arXiv subjects

Matthias J. Graf

Publications and source records attributed to Matthias J. Graf.

43 records · Page 3Linked to original sources

arXiv2005

Knight Shift in the FFLO State of a Two-Dimensional D-Wave Superconductor

We report on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in two-dimensional d-wave superconductors with magnetic field parallel to the superconducting planes. This state occurs at high magnetic field near the Pauli-Clogston limit and is a consequence of the competition between the pair condensation and Zeeman energy. We use the quasiclassical theory to self-consistently compute the spatially nonuniform order parameter. Our self-consistent calculations show that the FFLO state of a d-wave order parameter breaks translational symmetry along preferred directions. The orientation of the nodes in real space is pinned by the nodes of the basis function in momentum space. Here, we present results for the Knight shift and discuss the implications for recent nuclear magnetic resonance measurements on CeCoIn5.

cond-mat.supr-con↗

arXiv2003

High-pressure Debye-Waller and Grueneisen parameters of Au and Cu

The lattice vibrations are determined in the quasi-harmonic approximation for elemental Au and Cu to twice their normal density by first-principles electronic band-structure calculations. It is found for these materials that the important moments of the phonon density of states can be obtained to high accuracy from short-ranged force constant models. We discuss the implications for the Grueneisen parameters on the basis of calculated phonon moments and their approximations by using bulk moduli and Debye-Waller factors.

cond-mat.mtrl-sci↗

arXiv2002

Limits on phonon information extracted from neutron pair-density functions

We explore the possibility of extracting information about lattice dynamics in simple crystal structures from the neutron pair-density function (PDF) through inverse data analysis. Contrary to the claims by Dimitrov, Louca, and Roeder [Phys.Rev. B 60, 6204 (1999)], and in agreement with recent work by Reichardt and Pintschovius [Phys.Rev. B 63, 174302 (2001)], we find that the PDF alone is not sufficient for constructing accurate phonon dispersions in the entire Brillouin zone in systems with complex lattice dynamics. However, our numerical simulations show that for monatomic fcc and bcc crystal structures it is, in principle, possible to obtain phonon moments of complex metals as well as phonon frequencies of simple metals within a few percent accuracy.

cond-mat.mtrl-sci↗

arXiv2001

The Resonance Peak in Sr$_2$RuO$_4$: Signature of Spin Triplet Pairing

We study the dynamical spin susceptibility, $χ({\bf q}, ω)$, in the normal and superconducting state of Sr$_2$RuO$_4$. In the normal state, we find a peak in the vicinity of ${\bf Q}_i\simeq (0.72π,0.72π)$ in agreement with recent inelastic neutron scattering (INS) experiments. We predict that for spin triplet pairing in the superconducting state a {\it resonance peak} appears in the out-of-plane component of $χ$, but is absent in the in-plane component. In contrast, no resonance peak is expected for spin singlet pairing.

cond-mat.supr-con↗

arXiv2000

Antiferromagnetic Domains and Superconductivity in UPt3

We explore the response of an unconventional superconductor to spatially inhomogeneous antiferromagnetism (SIAFM). Symmetry allows the superconducting order parameter in the E-representation models for UPt3 to couple directly to the AFM order parameter. The Ginzburg-Landau equations for coupled superconductivity and SIAFM are solved numerically for two possible SIAFM configurations: (I) abutting antiferromagnetic domains of uniform size, and (II) quenched random disorder of `nanodomains' in a uniform AFM background. We discuss the contributions to the free energy, specific heat, and order parameter for these models. Neither model provides a satisfactory account of experiment, but results from the two models differ significantly. Our results demonstrate that the response of an E_{2u} superconductor to SIAFM is strongly dependent on the spatial dependence of AFM order; no conclusion can be drawn regarding the compatibility of E_{2u} superconductivity with UPt3 that is independent of assumptions on the spatial dependence of AFM

cond-mat.supr-con↗

arXiv2000

Identification of the Orbital Pairing Symmetry in UPt_3

This paper summarizes the results of a comprehensive analysis of the thermodynamic and transport data for the superconducting phases of UPt_3. Calculations of the transverse sound attenuation as a function of temperature, frequency, polarization, and disorder are presented for the leading models of the superconducting order parameter. Measurements of the specific heat, thermal conductivity, and transverse sound attenuation place strong constraints on the orbital symmetry of the superconducting order parameter. We show that the superconducting A and B phases are in excellent agreement with pairing states belonging to the odd-parity E_{2u} orbital representation.

cond-mat.supr-con↗

arXiv1998

Electronic Transport in Unconventional Superconductors

We investigate the electronic transport coefficients in unconventional superconductors at low temperatures, where charge and heat transport are dominated by electron scattering from random lattice defects. We discuss the features of the pairing symmetry, Fermi surface, and excitation spectrum which are reflected in the low temperature heat transport. For temperatures $k_B T \la γ\ll Δ_0$, where $γ$ is the bandwidth of impurity induced Andreev states, certain eigenvalues become {\it universal}, i.e., independent of the impurity concentration and phase shift. Deep in the superconducting phase ($k_B T \la γ$) the Wiedemann-Franz law, with Sommerfeld's value of the Lorenz number, is recovered. We compare our results for theoretical models of unconventional superconductivity in high-T$_c$ and heavy fermion superconductors with experiment. Our findings show that impurities are a sensitive probe of the low-energy excitation spectrum, and that the zero-temperature limit of the transport coefficients provides an important test of the order parameter symmetry.

cond-mat.supr-con↗