML) content, are studied. In these structures DL is separated from QW by GaAs spacer with the thickness ds = 2-5 nm. All structures possess a dielectric character of conductivity and demonstrate a maximum in the resistance temperature dependence Rxx(T) at the temperature {\approx} 46K which is usually associated with the Curie temperature Tc of ferromagnetic (FM) transition in DL. However, it is found that the Hall effect concentration of holes pH in QW does not decrease below TC as one ordinary expects in similar systems. On the contrary, the dependence pH(T) experiences a minimum at T = 80-100 K depending on the spacer thickness, then increases at low temperatures more strongly than ds is smaller and reaches a giant value pH = (1-2)\cdot10^13 cm^(-2). Obtained results are interpreted in the terms of magnetic proximity effect of DL on QW, leading to induce spin polarization of the holes in QW. Strong structural and magnetic disorder in DL and QW, leading to the phase segregation in them is taken into consideration. The high pH value is explained as a result of compensation of the positive sign normal Hall effect component by the negative sign anomalous Hall effect component.Comment: 19 pages, 6 figure

Spectral Properties of Holstein and Breathing Polarons

We calculate the spectral properties of the one-dimensional Holstein and breathing polarons using the self-consistent Born approximation. The Holstein model electron-phonon coupling is momentum independent while the breathing coupling increases monotonically with the phonon momentum. We find that for a linear or tight binding electron dispersion: i) for the same value of the dimensionless coupling the quasiparticle renormalization at small momentum in the breathing polaron is much smaller, ii) the quasiparticle renormalization at small momentum in the breathing polaron increases with phonon frequency unlike in the Holstein model where it decreases, iii) in the Holstein model the quasiparticle dispersion displays a kink and a small gap at an excitation energy equal to the phonon frequency w0 while in the breathing model it displays two gaps, one at excitation energy w0 and another one at 2w0. These differences have two reasons: first, the momentum of the relevant scattered phonons increases with increasing polaron momentum and second, the breathing bare coupling is an increasing function of the phonon momentum. These result in an effective electron-phonon coupling for the breathing model which is an increasing function of the total polaron momentum, such that the small momentum polaron is in the weak coupling regime while the large momentum one is in the strong coupling regime. However the first reason does not hold if the free electron dispersion has low energy states separated by large momentum, as in a higher dimensional system for example, in which situation the difference between the two models becomes less significant.Comment: 11 pages, 10 figure

On the angular distribution of extensive air showers

Angular distributions of extensive air showers with different number of charged particles in the range 2.5x10^5--4x10^7 are derived using the experimental data obtained with the EAS MSU array. Possible approximations of the obtained distributions with different empiric functions available in literature, are analysed. It is shown that the exponential function provides the best approximation of the angular distributions in the sense of the chi-squared criterion.Comment: 5 pages including 1 figur

Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model

The equilibrium structure, energy bands, phonon dispersions, and s- and d-channel electron-phonon interactions (EPIs) are calculated for the infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA and the linear-response full-potential LMTO method were used. In the equilibrium structure, oxygen is found to buckle slightly out of the plane and, as a result, the characters of the energy bands near EF are found to be similar to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in accord with previous LDA calculations for YBa2Cu3O7. This supports the common belief that the EPI mechanism alone is insufficient to explain HTSC. Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This is surprising and indicates that the EPI could enhance some other d-wave pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute significantly to the EPI, although these contributions are proportional to the static buckling and would vanish for flat planes. These numerical results can be understood from a generic tight-binding model originally derived from the LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the buckling-modes and the influence of the spin-fluctuations should be investigated.Comment: 19 pages, 9 Postscript figures, Late