Entropy per particle spikes in the transition metal dichalcogenides

We derive a general expression for the entropy per particle as a function of chemical potential, temperature and gap magnitude for the single layer transition metal dichalcogenides. The electronic excitations in these materials can be approximately regarded as two species of the massive or gapped Dirac fermions. Inside the smaller gap there is a region with zero density of states where the dependence of the entropy per particle on the chemical potential exhibits a huge dip-and-peak structure. The edge of the larger gap is accompanied by the discontinuity of the density of states that results in the peak in the dependence of the entropy per particle on the chemical potential. The specificity of the transition metal dichalcogenides makes possible the observation of these features at rather high temperatures order of 100 K. The influence of the uniaxial strain on the entropy per particle is discussed.Comment: 6 pages, 4 figures; Special Issue to the 90th birthday of A.A. Abrikoso

Strong compensation of the quantum fluctuation corrections in clean superconductor

The theory of fluctuation conductivity for an arbitrary impurity concentration including ultra-clean limit is developed. It is demonstrated that the formal divergency of the fluctuation density of states contribution obtained previously for the clean case is removed by the correct treatment of the non-local ballistic electron scattering. We show that in the ultra-clean limit ($T\tau \gg \sqrt{\frac{T_c}{T-T_c}}$) the density-of-states quantum corrections are canceled by the Maki-Thompson term and only quasi-classical paraconductivity remains.Comment: 7 pages 2 figure

Detection of topological phase transitions through entropy measurements: the case of germanene

We propose a characterization tool for studies of the band structure of new materials promising for the observation of topological phase transitions. We show that a specific resonant feature in the entropy per electron dependence on the chemical potential may be considered as a fingerprint of the transition between topological and trivial insulator phases. The entropy per electron in a honeycomb two-dimensional crystal of germanene subjected to the external electric field is obtained from the first principle calculation of the density of electronic states and the Maxwell relation. We demonstrate that, in agreement to the recent prediction of the analytical model, strong spikes in the entropy per particle dependence on the chemical potential appear at low temperatures. They are observed at the values of the applied bias both below and above the critical value that corresponds to the transition between the topological insulator and trivial insulator phases, while the giant resonant feature in the vicinity of zero chemical potential is strongly suppressed at the topological transition point, in the low temperature limit. In a wide energy range, the van Hove singularities in the electronic density of states manifest themselves as zeros in the entropy per particle dependence on the chemical potential.Comment: 8 pages, 5 figures; final version published in PR

Fluctuoscopy of Disordered Two-Dimensional Superconductors

We revise the long studied problem of fluctuation conductivity (FC) in disordered two-dimensional superconductors placed in a perpendicular magnetic field by finally deriving the complete solution in the temperature-magnetic field phase diagram. The obtained expressions allow both to perform straightforward (numerical) calculation of the FC surface $\delta\sigma_{xx}^{(\mathrm{tot})}(T,H)$ and to get asymptotic expressions in all its qualitatively different domains. This surface becomes in particular non-trivial at low temperatures, where it is trough-shaped with $\delta\sigma_{xx}^{(\mathrm{tot})}(T,H)<0$. In this region, close to the quantum phase transition, $\delta\sigma_{xx}^{(\mathrm{tot})}(T,H=\mathrm{const})$ is non-monotonic, in agreement with experimental findings. We reanalyzed and present comparisons to several experimental measurements. Based on our results we derive a qualitative picture of superconducting fluctuations close to $H_{\mathrm{c2}}(0)$ and T=0 where fluctuation Cooper pairs rotate with cyclotron frequency $\omega_{c}\sim\Delta_{\mathrm{BCS}}^{-1}$ and Larmor radius $\sim \xi_{\mathrm{BCS}}$, forming some kind of quantum liquid with long coherence length $\xi_{\mathrm{QF}}\gg\xi_{\mathrm{BCS}}$ and slow relaxation ($\tau_{\mathrm{QF}}\gg\hbar\Delta_{\mathrm{BCS}}^{-1}$).Comment: 26 pages, 13 figures, 3 tables, RevTex 4.

Gaussian superconducting fluctuations, thermal transport, and the Nernst effect

We calculate the contribution of superconducting fluctuations to thermal transport in the normal state, for low magnetic fields. We do so in the Gaussian approximation to their critical dynamics which is also the Aslamazov-Larkin approximation in the microscopics. Our results for the thermal conductivity tensor and the transverse thermoelectric response are new. The latter compare favorably with the data of Ong and collaborators on the Nernst effect in the cuprates.Comment: 4 pages, 1 figure; improved introduction, minor changes; published versio