arXiv subjects

Thomas Dahm

Publications and source records attributed to Thomas Dahm.

24 records · Page 2Linked to original sources

arXiv2013

Interaction of a Bose-Einstein Condensate and a Superconductor via Eddy Currents

We study center-of-mass oscillations of a dipolar Bose-Einstein condensate in the vicinity of a superconducting surface. We show that the magnetic field of the magnetic dipoles induces eddy currents in the superconductor, which act back on the Bose-Einstein condensate. This leads to a shift of its oscillation frequency and to an anharmonic coupling of the Bose-Einstein condensate with the superconductor. The anharmonicity creates a coupling to one of the collective modes of the condensate that can be resonantly enhanced, if the parameters of the condensate are chosen properly. This provides a new physical mechanism to couple a Bose-Einstein condensate and a superconductor which becomes significant for 52Cr, 168Er or 164Dy condensates in superconducting mircotraps.

cond-mat.quant-gas↗

arXiv2013

Magnetically robust topological edge states and flat bands

We study thin strips of three dimensional topological insulators in the presence of a spin-splitting Zeeman field. We show that under certain conditions the topological edge states at the sides of a strip remain robust against a time-reversal symmetry breaking Zeeman field. For a particle-hole symmetric system with Zeeman field perpendicular to the strip we strictly proof that the dispersion and the spin-orbital structure of the edge states remains unchanged. When the Zeeman field lies parallel to the strip, the Dirac spectrum becomes flat, but remains intact. Above a critical value of the Zeeman field a topological flat band appears at the edge. We present numerical calculations for a lattice model of Bi$_2$Se$_3$. These calculations show that even though particle-hole symmetry is not strictly fulfilled in this system, these special features are still present. The flat band is tunable by the Zeeman field and can be realistically achieved in Bi$_2$Se$_3$-ferromagnet heterosystems at room temperature.

cond-mat.mes-hall↗

arXiv2007

Theoretical study of NMR relaxation due to rattling phonons

We calculate the NMR relaxation rate due to quadrupolar coupling of the nucleus to a local, strongly anharmonic phonon mode. As a model potential for a rattling motion we consider a square-well potential. We calculate the free phonon Green's function analytically and derive the low and high temperature limits of the NMR relaxation rate. It is shown that the temperature dependence of the NMR relaxation rate possesses a peak in contrast to harmonic phonons but in qualitative agreement with a recent NMR study on KOs2O6. We discuss the influence of phonon renormalization due to electron-phonon interaction.

cond-mat.supr-con↗

arXiv2007

NMR relaxation and resistivity from rattling phonons in pyrochlore superconductors

We calculate the temperature dependence of NMR relaxation rate and electrical resistivity for coupling to a local, strongly anharmonic phonon mode. We argue that the two-phonon Raman process is dominating NMR relaxation. Due to the strong anharmonicity of the phonon an unusual temperature dependence is found having a low temperature peak and becoming constant towards higher temperatures. The electrical resistivity is found to vary like T^2 at low temperatures and following a sqrt{T} behavior at high temperatures. Both results are in qualitative agreement with recent observations on beta-pyrochlore oxide superconductors.

cond-mat.supr-con↗

arXiv2003

Anatomy of Gossamer Superconductivity

There are many systems in which two order parameters compete with each other. Of particular interest are systems in which these order parameters are both unconventional. In this contribution, we examine the representative example of a d-wave superconductor in the presence of a d-wave density wave, which has been suggested as a model for the pseudogap phase in the high- T_c superconductors. The physical properties of unconventional superconductivity in the presence of an anisotropic charge density wave are investigated within mean field theory. This model describes many features that were anticipated by an earlier phenomenological treatment of Tallon and Loram. In addition, the quasiparticle density of states in the presence of these two order parameters is calculated, which should be accessible by scanning tunneling microscopy.

cond-mat.supr-con↗

arXiv1999

Isotope effect in the presence of a pseudogap

We investigate the question whether the unusual doping dependence of the isotope exponent observed in underdoped high-Tc superconductors might be related to another unusual phenomenon observed in these systems: the pseudogap phenomenon. Within different approximations we study the influence of a phenomenological pseudogap on the isotope exponent and find that it generally strongly increases the isotope exponent, in qualitative agreement with experiments on underdoped high-Tc compounds. This result is stable against strong-coupling self-energy corrections and also holds for recently proposed spinfluctuation exchange models, if a weak additional electron-phonon coupling is considered.

cond-mat.supr-con↗