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T. Dahm

Publications and source records attributed to T. Dahm.

At least 19 recordsLinked to original sources
arXiv2012

Optimal thickness of rectangular superconducting microtraps for cold atomic gases

We study superconducting microtraps with rectangular shapes for cold atomic gases. We present a general argument why microtraps open, if brought close to the surface of the superconductor. We show that for a given width of the strips there exists an optimal thickness under which the closest distance of the microtrap from the superconductor can be achieved. The distance can be significantly improved, if the edge enhancement of the supercurrent near edges and corners is exploited. We compare numerical calculations with results from conformal mapping and show that conformal mapping can often give useful approximate results.

physics.atom-ph↗
arXiv2010

Strong surface contribution to the Nonlinear Meissner Effect

We demonstrate that in a d-wave superconductor the bulk nonlinear Meissner effect is dominated by a surface effect due to Andreev bound states at low temperatures. The contribution of this surface effect to the nonlinear response coefficient follows a 1/T^3 law with opposite sign compared to the bulk 1/T behavior. The cross-over from bulk dominated behavior to surface dominated behavior occurs at a temperature of T/T_c ~ 1/sqrt(kappa). We present an approximate analytical calculation, which supports our numerical calculations and provides a qualitative understanding of the effect. The effect can be probed by intermodulation distortion experiments.

cond-mat.supr-con↗
arXiv2008

Strength of the Spin-Fluctuation-Mediated Pairing Interaction in a High-Temperature Superconductor

Theories based on the coupling between spin fluctuations and fermionic quasiparticles are among the leading contenders to explain the origin of high-temperature superconductivity, but estimates of the strength of this interaction differ widely. Here we analyze the charge- and spin-excitation spectra determined by angle-resolved photoemission and inelastic neutron scattering, respectively, on the same crystals of the high-temperature superconductor YBa2Cu3O6.6. We show that a self-consistent description of both spectra can be obtained by adjusting a single parameter, the spin-fermion coupling constant. In particular, we find a quantitative link between two spectral features that have been established as universal for the cuprates, namely high-energy spin excitations and "kinks" in the fermionic band dispersions along the nodal direction. The superconducting transition temperature computed with this coupling constant exceeds 150 K, demonstrating that spin fluctuations have sufficient strength to mediate high-temperature superconductivity.

cond-mat.supr-con↗
arXiv2008

Modification of Born impurity scattering near the surface of d-wave superconductors and influence of external magnetic field

We study the influence of Born impurity scattering on the zero-energy Andreev bound states near the surface of a d-wave superconductor with and without an externally applied magnetic field. Without an external magnetic field we show that the effect of Born impurity scattering is stronger at the surface than in the bulk. In the presence of an external magnetic field the splitting of the zero-energy Andreev bound states is shown to have a nonmonotonous temperature dependence. Born impurity scattering does not wash out the peak splitting, but instead the peak splitting is shown to be quite robust against impurities. We also show that a nonzero gap renormalization appears near the surface.

cond-mat.supr-con↗
arXiv2008

Tc suppression and resistivity in cuprates with out of plane defects

Recent experiments introducing controlled disorder into optimally doped cuprate superconductors by both electron irradiation and chemical substitution have found unusual behavior in the rate of suppression of the critical temperature Tc vs. increase in residual resistivity. We show here that the unexpected discovery that the rate of Tc suppression vs. resistivity is stronger for out-of-plane than for in-plane impurities may be explained by consistent calculation of both Tc and resistivity if the potential scattering is assumed to be nearly forward in nature. For realistic models of impurity potentials, we further show that significant deviations from the universal Abrikosov-Gor'kov Tc suppression behavior may be expected for out of plane impurities.

cond-mat.supr-con↗
arXiv2007

Effect of Surface Andreev Bound States on the Bean-Livingston Barrier in d-Wave Superconductors

We study the influence of surface Andreev bound states in d-wave superconductors on the Bean-Livingston surface barrier for entry of a vortex line into a strongly type-II superconductor. Starting from Eilenberger theory we derive a generalization of London theory to incorporate the anomalous surface currents arising from the Andreev bound states. This allows us to find an analytical expression for the modification of the Bean-Livingston barrier in terms of a single parameter describing the influence of the Andreev bound states. We find that the field of first vortex entry is significantly enhanced. Also, the depinning field for vortices near the surface is renormalized. Both effects are temperature dependent and depend on the orientation of the surface relative to the d-wave gap function.

cond-mat.supr-con↗
arXiv2007

Microscopic Theory of Superconductor-Constriction-Superconductor Josephson Junctions in a Magnetic Field

Self-consistent solutions of microscopic Eilenberger theory are presented for a two-dimensional model of a superconducting channel with a geometric constriction. Magnetic fields, external ones as well as those caused by the supercurrents, are included and the relevant equations are solved numerically without further assumptions. Results concerning the influence of temperature, geometric parameters, of $κ=λ_L/ξ_0$ and of external magnetic fields on the Andreev bound states in the weak link and on the current-phase relation are presented. We find that the Andreev bound states within the junction obtain peculiar substructure when a finite supercurrent flows. As long as the London penetration depth is comparable to or bigger than the extension of the constriction, the Josephson effect is independent of $κ$. Furthermore, the weak link is very insensitive to external magnetic fields. Features restricted to a self-consistent calculation are discussed.

cond-mat.supr-con↗
arXiv2006

Transfer-matrix approach to multiband Josephson junctions

We study the influence of multiple bands on the properties of Josephson junctions. In particular we focus on the two gap superconductor magnesium diboride. We present a formalism to describe tunneling at a point contact between two MgB2 electrodes generalizing the transfer-matrix approach to multiple bands. A simple model is presented to determine the effective hopping amplitudes between the different energy bands as a function of the misorientation angle of the electrodes. We calculate the critical current and the current-voltage characteristics for N-I-S and S-I-S contacts with different orientation for junctions with both high and low transparency. We find that interband tunneling processes become increasingly important with increasing misorientation angle. This is reflected in certain features in the differential tunneling conductance in both the tunneling limit as well as for multiple Andreev reflections.

cond-mat.supr-con↗
arXiv2006

Magnetic Excitations and their energy change available to Superconducting Condensation in Optimally Doped YBa$_2$Cu$_3$O$_{6.95}$

Understanding the magnetic excitations in high-transition temperature (high-$T_c$) copper oxides is important because they may mediate the electron pairing for superconductivity. By determining the wavevector ({\bf Q}) and energy ($\hbarω$) dependence of the magnetic excitations, one can calculate the change in the exchange energy available to the superconducting condensation energy. For the high-$T_c$ superconductor YBa$_2$Cu$_3$O$_{6+x}$, the most prominent feature in the magnetic excitations is the resonance. Although the resonance has been suggested to contribute a major part of the superconducting condensation, the accuracy of such an estimation has been in doubt because the resonance is only a small portion of the total magnetic scattering. Here we report an extensive mapping of magnetic excitations for YBa$_2$Cu$_3$O$_{6.95}$ ($T_c\approx 93$ K). Using the absolute intensity measurements of the full spectra, we estimate the change in the magnetic exchange energy between the normal and superconducting states and find it to be about 15 times larger than the superconducting condensation energy. Our results thus indicate that the change in the magnetic exchange energy is large enough to provide the driving force for high-$T_c$ superconductivity in YBa$_2$Cu$_3$O$_{6.95}$.

cond-mat.supr-con↗
arXiv2006

Vortex core shrinkage in a two gap superconductor: application to MgB2

As a model for the vortex core in MgB2 we study a two band model with a clean sigma band and a dirty pi band. We present calculations of the vortex core size in both bands as a function of temperature and show that there exists a Kramer-Pesch effect in both bands even though only one of the bands is in the clean limit. We present calculations for different pi band diffusivities and coherence lengths.

cond-mat.supr-con↗
arXiv2005

Nodal Quasiparticle Lifetimes in Cuprate Superconductors

A new generation of angular-resolved photoemission spectroscopy (ARPES) measurements on the cuprate superconductors offer the promise of enhanced momentum and energy resolution. In particular, the energy and temperature dependence of the on-shell nodal (k_x=k_y) quasiparticle scattering rate can be studied. In the superconducting state, low temperature transport measurements suggest that one can describe nodal quasiparticles within the framework of a BCS d-wave model by including forward elastic scattering and spin-fluctuation inelastic scattering. Here, using this model, we calculate the temperature and frequency dependence of the on-shell nodal quasiparticle scattering rate in the superconducting state which determines the momentum width of the ARPES momentum distribution curves. For a zero-energy quasiparticle at the nodal momentum k_N, both the elastic and inelastic scattering rate show a sudden decrease as the temperature drops below Tc, reflecting the onset of the gap amplitude. At low temperatures the scattering rate decreases as T^3 and approaches a zero temperature value determined by the elastic impurity scattering. For T>T_c, we find a quasilinear dependence on T. At low reduced temperatures, the elastic scattering rate for the nodal quasiparticles exhibits a quasilinear increase at low energy which arises from elastic scattering processes. The inelastic spin-fluctuation scattering leads to a low energy omega^3 dependence which, for omega>~Delta_0, crosses over to a quasilinear behavior.

cond-mat.supr-con↗
arXiv2005

Induced Kramer-Pesch-Effect in a Two Gap Superconductor: Application to MgB2

The size of the vortex core in a clean superconductor is strongly temperature dependent and shrinks with decreasing temperature, decreasing to zero for T -> 0. We study this so-called Kramer-Pesch effect both for a single gap superconductor and for the case of a two gap superconductor using parameters appropriate for Magnesium Diboride. Usually, the Kramer-Pesch effect is absent in the dirty limit. Here, we show that the Kramer-Pesch effect exists in both bands of a two gap superconductor even if only one of the two bands is in the clean limit and the other band in the dirty limit, a case appropriate for MgB2. In this case an induced Kramer-Pesch effect appears in the dirty band. Besides numerical results we also present an analytical model for the spatial variation of the pairing potential in the vicinity of the vortex center that allows a simple calculation of the vortex core radius even in the limit T -> 0.

cond-mat.supr-con↗
arXiv2005

Observation of Andreev bound states in YBaCuO/Au/Nb ramp-type Josephson junctions

We report on Josephson and quasiparticle tunneling in YBa2Cu3O7-x(YBCO)/Au/Nb ramp junctions of several geometries. Macroscopically, tunneling occurs in the ab-plane of YBCO either in the (100) and (010) direction, or in the (110) direction. These junctions have a stable and macroscopically well defined geometry. This allows systematic investigations of both quasiparticle and Josephson tunneling over a wide range of temperature and magnetic field. With Nb superconducting, its gap appears in the quasiparticle conductance spectra as Nb coherence peaks and a dip at the center of a broadened zero-bias conductance peak (ZBCP). As we increase the temperature or an applied magnetic field both the Nb coherence peaks and the dip get suppressed and the ZBCP fully develops, while states are conserved. With Nb in the normal state the ZBCP is observed up to about 77 K and is almost unaffected by an increasing field up to 7 T. The measurements are consistent with a convolution of density of states with broadened Andreev bound states formed at the YBCO/Au/Nb junction interfaces. Since junctions with different geometries are fabricated on the same substrate under the same conditions one expects to extract reliable tunneling information that is crystallographic direction sensitive. In high contrast to Josephson tunneling, however, the quasiparticle conductance spectra are crystallographic orientation insensitive: independent whether the tunneling occurs in the (100) or (110) directions, a pronounced ZBCP is always observed, consistent with microscopic roughness of the junction interfaces. Qualitatively, all these particularities regarding quasiparticle spectra hold regardless whether the YBCO thin film is twinned or untwinned.

cond-mat.supr-con↗
arXiv2005

Local density of states at polygonal boundaries of d-wave superconductors

Besides the well-known existence of Andreev bound states, the zero-energy local density of states at the boundary of a d-wave superconductor strongly depends on the boundary geometry itself. In this work, we examine the influence of both a simple wedge-shaped boundary geometry and a more complicated polygonal or faceted boundary structure on the local density of states. For a wedge-shaped boundary geometry, we find oscillations of the zero-energy density of states in the corner of the wedge, depending on the opening angle of the wedge. Furthermore, we study the influence of a single Abrikosov vortex situated near a boundary, which is of either macroscopic or microscopic roughness.

cond-mat.supr-con↗
arXiv2005

Anomaly at Delta_k in the ARPES spectrum of dirty superconductors

Elastic forward scattering can lead to an anomaly in the ARPES spectrum of the cuprate superconductors. Here we discuss how this anomaly can be used to provide a measurement of the superconducting gap Delta_k for k values away from the Fermi surface.

cond-mat.supr-con↗
arXiv2004

Observation of Andreev bound states in bicrystal grain-boundary Josephson junctions of the electron doped superconductor LaCeCuO

We observe a zero-bias conductance peak (ZBCP) in the ab-plane quasiparticle tunneling spectra of thin film grain-boundary Josephson junctions made of the electron doped cuprate superconductor LaCeCuO. An applied magnetic field reduces the spectral weight around zero energy and shifts it non-linearly to higher energies consistent with a Doppler shift of the Andreev bound states (ABS) energy. For all magnetic fields the ZBCP appears simultaneously with the onset of superconductivity. These observations strongly suggest that the ZBCP results from the formation of ABS at the junction interfaces, and, consequently, that there is a sign change in the symmetry of the superconducting order parameter of this compound consistent with a d-wave symmetry.

cond-mat.supr-con↗
arXiv2004

Superconductivity of Magnesium Diboride: Theoretical Aspects

In this work we review our present understanding of superconductivity in Magnesium Diboride from the theoretical perspective as it evolves from band structure calculations. Particular emphasis is placed on two gap superconductivity. Some of its peculiar consequences are discussed, in particular upper critical field anisotropy and microwave conductivity.

cond-mat.supr-con↗