arXiv subjects

R. J. Radtke

Publications and source records attributed to R. J. Radtke.

15 recordsLinked to original sources

arXiv2000

Phonon Dispersion Effects and the Thermal Conductivity Reduction in GaAs/AlAs Superlattices

The experimentally observed order-of-magnitude reduction in the thermal conductivity along the growth axis of (GaAs)_n/(AlAs)_n (or n x n) superlattices is investigated theoretically for (2x2), (3x3) and (6x6) structures using an accurate model of the lattice dynamics. The modification of the phonon dispersion relation due to the superlattice geometry leads to flattening of the phonon branches and hence to lower phonon velocities. This effect is shown to account for a factor-of-three reduction in the thermal conductivity with respect to bulk GaAs along the growth direction; the remainder is attributable to a reduction in the phonon lifetime. The dispersion-related reduction is relatively insensitive to temperature (100 < T < 300K) and n. The phonon lifetime reduction is largest for the (2x2) structures and consistent with greater interface scattering. The thermal conductivity reduction is shown to be appreciably more sensitive to GaAs/AlAs force constant differences than to those associated with molecular masses.

cond-mat.mtrl-sci↗

arXiv1998

Thermoelectric Properties of Anisotropic Systems

The effective transport coefficients and figure of merit ZT for anisotropic systems are derived from a macroscopic formalism. The full tensorial structure of the transport coefficients and the effect of the sample boundaries are included. Induced transverse fields develop which can be larger than the applied fields and which reduce the effective transport coefficients. A microscopic model relevant for multi-valleyed materials is introduced which utilizes the effective-mass and relaxation-time approximations. The thermopower and Lorentz number are independent of the tensorial structure of the transport coefficients in this case and are therefore isotropic. ZT is also isotropic for vanishing lattice thermal conductivity κ_\ell. For non-vanishing but sufficiently isotropic κ_\ell, ZT is maximal along the direction of highest electrical conductivity σ. Numerical calculations suggest that maximal ZT generally occurs along the principal direction with the largest σ/κ_\ell. An explicit bound on ZT is derived. Several results for specific systems are obtained: (1) Bulk n-type Bi_2Te_3 exhibits easily observable induced transverse fields and anisotropic ZTs. (2) Increased anisotropy in HgTe/ Hg_{1-x}Cd_xTe superlattices (SLs) is associated with larger induced fields. (3) The valley degeneracy is split and the bulk masses modified in isolated Bi_2Te_3 quantum wells, resulting in optimal ZTs for wells grown along the trigonal direction. (4) Non-parabolic dispersion in SLs has little effect on the thermopower at the carrier concentrations which maximize ZT.

cond-mat.mtrl-sci↗

arXiv1997

Mode Mixing in Antiferromagnetically Correlated Double Quantum Wells

We examine the robustness of a recently predicted exchange-induced zero-field magnetic phase in semiconductor double quantum wells in which each well is spin-polarized and the polarization vectors are antiparallel. Magnetic instabilities are a general feature of Coulombic double quantum well systems at low densities. We argue that this antiferromagnetic phase is stabilized relative to ferromagnetic ones by an effective superexchange interaction between the wells. Detailed self-consistent Hartree-Fock calculations using a point-contact model for the interaction show that the antiferromagnetic phase survives intra-subband repulsion matrix elements neglected in earlier work in a large portion of the model's parameter space. We also examine the role of asymmetry due to biasing or to differences in the widths of the two quantum wells. The asymmetry creates a mode coupling between the intra- and inter-subband collective spin-density excitations (SDEs) which changes the Raman spectroscopy signature of the phase transition from a complete softening of the inter-subband SDE to a cusp as the density is tuned through the transition. This cusp may be detectable in inelastic light scattering experiments in samples of sufficient quality at low enough temperatures and densities.

cond-mat.mes-hall↗

arXiv1997

Spin-Excitation-Instability-Induced Quantum Phase Transitions in Double-Layer Quantum Hall Systems

We study intersubband spin density collective modes in double-layer quantum Hall systems at $ν=2$ within the time-dependent Hartree-Fock approximation. We find that these intersubband spin density excitations may soften under experimentally accessible conditions, signaling a phase transition to a new quantum Hall state with interlayer inplane antiferromagnetic spin correlations. We show that this novel canted antiferromagnetic phase is energetically stable and that the phase transition is continuous.

cond-mat.mes-hall↗

arXiv1996

Spin Instabilities in Coupled Semiconductor Quantum Wells

We study the magnetic phases of two coupled two-dimensional electron gases in order to determine under what circumstances these phases may occur in real semiconductor quantum wells and what the experimental properties of the broken-symmetry ground states may be. Within the local-density-approximation to time-dependent density functional theory (DFT), we find a phase transition signaled by the vanishing of the intersubband spin-density excitations at low but accessible (\sim 10^10-10^11 cm^{-2}) electron densities. Through a self-consistent Hartree-Fock calculation, we associate this transition with an antiferromagnetic phase and study the phase diagram, thermodynamics, and collective modes in it. The collective modes are in principle observable in inelastic light scattering experiments, and we discuss the implications of our calculations for these measurements. We also examine the ferromagnetic transition in both single and double quantum wells within the local-spin-density approximation to DFT and obtain a critical density which depends on the well width and which is far below that of the antiferromagnetic transition.

cond-mat↗

arXiv1996

Antiferromagnetic Interactions and the Superconducting Gap Function

Spin-fluctuation-mediated superconductivity is conventionally associated with d_{x^2-y^2} pairing. We show that a generalized model of antiferromagnetic spin fluctuations in three dimensions may also yield a state with formal ``s-wave'' (A_{1g}) symmetry but with line nodes at k_z \approx \pm π/ 2c. We study this new state within both BCS and Eliashberg theories using a realistic band structure and find that it is more stable than the d_{x^2-y^2} (B_{1g}) state over a wide range of parameters. Thus, models of spin-fluctuation-mediated superconductivity must consider both possibilities on an equal footing.

cond-mat↗

arXiv1995

Collective Modes in a Symmetry-Broken Phase: Antiferromagnetically Correlated Quantum Wells

We investigate the intersubband spin-density-excitation spectrum of a double quantum well in a low-density symmetry-broken phase with interwell antiferromagnetic correlations. This spectrum is related to the intensity measured in depolarized inelastic light scattering (ILS) experiments and therefore provides a means of empirically identifying the antiferromagnetic phase. Our computations reveal the existence of two collective modes, a damped Nambu-Goldstone (NG) mode arising from the broken spin symmetry and an undamped optical mode. Since the NG mode contains most of the spectral weight, ILS experiments will need to examine the low-frequency response for signatures of the antiferromagnetic phase.

cond-mat↗

arXiv1995

Theory of the c-Axis Penetration Depth in the Cuprates

Recent measurements of the London penetration depth tensor in the cuprates find a weak temperature dependence along the c-direction which is seemingly inconsistent with evidence for d-wave pairing deduced from in-plane measurements. We demonstrate in this paper that these disparate results are not in contradiction, but can be explained within a theory based on incoherent quasiparticle hopping between the CuO2 layers. By relating the calculated temperature dependence of the penetration depth λ_c(T) to the c-axis resistivity, we show how the measured ratio λ_c^2(0) / λ_c^2(T) can provide insight into the behavior of c-axis transport below Tc and the related issue of ``confinement.''

cond-mat↗

arXiv1995

c-Axis Penetration Depth in the Cuprates: Additional Evidence for Incoherent Hopping

Measurements of the $c$-axis penetration depth $λ_c$ in the cuprates reveal a low-temperature $T$ dependence which is inconsistent with simple models of coupling between the CuO$_2$ layers. In this paper, we examine whether a model based on {\it incoherent} hopping between the layers can account for this low-$T$ behavior. We compute $λ_c$ directly from linear response theory and compare our results with recent experimental measurements on $\rm YBa_2u_3O_{7-δ}$ as a function of temperature and doping. We find that the data can be reproduced within this model providing the inter-layer scattering is anisotropic and the pairing is $d$-wave. In addition, our calculations demonstrate that $1/λ_c^2$ is proportional to the $c$-axis critical current, which seems to be a generic feature in weakly coupled layered superconductors.

cond-mat↗

arXiv1995

Origin of Intrinsic Josephson Coupling in the Cuprates and Its Relation to Order Parameter Symmetry: An Incoherent Hopping Model

Experiments on the cuprate superconductors demonstrate that these materials may be viewed as a stack of Josephson junctions along the c-direction. In this paper, we present a model which describes this intrinsic Josephson coupling in terms of incoherent quasiparticle hopping along the c-axis arising from wave-function overlap, impurity-assisted hopping, and boson-assisted hopping. We use this model to compute the magnitude and temperature T dependence of the resulting Josephson critical current j_c (T) for s- and d-wave superconductors. Contrary to other approaches, d-wave pairing in this model is compatible with an intrinsic Josephson effect at all hole concentrations and leads to j_c (T) \propto T at low T. By parameterizing our theory with c-axis resistivity data from YBCO, we estimate j_c (T) for optimally doped and underdoped members of this family. Our estimates suggest that further experiments on this compound would be of great help in elucidating the validity of our model in general and the pairing symmetry in particular. We also discuss the implications of our model for LSCO and BSCCO.

cond-mat↗

arXiv1995

Mean-Field Theory for the Spin-Triplet Exciton Liquid in Quantum Wells

Using a mean-field theory, we study the possible existence of a spin-triplet intersubband exciton liquid ground state in semiconductor quantum well systems as a function of the electronic density and the strength of the intersubband Coulomb interaction matrix element at low temperatures. We find the excitonic phase to be stable over a large region of parameter space, and our calculated critical temperatures are attainable experimentally. In addition, we find that the transition to the excitonic phase can be either first- or second-order at zero temperature.

cond-mat↗

arXiv1994

Relation of Extended Van Hove Singularities to High-Temperature Superconductivity within Strong-Coupling Theory

Recent angle-resolved photoemission (ARPES) experiments have indicated that the electronic dispersion in some of the cuprates possesses an extended saddle point near the Fermi level which gives rise to a density of states that diverges like a power law instead of the weaker logarithmic divergence usually considered. We investigate whether this strong singularity can give rise to high transition temperatures by computing the critical temperature $T_c$ and isotope effect coefficient $α$ within a strong- coupling Eliashberg theory which accounts for the full energy variation of the density of states. Using band structures extracted from ARPES measurements, we demonstrate that, while the weak-coupling solutions suggest a strong influence of the strength of the Van Hove singularity on $T_c$ and $α$, strong-coupling solutions show less sensitivity to the singularity strength and do not support the hypothesis that band structure effects alone can account for either the large $T_c$'s or the different $T_c$'s within the copper oxide family. This conclusion is supported when our results are plotted as a function of the physically relevant self- consistent coupling constant, which show universal behavior at very strong coupling.

cond-mat↗

arXiv1993

Comment on "Theory of Impure Superconductors: Anderson versus Abrikosov and Gor'kov"

In a recent article, Kim and Overhauser [Phys. Rev. B 47, 8025 (1993)] have found fault with the theory of impure superconductors by Abrikosov and Gor'kov and have proposed an alternative formalism based on Green's functions which are not derivable from a Dyson equation. Although the corrections to the Abrikosov-Gor'kov theory found by Kim and Overhauser are correct for non-retarded interactions, I argue that these corrections do not appear when one treats realistic retarded interactions within the field-theoretic approach of Abrikosov and Gor'kov. Direct numerical computation of the impurity-induced suppression of the superconducting transition temperature Tc for both retarded and non-retarded interactions illustrates these points. I conclude that Abrikosov-Gor'kov theory applied to physical electron-electron interactions yields a tractable formalism which accurately predicts the effects of magnetic and non-magnetic impurities on Tc.

cond-mat↗

arXiv1993

Role of Van Hove Singularities and Momentum Space Structure in High-Temperature Superconductivity

There is a great deal of interest in attributing the high critical temperatures of the cuprates to either the proximity of the Fermi level to a van Hove singularity or to structure of the superconducting pairing potential in momentum space far from the Fermi surface. We examine these ideas by calculating the critical temperature Tc for model Einstein-phonon- and spin-fluctuation-mediated superconductors within both the standard, Fermi-surface-restricted Eliashberg theory and the exact mean field theory, which accounts for the full momentum structure of the pairing potential and the energy dependence of the density of states. By using two models of spin-fluctuation-mediated pairing in the cuprates, we demonstrate that our results are independent of the details of the dynamical susceptibility, which is taken to be the pairing potential. We also compare these two models against available neutron scattering data, since these data provide the most direct constraints on the susceptibility. We conclude that the van Hove singularity does not drastically alter Tc from its value when the density of states is constant and that the effect of momentum structure is significant but secondary in importance to that of the energy dependence in the density of states.

cond-mat↗

arXiv1993

Predictions for Impurity-Induced Tc Suppression in the High-Temperature Superconductors

We address the question of whether anisotropic superconductivity is compatible with the evidently weak sensitivity of the critical temperature Tc to sample quality in the high-Tc copper oxides. We examine this issue quantitatively by solving the strong-coupling Eliashberg equations numerically as well as analytically for s-wave impurity scattering within the second Born approximation. For pairing interactions with a characteristically low energy scale, we find an approximately universal dependence of the d-wave superconducting transition temperature on the planar residual resistivity which is independent of the details of the microscopic pairing. These results, in conjunction with future systematic experiments, should help elucidate the symmetry of the order parameter in the cuprates.

cond-mat↗