Tailoring Chirp in Spin-Lasers

The usefulness of semiconductor lasers is often limited by the undesired frequency modulation, or chirp, a direct consequence of the intensity modulation and carrier dependence of the refractive index in the gain medium. In spin-lasers, realized by injecting, optically or electrically, spin-polarized carriers, we elucidate paths to tailoring chirp. We provide a generalized expression for chirp in spin-lasers and introduce modulation schemes that could simultaneously eliminate chirp and enhance the bandwidth, as compared to the conventional (spin-unpolarized) lasers.Comment: 4 pages, 3 figure

Piezomagnetic Quantum Dots

We study the influence of deformations on magnetic ordering in quantum dots doped with magnetic impurities. The reduction of symmetry and the associated deformation from circular to elliptical quantum confinement lead to the formation of piezomagnetic quantum dots. The strength of elliptical deformation can be controlled by the gate voltage to change the magnitude of magnetization, at a fixed number of carriers and in the absence of applied magnetic field. We reveal a reentrant magnetic ordering with the increase of elliptical deformation and suggest that the piezomagnetic quantum dots can be used as nanoscale magnetic switches.Comment: 4 pages, 3 figure

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.Comment: 14 PRB-style pages, 10 figure

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.Comment: Minor Revisions. To appear in Phys. Rev. Let

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.Comment: 7 pages, 4 figures; revised and extended version, to appear in PR

Angular dependence of the penetration depth in unconventional superconductors

We examine the Meissner state nonlinear electrodynamic effects on the field and angular dependence of the low temperature penetration depth, $\lambda$, of superconductors in several kinds of unconventional pairing states, with nodes or deep minima (``quasinodes'') in the energy gap. Our calculations are prompted by the fact that, for typical unconventional superconducting material parameters, the predicted size of these effects for $\lambda$ exceeds the available experimental precision for this quantity by a much larger factor than for others. We obtain expressions for the nonlinear component of the penetration depth, $\Delta\lambda$, for different two- and three- dimensional nodal or quasinodal structures. Each case has a characteristic signature as to its dependence on the size and orientation of the applied magnetic field. This shows that $\Delta\lambda$ measurements can be used to elucidate the nodal or quasinodal structure of the energy gap. For nodal lines we find that $\Delta\lambda$ is linear in the applied field, while the dependence is quadratic for point nodes. For layered materials with $\rm{YBa_2Cu_3O_{7-\delta}}$ (YBCO) type anisotropy, our results for the angular dependence of $\Delta\lambda$ differ greatly from those for tetragonal materials and are in agreement with experiment. For the two- and three- dimensional quasinodal cases, $\Delta\lambda$ is no longer proportional to a power of the field and the field and angular dependences are not separable, with a suppression of the overall signal as the node is filled in.Comment: 16 pages plus nine figure

Calorimetric tunneling study of heat generation in metal-vacuum-metal tunnel junction

We have proposed novel calorimetric tunneling (CT) experiment allowing exact determination of heat generation (or heat sinking) in individual tunnel junction (TJ) electrodes which opens new possibilities in the field of design and development of experimental techniques for science and technology. Using such experiment we have studied the process of heat generation in normal-metal electrodes of the vacuum-barrier tunnel junction (VBTJ). The results show there exists dependence of the mutual redistribution of the heat on applied bias voltage and the direction of tunnel current, although the total heat generated in tunnel process is equal to Joule heat, as expected. Moreover, presented study indicates generated heat represents the energy of non-equilibrium quasiparticles coming from inelastic electron processes accompanying the process of elastic tunneling.Comment: 8 pages, 3 figures, LaTe

Low Frequency Nonlinear Magnetic Response of an Unconventional Superconductor

We consider an unconventional superconductor in a low frequency harmonic magnetic field. In the Meissner regime at low T a nonlinear magnetic response arises from quasiparticle excitations near minima in the energy gap. Various physical quantities then acquire higher harmonics of the frequency of the applied field. We discuss how examination of the field and angular dependence of these harmonics allows determination of the structure of the energy gap. We show how to distinguish nodes from small finite minima ("quasinodes"). Gaps with nodal lines give rise to universal power law field dependences for the nonlinear magnetic moment and the nonlinear torque. They both have separable temporal and angular dependences. In contrast, when there are quasinodes these quantities have more complicated and nonseparable field, temporal, and angular dependences. We illustrate this on the example of an s+id gap. We discuss how to perform measurements so as to maximize the nonlinear signal and how to investigate in detail the properties of the superconducting minima, thus determining the gap function symmetry.Comment: To appear in Phys Rev B. Ten figures, 13 text page

Coherent spin transport through a 350-micron-thick Silicon wafer

We use all-electrical methods to inject, transport, and detect spin-polarized electrons vertically through a 350-micron-thick undoped single-crystal silicon wafer. Spin precession measurements in a perpendicular magnetic field at different accelerating electric fields reveal high spin coherence with at least 13pi precession angles. The magnetic-field spacing of precession extrema are used to determine the injector-to-detector electron transit time. These transit time values are associated with output magnetocurrent changes (from in-plane spin-valve measurements), which are proportional to final spin polarization. Fitting the results to a simple exponential spin-decay model yields a conduction electron spin lifetime (T1) lower bound in silicon of over 500ns at 60K.Comment: Accepted in PR