arXiv subjects

A. J. Millis

Publications and source records attributed to A. J. Millis.

At least 127 records · Page 7Linked to original sources

arXiv1999

Gap to Transition Temperature Ratio in Density Wave Ordering: a Dynamical Mean Field Study

We use the dynamical mean-field method to determine the origin of the large ratio of the zero temperature gap to the transition temperature observed in most charge density wave materials. The method is useful because it allows an exact treatment of thermal fluctuations. We establish the relation of the dynamical mean-field results to conventional diagrammatics and thereby determine that in the physically relevant regime the origin of the large ratio is a strong inelastic scattering.

cond-mat.str-el↗

arXiv1999

Two-phase behavior in strained thin films of hole-doped manganites

We present a study of the effect of biaxial strain on the electrical and magnetic properties of thin films of manganites. We observe that manganite films grown under biaxial compressive strain exhibit island growth morphology which leads to a non-uniform distribution of the strain. Transport and magnetic properties of these films suggest the coexistence of two different phases, a metallic ferromagnet and an insulating antiferromagnet. We suggest that the high strain regions are insulating while the low strain regions are metallic. In such non-uniformly strained samples, we observe a large magnetoresistance and a field-induced insulator to metal transition.

cond-mat.str-el↗

arXiv1999

Quantum fluctuations and the c-axis optical conductivity of High-T_c Superconductors

A theory of the frequency dependence of the interplane conductivity of a strongly anisotropic superconductor is presented. The form of the conductivity is shown to be a sensitive probe of the strength of quantum and thermal fluctuations of the phase of the superconducting order parameter. The temperature dependence of the superfluid stiffness and of the form of the absorbtion at frequencies of the order of twice the superconducting gap is shown to depend on the interplay between superconducting pairing, phase coherence and the mechanism by which electrons are scattered. Measurements of the c-axis conductivity of high-T_c superconductors are interpreted in terms of the theory.

cond-mat.str-el↗

arXiv1999

Optical spectral weights and the ferromagnetic transition temperature of CMR manganites: relevance of double-exchange to real materials

We present a thorough and quantitative comparison of double-exchange models to experimental data on the colossal magnetoresistance manganese perovskites. Our results settle a controversy by showing that physics beyond double-exchange is important even in La$_{0.7}$Sr$_{0.3}$MnO$_3$, which has been regarded as a conventional double-exchange system. We show that the crucial quantity for comparisons of different calculations to each other and to data is the conduction band kinetic energy $K$, which is insensitive to the details of the band structure and can be experimentally determined from optical conductivity measurements. The seemingly complicated dependence of $T_c$ on the Hund's coupling $J$ and carrier concentration $n$ is shown to reflect the variation of $K$ with $J$, $n$ and temperature. We present results for the optical conductivity which allow interpretation of experiments and show that a feature previously interpreted in terms of the Hund's coupling was misidentified. We also correct minor errors in the phase diagram presented in previous work.

cond-mat.str-el↗

arXiv1999

On Pseudogaps in One-Dimensional Models with Quasi-Long-Ranged-Order

We use analytic and numerical methods to determine the density of states of a one-dimensional electron gas coupled to a spatially random quasi-static back-scattering potential of long correlation length. Our results provide insight into the 'pseudogap' phenomenon occurring in underdoped high-Tc superconductors, quasi-one-dimensional organic conductors and liquid metals. They demonstrate the important role played by amplitude fluctuations of the backscattering potential and by fluctuations in gradients of the potential, and confirm the importance of the self-consistency which is a key feature of the 'FLEX'-type approximations for the electron Green's function. Our results allow an assessment of the merits of different approximations: a previous approximate treatment presented by Sadovskii and, we show, justified by a WKB approximation gives a reasonably good representation, except for a ``central peak'' anomaly, of our numerically computed densities of states, whereas a previous approximation introduced by Lee, Rice and Anderson is not as accurate.

cond-mat.str-el↗

arXiv1999

Berry Phase Theory of Anomalous Hall Effect: Application to Colossal Magnetoresistance Manganites

We show that the Anomalous Hall Effect (AHE) observed in Colossal Magnetoresistance Manganites is a manifestation of Berry phase effects caused by carrier hopping in a non-trivial spin background. We determine the magnitude and temperature dependence of the Berry phase contribution to the AHE, finding that it increases rapidly in magnitude as the temperature is raised from zero through the magnetic transition temperature T_c, peaks at a temperature $T_{max} > T_c$ and decays as a power of T, in agreement with experimental data. We suggest that our theory may be relevant to the anomalous hall effect in conventional ferromagnets as well.

cond-mat.str-el↗

arXiv1999

Anisotropy and temperature dependence of the optical conductivity in LaMnO_3

A tight binding parameterization of the band structure, along with a mean field treatment of Hund, electron-electron, and electron-lattice couplings, is used to obtain the full optical conductivity tensor of LaMnO_3 as a function of temperature. We predict striking changes with temperature in the functional form and magnitude of the optical absorption. Comparison of our results to data will determine the Hund, electron-lattice, and electron-electron interactions.

cond-mat.str-el↗

arXiv1998

Resistivity saturation revisited: results from a dynamical mean field theory

We use the dynamical mean field method to study the high-temperature resistivity of electrons strongly coupled to phonons. The results reproduce the qualtiative behavior of the temperature and disorder dependence of the resistivity of the 'A-15' materials, which is commonly described in terms of saturation, but imply that the resistivity does not saturate. Rather, a change in temperature dependence occurs when the scattering becomes strong enough to cause a breakdown of the Migdal approximation.

cond-mat.str-el↗

arXiv1998

Phase Fluctuations and Spectral Properties of Underdoped Cuprates

We consider the effect of classical phase fluctuations on the quasiparticle spectra of underdoped high-$T_c$ cuprate superconductors in the pseudogap regime above $T_c$. We show that photoemission and tunneling spectroscopy data are well accounted for by a simple model in which mean field d-wave quasiparticles are semiclasically coupled to supercurrents induced by fluctuating unbound vortex-antivortex pairs. We argue that the data imply that transverse phase fluctuations are important at temperatures above $T_c$, while longitudinal fluctuations are unimportant at all temperatures.

cond-mat.supr-con↗

arXiv1998

Interplay of charge and orbital ordering in manganese perovskites

A model of localized classical electrons coupled to lattice degrees of freedom and, via the Coulomb interaction, to each other, has been studied to gain insight into the charge and orbital ordering observed in lightly doped manganese perovskites. Expressions are obtained for the minimum energy and ionic displacements caused by given hole and electron orbital configurations. The expressions are analyzed for several hole configurations, including that experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find that, although the preferred charge and orbital ordering depend sensitively on parameters, there are ranges of the parameters in which the experimentally observed hole configuration has the lowest energy. For these parameter values we also find that the energy differences between different hole configurations are on the order of the observed charge ordering transition temperature. The effects of additional strains are also studied. Some results for La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately describe this material because the high temperature phase is metallic.

cond-mat.str-el↗

arXiv1998

Low temperature transport in the XXZ model

We present evidence suggesting that spin transport in the gapless phase of the S=1/2 XXZ model is ballistic rather than diffusive. We map the model onto a spinless fermion model whose charge stiffness determines the spin transport of the original model. By means of exact numerical diagonalisation and finite size scaling we study both the stiffness and the level statistics. We show that the stiffness is non-zero at low temperatures so that the transport is ballistic. Our results suggest that the non-zero stiffness is due to the fact that even in the presence of Umklapp scattering a non-zero fraction of states remain degenerate in the thermodynamic limit.

cond-mat↗

arXiv1998

Anomalous Hall Effect in Double Exchange Magnets

We investigate the possible origin of anomalous Hall effect in the CMR (colossal magnetoresistance) materials - the doped rare earth manganites - observed recently by Matl et al. It is demonstrated that the spin-orbit interaction in the double exchange model couples magnetization to the Berry phase associated with three dimensional spin textures and induces a non-zero average topological flux which in turn generates an anomalous contribution to transverse resistivity. The same effect, but involving the orbital Berry phase, occurs in the model with orbital degeneracy and Coulomb repulsion.

cond-mat.str-el↗

arXiv1998

Optical Conductivity of Manganites: Crossover from Jahn-Teller Small Polaron to Coherent Transport in the Ferromagnetic State

We report on the optical properties of the hole-doped manganites Nd_{0.7}Sr _{0.3}MnO_{3}, La_{0.7}Ca_{0.3}MnO_{3}, and La_{0.7}Sr_{0.3}MnO_{3}. The low-energy optical conductivity in the paramagnetic-insulating state of these materials is characterized by a broad maximum near 1 eV. This feature shifts to lower energy and grows in optical oscillator strength as the temperature is lowered into the ferromagnetic state. It remains identifiable well below Tc and transforms eventually into a Drude-like response. This optical behavior and the activated transport in the paramagnetic state of these materials are consistent with a Jahn-Teller small polaron. The optical spectra and oscillator strength changes compare well with models that include both double exchange and the dynamic Jahn-Teller effect in the description of the electronic structure.

cond-mat.str-el↗

arXiv1998

``Cold spots'': a new model for transport in high $T_c$ cuprates

We present a Boltzmann equation analysis of the transport properties of a model of electrons with a lifetime which is short everywhere except near the Brillouin zone diagonals. The anomalous lifetime is directly implied by photoemission and c-axis transport data. We find quantitative agreement between calculations and ac and dc longitudinal and Hall resistivity, but the predicted longitudinal magnetoresistance disagrees with experiment. A possible microscopic origin of the anomalous lifetime is discussed

cond-mat.supr-con↗

arXiv1998

Theory of the Resistive Transition in Overdoped $Tl_2Ba_2CuO_{6+x}$: Implications for the angular dependence of the quasiparticle scattering rate in High-$T_c$ superconductors

We show that recent measurements of the magnetic field dependence of the magnetization, specific heat and resistivity of overdoped $T_c \sim 17K$ $Tl_{2}Ba_{2}CuO_{6+δ}$ in the vicinity of the superconducting $H_{c2}$ imply that the vortex viscosity is anomalously small and that the material studied is inhomogeneous with small, a few hundred $Å$, regions in which the local $T_{c}$ is much higher than the bulk $T_{c}$. The anomalously small vortex viscosity can be derived from a microscopic model in which the quasiparticle lifetime varies dramatically around the Fermi surface, being small everywhere except along the zone diagonal (``cold spot''). We propose experimental tests of our results.

cond-mat.supr-con↗

arXiv1997

Anomalous Charge Dynamics in the Superconducting State of Underdoped Cuprates

We present fermi liquid expressions for the low temperature behavior of the superfluid stiffness, explain why they differ from those suggested recently by Lee and Wen, and discuss their applicability to data on high-$T_c$ superconductors. We find that a consistent description requires a strong, doping dependent anisotropy, which affects states near the zone corners much more strongly than those near the zone diagonals.

cond-mat.str-el↗

arXiv1997

Possible explanation for the absence of bilayer splitting in YBCO

It has been claimed that the absence of the bilayer splitting in the high-temperature superconductor YBCO is a strong experimental indication that there are no coherent quasiparticles present in the CuO planes. We study a pair of strongly correlated planes which are connected by a hopping transfer integral $t_{\perp}$ in the limit of large in-plane coordination number. The effect of the correlation is incorporated in a dynamical mean-field theory, where the Weiss field is determined by a two-site Hubbard Hamiltonian. We have solved this problem by numerical techniques and present results for the spectral function $ρ$ for relevant parameters of the model. For small $t_{\perp}$ we find the coherent hoping between planes to be proportional to the in-plane quasiparticle renormalisation.

cond-mat.str-el↗

arXiv1996

Gauge drag between half-filled Landau levels

The transresistance (i.e. the voltage induced in one layer by a current in another) between composite fermions in double-layers of half-filled Landau levels is shown to be dominated by scattering due to singular gauge field fluctuations arising from the antisymmetric combination of density fluctuations in two layers. The drag rate is found to be proportional to $T^{4/3}$ for $T$ less than an energy scale $Ω_{cr}$ which depends on the spacing, $d$, between the layers. An observation of a $T^{4/3}$ temperature dependence in the transresistance would be a strong evidence for the existence of the gauge field fluctuations.

cond-mat.str-el↗