arXiv subjects

C. Morais Smith

Publications and source records attributed to C. Morais Smith.

At least 109 records · Page 6Linked to original sources

arXiv2006

Transport properties of a quantum wire: the role of extended time-dependent impurities

We study the transport properties of a quantum wire, described by the Tomonaga-Luttinger model, in the presence of a backscattering potential provided by several extended time-dependent impurities (barriers). Employing the B\" uttiker-Landauer approach, we first consider the scattering of noninteracting electrons ($g=1$) by a rectangular-like barrier and find an exact solution for the backscattering current, as well as a perturbative solution for a weak static potential with an arbitrary shape. We then include electron-electron interactions and use the Keldysh formalism combined with the bosonization technique to study oscillating extended barriers. We show that the backscattering current off time-dependent impurities can be expressed in terms of the current for the corresponding static barrier. Then we determine the backscattering current for a static extended potential, which, in the limit of noninteracting electrons ($g=1$), coincides with the result obtained using the B\" uttiker-Landauer formalism. In particular, we find that the conductance can be increased beyond its quantized value in the whole range of repulsive interactions $0<g<1$ already in the case of a single oscillating extended impurity, in contrast %contrary to the case of a point-like impurity, where this phenomenon occurs only for $0<g<1/2$.

cond-mat.str-el↗

arXiv2006

Transport properties of a Luttinger liquid in the presence of several time-dependent impurities

We study the transport properties of a Luttinger liquid in the presence of several time-dependent weak point-like impurities. Our starting point is the bosonized form of the Luttinger liquid Hamiltonian with a potential introduced by the impurities. We find the correction to the total current due to the backscattering of the electrons by the impurities in first order of perturbation theory for a general time-dependent impurity potential. We then apply this result to a single time-dependent impurity, and show that it reproduces the result obtained by Feldman and Gefen in Ref. [7], in particular that the current enhancement occurs only for strong repulsive electron-electron interactions. For the case of two impurities oscillating with the same frequency, we find a novel effect, namely, that the dc-component of the backscattering current is positive {\it even for weak repulsive interactions}, due to the presence of the interference term induced by spatial correlations in the Luttinger liquid. We show that the impurity frequencies at which the enhancement takes place form a band-like structure. In the case of several impurities, the bands become broader, because of interference effects.

cond-mat.str-el↗

arXiv2006

Optical conductivity of charge carriers interacting with a two-level systems reservoir

Using the functional-integral method we investigate the effective dynamics of a charged particle coupled to a set of two-level systems as a function of temperature and external electric field. The optical conductivity and the direct current (dc) resistivity induced by the reservoir are computed. Three different regimes are found depending on the two-level system spectral function, which may lead to a non-Drude optical conductivity in a certain range of parameters. Our results contrast to the behavior found when considering the usual bath of harmonic oscillators which we are able to recover in the limit of very low temperatures.

cond-mat.other↗

arXiv2006

Lightly doped La(2-x)Sr(x)CuO(4) as a Lifshitz helimagnet

We study the static magnetic correlations in lightly doped La(2-x)Sr(x)CuO(4) within the framework of a dipolar frustration model for a canted antiferromagnet. We show that the stability of the canted Neel state for x<2% is due to the Dzyaloshinskii-Moriya and XY anisotropies. For higher doping the ground state is unstable towards a helicoidal magnetic phase where the transverse components of the staggered magnetization rotate in a plane perpendicular to the orthorhombic b-axis. Our theory reconciles, for the first time, the incommensurate peaks observed in elastic neutron scattering with Raman and magnetic susceptibility experiments in La(2-x)Sr(x)CuO(4).

cond-mat.str-el↗

arXiv2006

A bosonization approach for bilayer quantum Hall systems at ν_T = 1

We develop a non-perturbative bosonization approach for bilayer quantum Hall systems at ν_T = 1, which allows us to systematically study the existence of an exciton condensate in these systems. An effective boson model is derived and the excitation spectrum is calculated both in the Bogoliubov and in the Popov approximations. In the latter case, we show that the ground state of the system is an exciton condensate only when the distance between the layers is very small compared to the magnetic length, indicating that the system possibly undergoes another phase transition before the incompressible-compressible one. The effect of a finite electron interlayer tunnelling is included and a quantitative phase diagram is proposed.

cond-mat.mes-hall↗

arXiv2006

Magnetic susceptibility anisotropies in a two-dimensional quantum Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions

The magnetic and thermodynamic properties of the two-dimensional quantum Heisenberg antiferromagnet that incorporates both a Dzyaloshinskii-Moriya and pseudo-dipolar interactions are studied within the framework of a generalized nonlinear sigma model (NLSM). We calculate the static uniform susceptibility and sublattice magnetization as a function of temperature and we show that: i) the magnetic-response is anisotropic and differs qualitatively from the expected behavior of a conventional easy-axis QHAF; ii) the Neel second-order phase transition becomes a crossover, for a magnetic field B perpendicular to the CuO(2) layers. We provide a simple and clear explanation for all the recently reported unusual magnetic anisotropies in the low-field susceptibility of La(2)CuO(4), L. N. Lavrov et al., Phys. Rev. Lett. 87, 017007 (2001), and we demonstrate explicitly why La(2)CuO(4) can not be classified as an ordinary easy-axis antiferromagnet.

cond-mat.str-el↗

arXiv2005

Derivation of the generalized Non Linear Sigma Model in the presence of the Dzyaloshinskii-Moriya interaction

We derive the long-wavelength non-linear sigma model for a two-dimensional Heisenberg system in the presence of the Dzyaloshinskii-Moriya and pseudodipolar interactions. We show that the system is a non-conventional easy-axis antiferromagnet, displaying an anomalous coupling between the magnetic field and the staggered order parameter. Our results are in good agreement with recent experimental data for undoped La2CuO4 compounds.

cond-mat.str-el↗

arXiv2005

Spin-excitations of the quantum Hall ferromagnet of composite fermions

The spin-excitations of a fractional quantum Hall system are evaluated within a bosonization approach. In a first step, we generalize Murthy and Shankar's Hamiltonian theory of the fractional quantum Hall effect to the case of composite fermions with an extra discrete degree of freedom. Here, we mainly investigate the spin degrees of freedom, but the proposed formalism may be useful also in the study of bilayer quantum-Hall systems, where the layer index may formally be treated as an isospin. In a second step, we apply a bosonization scheme, recently developed for the study of the two-dimensional electron gas, to the interacting composite-fermion Hamiltonian. The dispersion of the bosons, which represent quasiparticle-quasihole excitations, is analytically evaluated for fractional quantum Hall systems at ν= 1/3 and ν= 1/5. The finite width of the two-dimensional electron gas is also taken into account explicitly. In addition, we consider the interacting bosonic model and calculate the lowest-energy state for two bosons. Besides a continuum describing scattering states, we find a bound-state of two bosons. This state is interpreted as a pair excitation, which consists of a skyrmion of composite fermions and an antiskyrmion of composite fermions. The dispersion relation of the two-boson state is evaluated for ν= 1/3 and ν= 1/5. Finally, we show that our theory provides the microscopic basis for a phenomenological non-linear sigma-model for studying the skyrmion of composite fermions.

cond-mat.mtrl-sci↗

arXiv2005

Dissipative dynamics of topological defects in frustrated Heisenberg spin systems

We study the dynamics of topological defects of a frustrated spin system displaying spiral order. As a starting point we consider the SO(3) nonlinear sigma model to describe long-wavelength fluctuations around the noncollinear spiral state. Besides the usual spin-wave magnetic excitations, the model allows for topologically non-trivial static solutions of the equations of motion, associated with the change of chirality (clockwise or counterclockwise) of the spiral. We consider two types of these topological defects, single vortices and vortex-antivortex pairs, and quantize the corresponding solutions by generalizing the semiclassical approach to a non-Abelian field theory. The use of the collective coordinates allows us to represent the defect as a particle coupled to a bath of harmonic oscillators, which can be integrated out employing the Feynman-Vernon path-integral formalism. The resulting effective action for the defect indicates that its motion is damped due to the scattering by the magnons. We derive a general expression for the damping coefficient of the defect, and evaluate its temperature dependence in both cases, for a single vortex and for a vortex-antivortex pair. Finally, we consider an application of the model for cuprates, where a spiral state has been argued to be realized in the spin-glass regime. By assuming that the defect motion contributes to the dissipative dynamics of the charges, we can compare our results with the measured inverse mobility in a wide range of temperature. The relatively good agreement between our calculations and the experiments confirms the possible relevance of an incommensurate spiral order for lightly doped cuprates.

cond-mat.str-el↗

arXiv2004

Possible Reentrance of the Fractional Quantum Hall Effect in the Lowest Landau Level

In the framework of a recently developed model of interacting composite fermions, we calculate the energy of different solid and Laughlin-type liquid phases of spin-polarized composite fermions. The liquid phases have a lower energy than the competing solids around the electronic filling factors nu=4/11,6/17, and 4/19 and may thus be responsible for the fractional quantum Hall effect at nu=4/11. The alternation between solid and liquid phases when varying the magnetic field may lead to reentrance phenomena in analogy with the observed reentrant integral quantum Hall effect.

cond-mat.mes-hall↗

arXiv2004

Second Generation of Composite Fermions and the Self-Similarity of the Fractional Quantum Hall Effect

A recently developed model of interacting composite fermions, is used to investigate different composite-fermion phases. Their interaction potential allows for the formation of both solid and new quantum-liquid phases, which are interpreted in terms of second-generation composite fermions and which may be responsible for the fractional quantum Hall states observed at unusual filling factors, such as nu=4/11. Projection of the composite-fermion dynamics to a single level, involved in the derivation of the Hamiltonian of interacting composite fermions, reveals the underlying self-similarity of the model.

cond-mat.mes-hall↗

arXiv2004

Second Generation of Composite Fermions in the Hamiltonian Theory

In the framework of a recently developed model of interacting composite fermions restricted to a single level, we calculate the activation gaps of a second generation of spin-polarized composite fermions. These composite particles consist each of a composite fermion of the first generation and a vortex-like excitation and may be responsible for the recently observed fractional quantum Hall states at unusual filling factors such as nu=4/11,5/13,5/17, and 6/17. Because the gaps of composite fermions of the second generation are found to be more than one order of magnitude smaller than those of the first generation, these states are less visible than the usual states observed at filling factors nu=p/(2ps+1). Their stability is discussed in the context of a pseudopotential expansion of the composite-fermion interaction potential.

cond-mat.mes-hall↗

arXiv2004

Quantum Phases in Partially Filled Landau Levels

We compare the energies of different electron solids, such as bubble crystals with triangular and square symmetry and stripe phases, to those of correlated quantum liquids in partially filled intermediate Landau levels. Multiple transitions between these phases when varying the filling of the top-most partially filled Landau level explain the observed reentrance of the integer quantum Hall effect. The phase transitions are identified as first-order. This leads to a variety of measurable phenomena such as the phase coexistence between a Wigner crystal and a two-electron bubble phase in a Landau-level filling-factor range $4.15 < nu < 4.26$, which has recently been observed in transport measurements under micro-wave irradiation.

cond-mat.mes-hall↗

arXiv2004

On the self-similarity in quantum Hall systems

The Hall-resistance curve of a two-dimensional electron system in the presence of a strong perpendicular magnetic field is an example of self-similarity. It reveals plateaus at low temperatures and has a fractal structure. We show that this fractal structure emerges naturally in the Hamiltonian formulation of composite fermions. After a set of transformations on the electronic model, we show that the model, which describes interacting composite fermions in a partially filled energy level, is self-similar. This mathematical property allows for the construction of a basis of higher generations of composite fermions. The collective-excitation dispersion of the recently observed 4/11 fractional-quantum-Hall state is discussed within the present formalism.

cond-mat.mes-hall↗

arXiv2004

Dynamics of topological defects in a spiral: a scenario for the spin-glass phase of cuprates

We propose that the dissipative dynamics of topological defects in a spiral state is responsible for the transport properties in the spin-glass phase of cuprates. Using the collective-coordinate method, we show that topological defects are coupled to a bath of magnetic excitations. By integrating out the bath degrees of freedom, we find that the dynamical properties of the topological defects are dissipative. The calculated damping matrix is related to the in-plane resistivity, which exhibits an anisotropy and linear temperature dependence in agreement with experimental data.

cond-mat.str-el↗

arXiv2003

Competition between quantum-liquid and electron-solid phases in intermediate Landau levels

On the basis of energy calculations we investigate the competition between quantum-liquid and electron-solid phases in the Landau levels n=1,2, and 3 as a function of their partial filling factor. Whereas the quantum-liquid phases are stable only in the vicinity of quantized values 1/(2s+1) of the partial filling factor, an electron solid in the form of a triangular lattice of clusters with a few number of electrons (bubble phase) is energetically favorable between these fillings. This alternation of electron-solid phases, which are insulating because they are pinned by the residual impurities in the sample, and quantum liquids displaying the fractional quantum Hall effect explains a recently observed reentrance of the integral quantum Hall effect in the Landau levels n=1 and 2. Around half-filling of the last Landau level, a uni-directional charge density wave (stripe phase) has a lower energy than the bubble phase.

cond-mat.mes-hall↗

arXiv2003

Microscopic Theory of the Reentrant IQHE in the First and Second Excited LLs

We present a microscopic theory for the recently observed reentrant integral quantum Hall effect in the n=1 and n=2 Landau levels. Our energy investigations indicate an alternating sequence of M-electron-bubble and quantum-liquid ground states in a certain range of the partial filling factor of the n-th level. Whereas the quantum-liquid states display the fractional quantum Hall effect, the bubble phases are insulating, and the Hall resistance is thus quantized at integral values of the total filling factor.

cond-mat.mes-hall↗

arXiv2003

Comment faire fondre un cristal d'electrons bidimensionnel sous champ magnetique

C'est un phenomene connu de la vie quotidienne : si l'on augmente la temperature, la glace fond. Ceci est le paradigme d'une transition de phase solide-liquide, qu'on observe egalement dans d'autres materiaux : tout cristal fond a temperature suffisamment elevee. Or de telles transitions de phase existent aussi dans des structures moins bien connues a temperature zero, ou l'on fait varier un parametre physique different de la temperature. C'est le cas par exemple dans un systeme d'electrons dont le mouvement est contraint dans un plan sous champ magnetique perpendiculaire. Nous avons montre qu'une partie de ces electrons peut faire une telle transition de phase solide-liquide. Le parametre que l'on varie est le champ magnetique meme, et l'on observe un phenomene etrange : quand on augmente le champ magnetique, un cristal d'electrons fond pour former un liquide electronique. Contrairement a notre intuition acquise par la fusion de la glace en la chauffant, les electrons liquides forment a nouveau un cristal si l'on augmente davantage le champ magnetique. Une telle alternance de phases en fonction du champ a ete recemment observee dans des experiences par Eisenstein et al. au California Institute of Technology.

cond-mat.mes-hall↗