arXiv subjects

S. Das Sarma

Publications and source records attributed to S. Das Sarma.

At least 631 records · Page 35Linked to original sources

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.

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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.

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arXiv1996

Calculated Schwoebel barriers on Si(111) steps using an empirical potential

Motivated by the recent investigations on instabilities caused by Schwoebel barriers during growth and their effects on growth or sublimation by step flows, we have investigated, using the Stillinger-Weber potential, how this step edge barrier arises for the two high symmetry steps on 1$\times$1 reconstructed Si(111). Relative to a barrier of 0.97 $\pm$ 0.07 eV on the surface, we find additional (Schwoebel) barriers of 0.61 $\pm$ 0.07 eV and 0.16 $\pm$ 0.07 eV for adatom migration over the [$\overline{2}11$] and the [$\overline{1}\overline{1}2$] steps respectively. The adatom potential energy is found to be strongly correlated with that derived from the local geometry of atoms on the adatom-free surface or step edges. This correlation preserves a strict correspondence between the barrier determining features in the spatial variation of the adatom potential energy and the same derived from the local geometry for the Si(111) surface and the [$\overline{2}11$] step. It is therefore argued that the Schwoebel barrier on the [$\overline{2}11$] step is robust {\it i.e.} a feature that would survive in more satisfactory {\it ab initio} or tight binding calculations. Using a diffusion equation for the adatom concentration the relevance of the barrier to electromigration of steps has been explored. Data from such experiments on Si(111) has been used to place an upper bound on the Schwoebel barrier and a lower bound on the electromigration force.

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arXiv1996

Schwoebel barriers on Si(111) steps and kinks

Motivated by our previous work using the Stillinger-Weber potential, which shows that the [$\overline{2}11$] step on 1$\times$1 reconstructed Si(111) has a Schwoebel barrier of 0.61$\pm$0.07 eV, we calculate here the same barrier corresponding to two types of kinks on this step - one with rebonding between upper and lower terrace atoms (type B) and the other without (type A). From the binding energy of an adatom, without additional relaxation of other atoms, we find that the Schwoebel barrier must be less than 0.39 eV (0.62 eV) for the kink of type A (type B). From the true adatom binding energy we determine the Schwoebel barrier to be 0.15$\pm$0.07eV (0.50$\pm$0.07 eV). The reduction of the Schwoebel barrier due to the presence of rebonding along the step edge or kink site is argued to be a robust feature. However, as the true binding energy plots show discontinuities due to significant movement of atoms at the kink site, we speculate on the possibility of multi-atom processes having smaller Schwoebel barriers.

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arXiv1996

Dynamical response of a one dimensional quantum wire electron system

We provide a self-contained theoretical analysis of the dynamical response of a one dimensional electron system, as confined in a semiconductor quantum wire, within the random phase approximation. We carry out a detailed comparison with the corresponding two and three dimensional situations, and discuss the peculiarities arising in the one dimensional linear response from the non-existence of low energy single-particle excitations and from the linear nature of the long wavelength plasmon mode. We provide a critical discussion of the analytic properties of the complex dielectric function in the complex frequency plane. We investigate the zeros of the complex dielectric function, and calculate the plasmon dispersion, damping, and plasmon spectral weight in one dimension. We consider finite temperature and impurity scattering effects on one dimensional plasmon dispersion and damping.

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arXiv1996

Quasiparticle properties of a coupled quantum wire electron-phonon system

We study leading-order many-body effects of longitudinal optical (LO) phonons on electronic properties of one-dimensional quantum wire systems. We calculate the quasiparticle properties of a weakly polar one dimensional electron gas in the presence of both electron-phonon and electron-electron interactions. The leading-order dynamical screening approximation (GW approximation) is used to obtain the electron self-energy, the quasiparticle spectral function, and the quasiparticle damping rate in our calculation by treating electrons and phonons on an equal footing. Our theory includes effects (within the random phase approximation) of Fermi statistics, Landau damping, plasmon-phonon mode coupling, phonon renormalization, dynamical screening, and impurity scattering. In general, electron-electron and electron-phonon many-body renormalization effects are found to be nonmultiplicative and nonadditive in our theoretical results for quasiparticle properties.

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arXiv1996

Realistic Calculations of Correlated Incompressible Electronic States in GaAs--Al_{x}Ga_{1-x}As Heterostructures and Quantum Wells

We perform an exact spherical geometry finite-size diagonalization calculation for the fractional quantum Hall ground state in three different experimentally relevant GaAs-Al_{x}Ga_{1-x}As systems: a wide parabolic quantum well, a narrow square quantum well, and a heterostructure. For each system we obtain the Coulomb pseudopotential parameters entering the exact diagonalization calculation by using the realistic subband wave function from a self-consistent electronic structure calculation within the local density approximation (LDA) for a range of electron densities. We compare our realistic LDA pseudopotential parameters with those from widely used simpler model approximations in order to estimate the accuracies of the latter. We also calculate the overlap between the exact numerical ground state and the analytical Laughlin state as well as the excitation gap as a function of density. For the three physical systems we consider the calculated overlap is found to be large in the experimental electron density range. We compare our calculated excitation gap energy to the experimentally obtained activated transport energy gaps after subtracting out the effect of level broadening due to collisions. The agreement between our calculated excitation gaps and the experimental measurements is excellent.

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arXiv1996

Coulomb scattering lifetime of a two-dimensional electron gas

Motivated by a recent tunneling experiment in a double quantum-well system, which reports an anomalously enhanced electronic scattering rate in a clean two-dimensional electron gas, we calculate the inelastic quasiparticle lifetime due to electron-electron interaction in a single loop dynamically screened Coulomb interaction within the random-phase-approximation. We obtain excellent quantitative agreement with the inelastic scattering rates in the tunneling experiment without any adjustable parameter, finding that the reported large ($\geq$ a factor of six) disagreement between theory and experiment arises from quantitative errors in the existing theoretical work and from the off-shell energy dependence of the electron self-energy.

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arXiv1996

Energy relaxation of an excited electron gas in quantum wires: many-body electron LO-phonon coupling

We theoretically study energy relaxation via LO-phonon emission in an excited one-dimensional electron gas confined in a GaAs quantum wire structure. We find that the inclusion of phonon renormalization effects in the theory extends the LO-phonon dominated loss regime down to substantially lower temperatures. We show that a simple plasmon-pole approximation works well for this problem, and discuss implications of our results for low temperature electron heating experiments in quantum wires.

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arXiv1996

Unusual temperature dependent resistivity of a semiconductor quantum wire

We calculate the electronic resistivity of a GaAs-based semiconductor quantum wire in the presence of acoustic phonon scattering. We find that the usual Drude-Boltzmann transport theory leads to a low temperature activated behavior instead of the well-known Bloch-Grüneisen power law. Many-body electron-phonon renormalization, which is entirely negligible in higher dimensional systems, has a dramatic effect on the low temperature quantum wire transport properties as it qualitatively modifies the temperature dependence of the resistivity from the exponentially activated behavior to an approximate power law behavior at sufficiently low temperatures.

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arXiv1996

Theory of Phonon Shakeup Effects on Photoluminescence from the Wigner Crystal in a Strong Magnetic Field

We develop a method to compute shakeup effects on photoluminescence from a strong magnetic field induced two-dimensional Wigner crystal. Only localized holes are considered. Our method treats the lattice electrons and the tunneling electron on an equal footing, and uses a quantum-mechanical calculation of the collective modes that does not depend in any way on a harmonic approximation. We find that shakeup produces a series of sidebands that may be identified with maxima in the collective mode density of states, and definitively distinguishes the crystal state from a liquid state in the absence of electron-hole interaction. In the presence of electron-hole interaction, sidebands also appear in the liquid state coming from short-range density fluctuations around the hole. However, the sidebands in the liquid state and the crystal state have different qualitative behaviors. We also find a shift in the main luminescence peak, that is associated with lattice relaxation in the vicinity of a vacancy. The relationship of the shakeup spectrum with previous mean-field calculations is discussed.

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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.

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arXiv1995

An exchange-correlation energy for a two-dimensional electron gas in a magnetic field

We present the results of a variational Monte Carlo calculation of the exchange-correlation energy for a spin-polarized two-dimensional electron gas in a perpendicular magnetic field. These energies are a necessary input to the recently developed current-density functional theory. Landau-level mixing is included in a variational manner, which gives the energy at finite density at finite field, in contrast to previous approaches. Results are presented for the exchange-correlation energy and excited-state gap at $ν=$ 1/7, 1/5, 1/3, 1, and 2. We parameterize the results as a function of $r_s$ and $ν$ in a form convenient for current-density functional calculations.

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arXiv1995

Coherent Magnetotransport Through an Artificial Molecule

The conductance in an extended multiband Hubbard model describing linear arrays of up to ten quantum dots is calculated via a Lanczos technique. A pronounced suppression of certain resonant conductance peaks in an applied magnetic field due to a density-dependent spin-polarization transition is predicted to be a clear signature of a coherent ``molecular'' wavefunction in the array. A many-body enhancement of localization is predicted to give rise to a {\em giant magnetoconductance} effect in systems with magnetic scattering.

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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.

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arXiv1995

Localization in Semiconductor Quantum Wire Nanostructures

Localization properties of quasi-one dimensional quantum wire nanostructures are investigated using the transfer matrix-Lyapunov exponent technique. We calculate the localization length as a function of the effective mean-field mobility assuming the random disorder potential to be arising from dopant-induced short-range $δ$-function or finite-range Gaussian impurity scattering. The localization length increases approximately linearly with the effective mobility, and is also enhanced by finite-range disorder. There is a sharp reduction in the localization length when the chemical potential crosses into the second subband.

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arXiv1994

Laughlin liquid - Wigner solid transition at high density in wide quantum wells

Assuming that the phase transition between the Wigner solid and the Laughlin liquid is first-order, we compare ground-state energies to find features of the phase diagram at fixed $ν$. Rather than use the Coulomb interaction, we calculate the effective interaction in a square quantum well, and fit the results to a model interaction with length parameter $λ$ roughly proportional to the width of the well. We find a transition to the Wigner solid phase at high density in very wide wells, driven by the softening of the interaction at short distances, as well as the more well-known transition to the Wigner solid at low density, driven by Landau-level mixing.

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arXiv1994

Electron Localization in a 2D System with Random Magnetic Flux

Using a finite-size scaling method, we calculate the localization properties of a disordered two-dimensional electron system in the presence of a random magnetic field. Below a critical energy $E_c$ all states are localized and the localization length $ξ$ diverges when the Fermi energy approaches the critical energy, {\it i.e.} $ξ(E)\propto |E-E_c|^{-ν}$. We find that $E_c$ shifts with the strength of the disorder and the amplitude of the random magnetic field while the critical exponent ($ν\approx 4.8$) remains unchanged indicating universality in this system. Implications on the experiment in half-filling fractional quantum Hall system are also discussed.

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