arXiv subjects

F. Marsiglio

Publications and source records attributed to F. Marsiglio.

At least 91 records · Page 5Linked to original sources

arXiv2006

Temperature Dependence of the Conductivity Sum Rule in the Normal State due to Inelastic Scattering

We examine the temperature dependence of the optical sum rule in the normal state due to interactions. To be concrete we adopt a weak coupling approach which uses an electron-boson exchange model to describe inelastic scattering of the electrons with a boson, in the Migdal approximation. While a number of recent works attribute the temperature dependence in the normal state to that which arises in a Sommerfeld expansion, we show that in a wide parameter regime this contribution can be quite small. Instead, most of the temperature dependence arises from the zeroth order term in the `expansion', through the temperature dependence of the spectral function, and the interaction parameters contained therein. For low boson frequencies this circumstance causes a linear T-dependence in the sum rule. We develop some analytical expressions and understanding of the temperature dependence.

cond-mat.str-el↗

arXiv2006

Observation of phonon structure in electron density of states of normal metal

Within standard treatments of the interacting electron--phonon system the electron density of states (EDOS) shows no sign of the phonons when the system is in the normal state. On the other hand, emergence of a fine energy scale in the superconducting state makes the phonons readily observable in the EDOS and other spectroscopic quantities. Here we wish to re-examine the conditions under which phonon structure might be present in the normal state. Accounting for a finite, i.e. non-infinite, electronic bandwidth results in easily distinguishable phonon structure in the EDOS. We argue that for narrow band metals, the resolution of photoemission spectroscopy is sufficient for resolving these structures. Manifestations of finite band effects in $K_3C_{60}$ are discussed.

cond-mat.mtrl-sci↗

arXiv2006

Off-Fermi surface cancellation effects in spin-Hall conductivity of a two-dimensional Rashba electron gas

We calculate the spin-Hall conductivity of a disordered two-dimensional Rashba electron gas within the self-consistent Born approximation and for arbitrary values of the electron density, parametrized by the ratio $E_F /E_0$, where $E_F$ is the Fermi level and $E_0$ is the spin-orbit energy. We confirm earlier results indicating that in the limit $E_F/E_0 \gg 1$ the vertex corrections suppress the spin-Hall conductivity. However, for sufficiently low electron density such that $E_F\lesssim E_0$, we find that the vertex corrections no longer cancel contributions from the Fermi surface, and they cannot therefore suppress the spin current. This is instead achieved by contributions away from the Fermi surface, disregarded in earlier studies, which become large when $E_F\lesssim E_0$.

cond-mat.mes-hall↗

arXiv2006

Hidden Symmetries of Electronic Transport in a Disordered One-Dimensional Lattice

Correlated, or extended, impurities play an important role in the transport properties of dirty metals. Here, we examine, in the framework of a tight-binding lattice, the transmission of a single electron through an array of correlated impurities. In particular we show that particles transmit through an impurity array in identical fashion, regardless of the direction of transversal. The demonstration of this fact is straightforward in the continuum limit, but requires a detailed proof for the discrete lattice. We also briefly demonstrate and discuss the time evolution of these scattering states, to delineate regions (in time and space) where the aforementioned symmetry is violated.

cond-mat.other↗

arXiv2004

How many electrons are needed to flip a local spin?

Considering the spin of a local magnetic atom as a quantum mechanical operator, we illustrate the dynamics of a local spin interacting with a ballistic electron represented by a wave packet. This approach improves the semi-classical approximation and provides a complete quantum mechanical understanding for spin transfer phenomena. Sending spin-polarized electrons towards a local magnetic atom one after another, we estimate the minimum number of electrons needed to flip a local spin.

cond-mat.mes-hall↗

arXiv2004

Pairing Fluctuations in The Attractive Hubbard Model in the Atomic Limit

BCS theory accounts for the pairing instability in the weak coupling limit, but fails to describe pairing fluctuations above $T_c$. One possibility for describing these fluctuations in the dilute limit is the T-matrix approximation. We critically examine various degrees of self-consistency in the T-matrix formalism, along with a non-diagrammatic two-particle self-consistent (TPSC) formulation, in the strong coupling regime, where an exact solution is readily available. We find that one particular degree of self-consistency is quite accurate, particularly at low temperature as evidenced by examining both static and dynamic properties.

cond-mat.supr-con↗

arXiv2004

Quantum Mechanics of Spin Transfer in Ferromagnetic Multilayers

We use a quantum mechanical treatment of a ballistic spin current to describe novel aspects of spin transfer to a ferromagnetic multilayer. We demonstrate quantum phenomena from spin transmission resonance (STR) to magnetoelectric spin echo (MESE), depending on the coupling between the magnetic moments in the ferromagnetic thin films. Our calculation reveals new channels through which the zero spin transfer occurs in multilayers: the STR and MESE. We also illustrate that counter-intuitively, a negative spin torque can act initially on the second moment in a bilayer system.

cond-mat.mes-hall↗

arXiv2004

Optical sum increase due to electron undressing

For a system with a fixed number of electrons, the total optical sum is a constant, independent of many-body interactions, of impurity scattering and of temperature. For a single band in a metal, such a sum rule is no longer independent of the interactions or temperature, when the dispersion and/or finite bandwidth is accounted for. We adopt such a model, with electrons coupled to a single Einstein oscillator of frequency $ω_{E}$, and study the optical spectral weight. The optical sum depends on both the strength of the coupling and on the characteristic phonon frequency, $ω_{E}$. A hardening of $ω_{E}$, due, for example, to a phase transition, leads to electron undressing and translates into a decrease in the electron kinetic energy and an increase in the total optical sum, as observed in recent experiments in the cuprate superconductors.

cond-mat.supr-con↗

arXiv2004

Microwave Conductivity of a High Purity d-wave Superconductor

The cusp-like behavior of the microwave conductivity observed in clean ortho-II YB$_{2}$Cu$_{3}$O$_{6.50}$ at low temperature and low frequency is shown to be related directly to a linear in frequency dependence of the impurity scattering rate with a negligibly small value at zero frequency. In the weak scattering limit, the conductivity decreases linearly with the frequency. In the vortex state, assuming a random (Gaussian) distribution of vortices, we show that the magnetic field profoundly alters the impurity scattering rate, which now acquires a finite zero frequency value. As a consequence, we predict a Drude-like line shape in the microwave conductivity at low frequency.

cond-mat.supr-con↗

arXiv2004

Demonstration of a robust pseudogap in a three-dimensional correlated electronic system

We outline a partial-fractions decomposition method for determining the one-particle spectral function and single-particle density of states of a correlated electronic system on a finite lattice in the non self-consistent T-matrix approximation to arbitrary numerical accuracy, and demonstrate the application of these ideas to the attractive Hubbard model. We then demonstrate the effectiveness of a finite-size scaling ansatz which allows for the extraction of quantities of interest in the thermodynamic limit from this method. In this approximation, in one or two dimensions, for any finite lattice or in the thermodynamic limit, a pseudogap is present and its energy diverges as Tc is approached from above; this is an unphysical manifestation of using an approximation that predicts a spurious phase transition in one or two dimensions. However, in three dimensions one expects the transition predicted by this approximation to represent a true continuous phase transition, and in the thermodynamic limit any pseudogap predicted by this formulation will remain finite. We have applied our method to the attractive Hubbard model on a three-dimensional simple cubic lattice, and find that for intermediate coupling a prominent pseudogap is found in the single-particle density of states, and this gap persists over a large temperature range. In addition, we also show that for weak coupling a pseudogap is also present. The pseudogap energy at the transition temperature is almost a factor of three larger than the T=0 BCS gap for intermediate coupling, whereas for weak coupling the pseudogap and BCS gap energies are essentially equal.

cond-mat.str-el↗

arXiv2003

Spin Torque and its Relation to Spin Filtering

The spin torque exerted on a magnetic moment is a reaction to spin filtering when spin-polarized electrons interact with a thin ferromagnetic film. We show that, for certain conditions, a spin transmission resonance (STR) gives rise to a failure of spin filtering. As a consequence, no spin is transfered to the ferromagnet. The condition for STR depends on the incoming energy of electrons and the thickness of the film. For a simple model we find that when the STR condition is satisfied, the ferromagnetic film is transparent to the incoming electrons.

cond-mat.mes-hall↗

arXiv2003

Transport in vortex state of d-wave superconductors at zero temperature: Wiedemann-Franz violation

We show that the Wiedemann-Franz law is violated at zero temperature in the vortex state of a d-wave BCS superconductor with isotropic impurity scattering. We use a semiclassical approach to include the Doppler shift experienced by the quasiparticles due to the circulating supercurrents and consider as well the Andreev scattering from an array of vortices assumed to be randomly distributed. We also show that the vertex corrections to the electric conductivity, which can be large when there is significant anisotropy in the impurity scattering, become unimportant as the magnetic field is increased. For the thermal conductivity, the corrections remain negligible as in the absence of a magnetic field.

cond-mat.supr-con↗

arXiv2003

Dynamics of a magnetic moment induced by a spin-polarized current

Effects of an incoming spin-polarized current on a magnetic moment are explored. We found that the spin torque occurs only when the incoming spin changes as a function of time inside of the magnetic film. This implies that some modifications are necessary in a phenomenological model where the coefficient of the spin torque term is a constant, and the coefficient is determined by dynamics instead of geometrical details. The precession of the magnetization reversal depends on the incoming energy of electrons in the spin-polarized current. If the incoming energy is smaller than the interaction energy, the magnetization does not precess while reversing its direction. We also found that the relaxation time associated with the reversal depends on the incoming energy. The coupling between an incoming spin and a magnetic moment can be estimated by measuring the relaxation time.

cond-mat.mtrl-sci↗

arXiv2003

Dynamic Hubbard Model: Effect of Finite Boson Frequency

Dynamic Hubbard models describe coupling of a boson degree of freedom to the on-site electronic double occupancy. In the limit of infinite boson frequency this coupling gives rise to a correlated hopping term in the effective Hamiltonian and to superconductivity when the Fermi level is near the top of the band. Here we study the effect of finite boson frequency through a generalized Lang-Firsov transformation and a high frequency expansion. It is found that finite frequency enhances the tendency to superconductivity in this model.

cond-mat.supr-con↗

arXiv2003

Low Temperature Thermal Conductivity of High Purity YBa$_{2}$Cu$_{3}$O$_{6.99}$ in the Vortex State

The low temperature thermal conductivity of a d-wave superconductor is considered for arbitrary strength of the impurity potential. A random distribution of vortices is included within the usual semiclassical approach. The vortex-quasiparticle scattering is accounted for phenomenologically, in addition to impurity scattering. Application of the theory to recent data on high purity YBa$_{2}$Cu$_{3}$O$_{6.99}$ shows excellent agreement with experiment, both in zero field and in the presence of an applied magnetic field. We find that the strength of the impurity potential required to describe the data deviates qualitatively from either the unitary or Born limits.

cond-mat.supr-con↗

arXiv2003

Wiedemann-Franz violation in the vortex state of a d-wave superconductor

We show that the Wiedemann-Franz law is violated in the vortex state of a d-wave superconductor at zero temperature. We use a semiclassical approach, which includes the Doppler shift on the quasiparticles as well as the Andreev scattering from a random distribution of vortices. We also show that the vertex corrections to the electrical conductivity due to the anisotropy of impurity scattering become unimportant in the presence of a sufficiently large magnetic field.

cond-mat.supr-con↗

arXiv2003

S-wave superconductivity near a surface

We study the superconducting order parameter near a surface with the Bogoliubov-de Gennes formalism. For definiteness we use the attractive Hubbard model. Near a surface, the order parameter and the density distribution exhibit ``Friedel-like'' oscillations. Although the local density of states is quite different from that in the bulk, the energy gap in the spectrum on a surface is almost the same as the bulk value. In the low-density limit, however, the energy gap tends to vanish on a surface.

cond-mat.supr-con↗

arXiv2003

Inversion of ARPES measurements in high Tc cuprates

Recent energy dispersion measurements in several families of the hole-doped copper oxides have revealed a kink in the energy vs. momentum relation. These have tentatively been identified as due to electron phonon coupling. We invert this data directly to determine the bosonic spectral function; the kink gives rise to a singular function in the phonon energy region.

cond-mat.supr-con↗