arXiv subjects

Jaroslav Fabian

Publications and source records attributed to Jaroslav Fabian.

At least 181 records · Page 10Linked to original sources

arXiv2004

Theory of spin-polarized transport in semiconductor heterojunctions: Proposal for spin injection and detection in silicon

Spin injection and detection in silicon is a difficult problem, in part because the weak spin-orbit coupling and indirect gap preclude using standard optical techniques. We propose two ways to overcome this difficulty, and illustrate their operation by developing a model for spin-polarized transport across a heterojunction. We find that equilibrium spin polarization of holes leads to a strong modification of the spin and charge dynamics of electrons, and we show how the symmetry properties of the charge current can be exploited to detect spin injection in silicon using currently available techniques.

cond-mat.other↗

arXiv2004

Entanglement distillation by adiabatic passage in coupled quantum dots

Adiabatic passage of two correlated electrons in three coupled quantum dots is shown to provide a robust and controlled way of distilling, transporting and detecting spin entanglement, as well as of measuring the rate of spin disentanglement. Employing tunable interdot coupling the scheme creates, from an unentangled two-electron state, a superposition of spatially separated singlet and triplet states. A single measurement of a dot population (charge) collapses the wave function to either of these states, realizing entanglement to charge conversion. The scheme is robust, with the efficiency close to 100%, for a large range of realistic spectral parameters.

cond-mat.mes-hall↗

arXiv2004

Spin switch and spin amplifier: magnetic bipolar transistor in the saturation regime

It is shown that magnetic bipolar transistors (MBT) can amplify currents even in the saturation regime, in which both the emitter-base and collector-base junctions are forward biased. The collector current and the current gain can change sign as they depend on the relative orientation of the equilibrium spin in the base and on the nonequilibrium spin in the emitter and collector. The predicted phenomena should be useful for electrical detection of nonequilibrium spins in semiconductors, as well as for magnetic control of current amplification and for current switching.

cond-mat.other↗

arXiv2004

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

cond-mat.other↗

arXiv2004

Magnetic bipolar transistor

A magnetic bipolar transistor is a bipolar junction transistor with one or more magnetic regions, and/or with an externally injected nonequilibrium (source) spin. It is shown that electrical spin injection through the transistor is possible in the forward active regime. It is predicted that the current amplification of the transistor can be tuned by spin.

cond-mat.other↗

arXiv2003

Spin-polarized current amplification and spin injection in magnetic bipolar transistors

The magnetic bipolar transistor (MBT) is a bipolar junction transistor with an equilibrium and nonequilibrium spin (magnetization) in the emitter, base, or collector. The low-injection theory of spin-polarized transport through MBTs and of a more general case of an array of magnetic {\it p-n} junctions is developed and illustrated on several important cases. Two main physical phenomena are discussed: electrical spin injection and spin control of current amplification (magnetoamplification). It is shown that a source spin can be injected from the emitter to the collector. If the base of an MBT has an equilibrium magnetization, the spin can be injected from the base to the collector by intrinsic spin injection. The resulting spin accumulation in the collector is proportional to $\exp(qV_{be}/k_BT)$, where $q$ is the proton charge, $V_{be}$ is the bias in the emitter-base junction, and $k_B T$ is the thermal energy. To control the electrical current through MBTs both the equilibrium and the nonequilibrium spin can be employed. The equilibrium spin controls the magnitude of the equilibrium electron and hole densities, thereby controlling the currents. Increasing the equilibrium spin polarization of the base (emitter) increases (decreases) the current amplification. If there is a nonequilibrium spin in the emitter, and the base or the emitter has an equilibrium spin, a spin-valve effect can lead to a giant magnetoamplification effect, where the current amplifications for the parallel and antiparallel orientations of the the equilibrium and nonequilibrium spins differ significantly. The theory is elucidated using qualitative analyses and is illustrated on an MBT example with generic materials parameters.

cond-mat↗

arXiv2003

Spin-Voltaic Effect and its Implications

In an inhomogeneously doped magnetic semiconductor, an interplay between an equilibrium magnetization and injected nonequilibrium spin leads to the spin-voltaic effect--a spin analogue of the photo-voltaic effect. By reversing either the sign of the equilibrium magnetization or the direction of injected spin polarization it is possible to switch the direction of charge current in a closed circuit or, alternatively, to switch the sign of the induced open-circuit voltage. Properties of the spin-voltaic effect can be used to perform all-electrical measurements of spin relaxation time and injected spin polarization, as well as to design devices with large magnetoresistance and spin-controlled amplification.

cond-mat↗

arXiv2003

Numerical study of anharmonic vibrational decay in amorphous and paracrystalline silicon

The anharmonic decay rates of atomic vibrations in amorphous silicon (a-Si) and paracrystalline silicon (p-Si), containing small crystalline grains embedded in a disordered matrix, are calculated using realistic structural models. The models are 1000-atom four-coordinated networks relaxed to a local minimum of the Stillinger-Weber interatomic potential. The vibrational decay rates are calculated numerically by perturbation theory, taking into account cubic anharmonicity as the perturbation. The vibrational lifetimes for a-Si are found to be on picosecond time scales, in agreement with the previous perturbative and classical molecular dynamics calculations on a 216-atom model. The calculated decay rates for p-Si are similar to those of a-Si. No modes in p-Si reside entirely on the crystalline cluster, decoupled from the amorphous matrix. The localized modes with the largest (up to 59%) weight on the cluster decay primarily to two diffusons. The numerical results are discussed in relation to a recent suggestion by van der Voort et al. [Phys. Rev. B {\bf 62}, 8072 (2000)] that long vibrational relaxation inferred experimentally may be due to possible crystalline nanostructures in some types of a-Si.

cond-mat.dis-nn↗

arXiv2002

Theory of magnetic bipolar transistors

The concept of a magnetic bipolar transistor (MBT) is introduced. The transistor has at least one magnetic region (emitter, base, or collector) characterized by spin-splitting of the carrier bands. In addition, nonequilibrium (source) spin in MBTs can be induced by external means (electrically or optically). The theory of ideal MBTs is developed and discussed in the forward active regime where the transistors can amplify signals. It is shown that source spin can be injected from the emitter to the collector. It is predicted that electrical current gain (amplification) can be controlled effectively by magnetic field and source spin.

cond-mat↗

arXiv2002

Spin-polarized bipolar transport and its applications

In spin-polarized bipolar transport both electrons and holes in doped semiconductors contribute to spin-charge coupling. The current conversion between the minority (as referred to carriers and not spin) and majority carriers leads to novel spintronic schemes if nonequilibrium spin is present. Most striking phenomena occur in inhomogeneously doped magnetic {\it p-n} junctions, where the presence of nonequilibrium spin at the depletion layer leads to the spin-voltaic effect: electric current flows without external bias, powered only by spin. The spin-voltaic effect manifests itself in giant magnetoresistance of magnetic {\it p-n} junctions, where the relative change of the magnitude of electric current upon reversing magnetic field can be more than 1000%. The paper reviews nonmagnetic and magnetic spin-polarized {\it p-n} junctions, formulates the essentials of spin-polarized bipolar transport as carrier recombination and spin relaxation limited drift and diffusion, and discusses specific device schemes of spin-polarized solar cells and magnetic diodes.

cond-mat↗

arXiv2002

Spin transport in inhomogeneous magnetic fields: a proposal for Stern-Gerlach-like experiments with conduction electrons

Spin dynamics in spatially inhomogeneous magnetic fields is studied within the framework of Boltzmann theory. Stern-Gerlach-like separation of spin up and spin down electrons occurs in ballistic and diffusive regimes, before spin relaxation sets in. Transient dynamics and spectral response to time-dependent inhomogeneous magnetic fields are investigated, and possible experimental observations of our findings are discussed.

cond-mat↗

arXiv2002

Theory of spin-polarized bipolar transport in magnetic p-n junctions

The interplay between spin and charge transport in electrically and magnetically inhomogeneous semiconductor systems is investigated theoretically. In particular, the theory of spin-polarized bipolar transport in magnetic p-n junctions is formulated, generalizing the classic Shockley model. The theory assumes that in the depletion layer the nonequilibrium chemical potentials of spin up and spin down carriers are constant and carrier recombination and spin relaxation are inhibited. Under the general conditions of an applied bias and externally injected (source) spin, the model formulates analytically carrier and spin transport in magnetic p-n junctions at low bias. The evaluation of the carrier and spin densities at the depletion layer establishes the necessary boundary conditions for solving the diffusive transport equations in the bulk regions separately, thus greatly simplifying the problem. The carrier and spin density and current profiles in the bulk regions are calculated and the I-V characteristics of the junction are obtained. It is demonstrated that spin injection through the depletion layer of a magnetic p-n junction is not possible unless nonequilibrium spin accumulates in the bulk regions--either by external spin injection or by the application of a large bias. Implications of the theory for majority spin injection across the depletion layer, minority spin pumping and spin amplification, giant magnetoresistance, spin-voltaic effect, biasing electrode spin injection, and magnetic drift in the bulk regions are discussed in details, and illustrated using the example of a GaAs based magnetic p-n junction.

cond-mat↗

arXiv2002

Proposal for all-electrical measurement of T_1 in semiconductors

In an inhomogeneously doped magnetic semiconductor spin relaxation time T_1 can be determined by all-electrical measurements. Nonequilibrium spin injected in a magnetic p-n junction gives rise to the spin-voltaic effect where the nonequilibrium spin-induced charge current is very sensitive to T_1 and can flow even at no applied bias. It is proposed that T_1 can be determined by measuring the I-V characteristics in such a geometry. For a magnetic p-n junction where the results can be calculated analytically, it is in addition possible to extract the g-factor and the degree of injected carrier spin polarization.

cond-mat↗

arXiv2001

Spin-polarized transport in inhomogeneous magnetic semiconductors: theory of magnetic/nonmagnetic p-n junctions

A theory of spin-polarized transport in inhomogeneous magnetic semiconductors is developed and applied to magnetic/nonmagnetic p-n junctions. Several phenomena with possible spintronic applications are predicted, including spinvoltaic effect, spin valve effect, and giant magnetoresistance. It is demonstrated that only nonequilibrium spin can be injected across the space-charge region of a p-n junction, so that there is no spin injection (or extraction) at low bias.

cond-mat↗

arXiv2001

Spin Electronics and Spin Computation

We review several proposed spintronic devices that can provide new functionality or improve available functions of electronic devices. In particular, we discuss a high mobility field effect spin transistor, an all-metal spin transistor, and our recent proposal of an all-semiconductor spin transistor and a spin battery. We also address some key issues in spin-polarized transport, which are relevant to the feasibility and operation of hybrid semiconductor devices. Finally, we discuss a more radical aspect of spintronic research--the spin-based quantum computation and quantum information processing.

cond-mat↗

arXiv2001

Spectral fluctuations of the atomic vibrations in glasses: random matrix theory and beyond

It is demonstrated on a realistic model of amorphous alloy Si$_{0.9}$Ge$_{0.1}$ with 1000 atoms, that short-range spectral fluctuations of propagons and diffusons are universal and in agreement with random matrix theory. The universality ceases at distances greater than Thouless number $N_T$, where propagons obey the Altshuler-Shklovskii power law, $Σ_2\sim N^{3/2}$ for the variance $Σ_2$ of the number of levels $N$, while a new power law, $Σ_2\sim N$, is observed for diffusons.

cond-mat↗

arXiv2001

A proposal for a spin-polarized solar battery

A solar cell illuminated by circularly-polarized light generates charge and spin currents. We show that the spin polarization of the current significantly exceeds the spin polarization of the carrier density for the majority carriers. Based on this principle we propose a semiconductor spin-polarized solar battery and substantiate our proposal using analytical arguments and numerical modeling.

cond-mat.mtrl-sci↗

arXiv2001

Spin injection through the depletion layer: a theory of spin-polarized p-n junctions and solar cells

A drift-diffusion model for spin-charge transport in spin-polarized {\it p-n} junctions is developed and solved numerically for a realistic set of material parameters based on GaAs. It is demonstrated that spin polarization can be injected through the depletion layer by both minority and majority carriers, making all-semiconductor devices such as spin-polarized solar cells and bipolar transistors feasible. Spin-polarized {\it p-n} junctions allow for spin-polarized current generation, spin amplification, voltage control of spin polarization, and a significant extension of spin diffusion range.

cond-mat.mtrl-sci↗