arXiv subjects

M. J. Graf

Publications and source records attributed to M. J. Graf.

At least 73 records · Page 4Linked to original sources

arXiv2003

Nuclear spin-lattice relaxation rate in the D+iD superconducting state: implications for CoO superconductor

We calculated the nuclear spin-lattice relaxation rate $1/T_1$ for the D+iD superconducting state with impurities. We found that small amount of unitary impurities quickly produces the residual density of states inside the gap. As a result, the T-linear behavior in 1/T$_1$ is observed at low temperatures. Our results show that the D+iD pairing symmetry of the superconducting state of Na$_{0.35}$CoO$_{2} \cdot y$H$_2$ O is compatible with recent $^{59}$Co 1/T$_1$ experiments of several groups.

cond-mat.str-el↗

arXiv2003

$1/T_1$ in the D-wave superconducting state with coexisting antiferromagnetism

We consider a general D-wave superconducting gap function in the presence of coexisting antiferromagnetic (AFM) long-rang order. We find that the D-wave order parameter develops additional nodes without lowering the symmetry of the state due to the interplay of the superconducting and AFM correlations. This AFM correlated D-wave gap has a small gap structure inside the generic D-wave gap. As a result of this additional structure it shows a quite different response to impurities compared to a standard D-wave gap. We calculate the nuclear spin-lattice relaxation rate $1/T_1$ of this gap in the presence of impurities and discuss the implications for the experiments of some AFM heavy- fermion superconductors and in particular for CeRhIn5.

cond-mat.supr-con↗

arXiv2003

Quasiparticle relaxation dynamics in heavy fermion compounds

We present the first femtosecond studies of electron-phonon (e-ph) thermalization in heavy fermion compounds. The e-ph thermalization time tau_{ep} increases below the Kondo temperature by more than two orders of magnitude as T = 0 K is approached. Analysis using the two-temperature model and numerical simulations based on Boltzmann's equations suggest that this anomalous slowing down of the e-ph thermalization derives from the large electronic specific heat and the suppresion of scattering between heavy electrons and phonons.

cond-mat.str-el↗

arXiv2002

Lattice dynamics and correlated atomic motion from the atomic pair distribution function

The mean-square relative displacements (MSRD) of atomic pair motions in crystals are studied as a function of pair distance and temperature using the atomic pair distribution function (PDF). The effects of the lattice vibrations on the PDF peak widths are modelled using both a multi-parameter Born von-Karman (BvK) force model and a single-parameter Debye model. These results are compared to experimentally determined PDFs. We find that the near-neighbor atomic motions are strongly correlated, and that the extent of this correlation depends both on the interatomic interactions and crystal structure. These results suggest that proper account of the lattice vibrational effects on the PDF peak width is important in extracting information on static disorder in a disordered system such as an alloy. Good agreement is obtained between the BvK model calculations of PDF peak widths and the experimentally determined peak widths. The Debye model successfully explains the average, though not detailed, natures of the MSRD of atomic pair motion with just one parameter. Also the temperature dependence of the Debye model largely agrees with the BvK model predictions. Therefore, the Debye model provides a simple description of the effects of lattice vibrations on the PDF peak widths.

cond-mat.mtrl-sci↗

arXiv2000

Magnetic quantum critical point and superconductivity in UPt3 doped with Pd

Transverse-field muon spin relaxation measurements have been carried out on the heavy-fermion superconductor UPt3 doped with small amounts of Pd. We find that the critical Pd concentration for the emergence of the large-moment antiferromagnetic phase is ~0.6 at.%Pd. At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity, provides evidence for ferromagnetic spin-fluctuation mediated odd-parity superconductivity, which competes with antiferromagnetic order.

cond-mat.str-el↗

arXiv2000

Identifying the pairing symmetry in the Sr2RuO4 superconductor

We have analyzed heat capacity and thermal conductivity measurements of Sr2RuO4 in the normal and superconducting state and come to the conclusion that an order parameter with nodal lines on the Fermi surface is required to account for the observed low-temperature behavior. A gapped order parameter is inconsistent with the reported thermodynamic and transport data. Guided by a strongly peaked dynamical susceptibility along the diagonals of the Brillouin zone in neutron scattering data, we suggest a spin-fluctuation mechanism that would favor the pairing state with the gap maxima along the zone diagonals (such as for a d_{xy} gap). The most plausible candidates are an odd parity, spin-triplet, f-wave pairing state, or an even parity, spin-singlet, d-wave state. Based on our analysis of possible pairing functions we propose measurements of the ultrasound attenuation and thermal conductivity in the magnetic field to further constrain the list of possible pairing states.

cond-mat.supr-con↗

arXiv2000

Muon localization site in U(Pt,Pd)3

The angular and temperature (10-250 K) variation of the Knight shift of single-crystalline U(Pt0.95Pd0.05)3 has been measured in transverse field (B=0.6 T) mSR experiments. By analysing the temperature variation of the Knight shift with a modified Curie-Weiss expression the muon localization site in this hexagonal material is determined at (0,0,0).

cond-mat.str-el↗

arXiv2000

Interlayer Tunneling of Quasiparticles and Cooper Pairs in Bi-2212 Single Crystal Whiskers

The interlayer tunneling has been studied on high quality Bi-2212 stacks of micron to the submicron lateral size. We found that low temperature and low voltage tunneling I-V characteristics can be self-consistently described by Fermi-liquid model for a d-wave superconductor with a significant contribution from coherent interlayer tunneling. The gap and pseudogap interplay with variation of temperature and magnetic field has been extracted from the I-V characteristics. We consider also the role of charging effects for submicron stacks.

cond-mat.supr-con↗

arXiv1999

Electronic Transport in a Three-dimensional Network of 1-D Bismuth Quantum Wires

The resistance R of a high density network of 6 nm diameter Bi wires in porous Vycor glass is studied in order to observe its expected semiconductor behavior. R increases from 300 K down to 0.3 K. Below 4 K, where R varies approximately as ln(1/T), the order-of-magnitude of the resistance rise, as well as the behavior of the magnetoresistance are consistent with localization and electron-electron interaction theories of a one-dimensional disordered conductor in the presence of strong spin-orbit scattering. We show that this behaviour and the surface-enhanced carrier density may mask the proposed semimetal-to-semiconductor transition for quantum Bi wires.

cond-mat.mtrl-sci↗

arXiv1999

Search for a quantum phase transition in U(Pt_(1-x)Pd_x)_3

Pd in U(Pt_{1-x}Pd_x)_3 suppresses the superconducting T_c to 0 K at critical concentration x_c of 0.007 and induces a conventional AFM state for x > x_c. The resistivity below 1 K shows a deviation from Fermi liquid behavior described by a power law where the exponent ranges from 2 at x=0 to 1.6 for x = x_c. This suggests that a quantum phase transition (QPT) may exist near x_c associated with either the magnetic or superconducting transition temperature = 0 K. Transport for a sample with x = 0.004 < x_c has constant exponent of 1.77 as increasing pressure suppresses T_c to 0 K, suggesting that if a QPT exists it may be associated with the magnetic transition.

cond-mat.str-el↗

arXiv1999

Local Time-Reversal Symmetry Breaking in d_(x2-y2) Superconductors

We show that an isolated impurity in a spin singlet d_{x2-y2} superconductor generates a d_{xy} order parameter with locally broken time-reversal symmetry. The origin of this effect is a coupling between the d_{x2-y2} and the d_{xy} order parameter induced by spin-orbit scattering off the impurity. The signature of locally broken time-reversal symmetry is an induced orbital charge current near the impurity, which generates a localized magnetic field in the vicinity of the impurity. We present a microscopic theory for the impurity induced d_{xy} component, discuss its spatial structure as well as the pattern of induced current and local magnetic field near the localized impurity spin.

cond-mat.supr-con↗

arXiv1999

Transport in the Heavy Fermion Superconductor UPt3

We report new theoretical results and analysis for the transport properties of superconducting UPt3 based on the leading models for the pairing symmetry. We use Fermi surface data and the measured inelastic scattering rate to show that the low-temperature thermal conductivity and transverse sound attenuation in the A and B phase of UPt3 are in excellent agreement with pairing states belonging to the two-dimensional orbital E2u representation.

cond-mat.supr-con↗

arXiv1999

The effect of an in-plane magnetic field on the interlayer transport of quasiparticles in layered superconductors

We consider the quasiparticle c-axis conductivity in highly anisotropic layered compounds in the presence of the magnetic field parallel to the layers. We show that at low temperatures the quasiparticle interlayer conductivity depends strongly on the orientation of the in-plane magnetic field if the excitation gap has nodes on the Fermi surface. Thus measurements of the angle-dependent c-axis (out-of-plane) magnetoresistance, as a function of the orientation of the magnetic field in the layers, provide information on the momentum dependence of the superconducting gap (or pseudogap) on the Fermi surface. Clean and highly anisotropic layered superconductors seem to be the best candidates for probing the existence and location of the nodes on the Fermi surface.

cond-mat.supr-con↗

arXiv1999

Superconductivity in heavy-fermion U(Pt,Pd)3 and its interplay with magnetism

The effect of Pd doping on the superconducting phase diagram of the unconventional superconductor UPt3 has been measured by (magneto)resistance, specific heat, thermal expansion and magnetostriction. Experiments on single- and polycrystalline U(Pt1-xPdx)3 for x<= 0.006 show that the superconducting transition temperatures of the A phase, Tc+, and of the B phase, Tc-, both decrease, while the splitting DTc increases at a rate of 0.30(2)K/at.%Pd. We find that DTc(x) correlates with an increase of the weak magnetic moment m(x) upon Pd doping. This provides further evidence for Ginzburg-Landau scenarios with magnetism as the symmetry breaking field, i.e. the 2D E representation and the 1D odd parity model. Only for small splittings DTc is proportional to m^2(Tc+) (DTc<= 0.05 K) as predicted. The results at larger splittings call for Ginzburg-Landau expansions beyond 4th order. The tetracritical point in the B-T plane persists till at least x= 0.002 for B perpendicular to c, while it is rapidly suppressed for B||c. Upon alloying the A and B phases gain stability at the expense of the C phase.

cond-mat.supr-con↗

arXiv1999

Interlayer Transport of Quasiparticles and Cooper pairs in Bi2Sr2CaCu2O8+y Superconductors

We study the c-axis transport of stacked, intrinsic junctions in Bi2Sr2CaCu2O8+y single crystals, fabricated by the double-sided ion beam processing technique from single crystal whiskers. Measurements of the I-V characteristics of these samples allow us to obtain the temperature and voltage dependence of the quasiparticle c-axis conductivity in the superconducting state, the Josephson critical current, and the superconducting gap. We show that the BCS d-wave model in the clean limit for resonant impurity scattering with a significant contribution from coherent interlayer tunneling, describes satisfactorily the low temperature and low energy c-axis transport of both quasiparticles and Cooper pairs.

cond-mat.supr-con↗

arXiv1999

Studies of superconductivity in U(Pt1-xPdx)3 for x < 0.006

We report measurements of the specific heat and resistivity (T < 1 K) for high quality polycrystals of U(Pt1-xPdx)3 with x < 0.006. The Tc-x phase diagram can be constructed, and superconductivity is destroyed for x = 0.006; this is approximately the same concentration above which the onset of large-moment antiferromagnetism is observed to occur. The splitting of the double superconducting transition increases smoothly with increasing Pd content, and is large enough that for Pd concentrations 0.004 < x < 0.006 only the superconducting A-phase will be present.

cond-mat.str-el↗

arXiv1999

Suppression of Superconductivity in Single Crystals of UPt_3 by Pd Substitution

The suppression of superconductivity by substitution effects has been measured in high quality single crystals of U(Pt_{1-x} Pd_x)_3 with 0 <= x <= 0.002. While the superconducting transition temperature T_c varies linearly with residual resistivity r_0, consistent with pair-breaking by impurity potential scattering, the rate of suppression of T_c with r_0 is much larger for Pd substitution than for other impurity substitutions or by increased defect density. This effect is correlated with an increase in the inelastic scattering coefficient, and may be related to Pd-induced changes in the magnetic fluctuation spectrum.

cond-mat.supr-con↗

arXiv1998

Thermal Conductivity of the Accidental Degeneracy and Enlarged Symmetry Group Models for Superconducting UPt 3

We present theoretical calculations of the thermal conductivity for the accidental degeneracy and enlarged symmetry group models that have been proposed to explain the phase diagram of UPt3. The order parameters for these models possess point nodes or cross nodes, reflecting the broken symmetries of the ground state. These broken symmetries lead to robust predictions for the ratio of the low-temperature thermal conductivity for heat flow along the c axis and in the basal plane. The anisotropy of the heat current response at low temperatures is determined by the phase space for scattering by impurities. The measured anisotropy ratio, $κ_c/κ_b$, provides a strong constraint of theoretical models for the ground state order parameter. The accidental degeneracy and enlarged symmetry group models based on no spin-orbit coupling do not account for the thermal conductivity of UPt3. The models for the order parameter that fit the experimental data for the c and b directions of the heat current are the 2D E1g and E2u models, for which the order parameters possess line nodes in the ab-plane and point nodes along the c axis, and the A1g+E1g model of Zhitomirsky and Ueda. This model spontaneously breaks rotational symmetry in theab-plane below Tc2 and predicts a large anisotropy for the $ab$-plane heat current.

cond-mat.supr-con↗