arXiv subjects

J. Bok

Publications and source records attributed to J. Bok.

22 records · Page 2Linked to original sources

arXiv2002

Transport properties of electrons and holes in a CuO2 layer doped by field effect

We propose a model for explaining the recent results obtained by J. Schon et al (1,2) on transport properties of electrons and holes in one plane of CuO2 in a layered cuprate (CaCuO2) where the carriers are created using a field effect transistor (FET). We use the known band structure of a CuO2 plane showing a van Hove singularity (vHs). When the energy of the hole lies near the vHs a variation of the resistivity linear with temperature (T) is calculated and when the FL lies far from the vHs, a quadratic law is obtained at low temperature and becomes linear at higher T. We find that the transition temperature T* is simply related to the distance between the Fermi level and the vHs. The behavior of the Hall coefficient is explained by the existence of hole type and electron type orbits in hole doped CuO2 planes. The fit with the experimental results is excellent.

cond-mat.supr-con↗

arXiv2002

Granularity-induced gapless superconductivity in NbN films: evidence of thermal phase fluctuations

Using a single coil mutual inductance technique, we measure the low temperature dependence of the magnetic penetration depth in superconducting NbN films prepared with similar critical temperatures around 16 K but with different microstructures. Only (100) epitaxial and weakly granular (100) textured films display the characteristic exponential dependence of conventional BCS s-wave superconductors. More granular (111) textured films exhibit a linear dependence, indicating a gapless state in spite of the s-wave gap. This result is quantitatively explained by a model of thermal phase fluctuations favored by the granular structure.

cond-mat.supr-con↗

arXiv2000

A Thought Construction of Working Perpetuum Mobile of the Second Kind

A previously published model of the isothermal Maxwell demon as one of models of open quantum systems endowed with faculty of selforganization is reconstructed here. It describes an open quantum system interacting with a single thermodynamic bath but otherwise not aided from outside. Its activity is given by the standard linear Liouville equation for the system and bath. Owing to its selforganization property, the model then yields cyclic conversion of heat from the bath into mechanical work without compensation. Hence, it provides an explicit thought construction of perpetuum mobile of the second kind, contradicting thus the Thomson formulation of the second law of thermodynamics. No approximation is involved as a special scaling procedure is used which makes the kinetic equations employed exact.

cond-mat.stat-mech↗

arXiv2000

Effect of doping on the linear temperature dependence of the magnetic penetration depth in cuprate superconductors

We report on measurements of the low temperature dependence of the magnetic penetration depth lambda_ab in several single crystals of La_2-xSr_xCuO_4, Bi_2Sr_2CaCu_2O_8 and YBa_2Cu_3O_6+x at various doping levels ranging from the under- to the overdoped regimes by using a novel single-coil technique achieving 10 pm resolution. We have found a linear dependence of lambda_ab in all samples with a rapidly increasing slope dlambda_ab/dT as doping decreases. Our analysis of the data indicates that a superconducting gap with d-wave symmetry is sufficient to quantitatively account for the above slope values in the optimally or over- doped samples. In the underdoped samples, the d-wave model predicts much smaller values than those measured by assuming realistic values for the zero-temperature lambda_ab and gap delta. The experimental data are compatible with a model of thermodynamic phase fluctuations of the order parameter. Therefore, we put forward the hypothesis that the gapless properties observed in cuprates may have qualitatively different physical origins depending on the doping level.

cond-mat.supr-con↗