Showing papers on "Spin-½ published in 2005"

Real-Time Dynamics in Quantum-Impurity Systems: A Time-Dependent Numerical Renormalization-Group Approach

Abstract: We develop a general approach to the nonequilibrium dynamics of quantum-impurity systems for arbitrary coupling strength. The numerical renormalization group is used to generate a complete basis set necessary for the correct description of the time evolution. We benchmark our method with the exact analytical solution for the resonant-level model. As a first application, we investigate the equilibration of an ultrasmall quantum dot subject to a sudden change of gate voltage and external magnetic field. Two distinct relaxation times are identified for the spin and charge dynamics.

Spin-charge separation and localization in one dimension.

Abstract: We report on measurements of quantum many-body modes in ballistic wires and their dependence on Coulomb interactions, obtained by tunneling between two parallel wires in an GaAs/AlGaAs heterostructure while varying electron density. We observed two spin modes and one charge mode of the coupled wires and mapped the dispersion velocities of the modes down to a critical density, at which spontaneous localization was observed. Theoretical calculations of the charge velocity agree well with the data, although they also predict an additional charge mode that was not observed. The measured spin velocity was smaller than theoretically predicted.

Coherent spinor dynamics in a spin-1 Bose condensate

Abstract: Collisions in a thermal gas are perceived as random or incoherent as a consequence of the large numbers of initial and final quantum states accessible to the system In a quantum gas, for example, a Bose–Einstein condensate or a degenerate Fermi gas, the phase space accessible to low-energy collisions is so restricted that collisions become coherent and reversible Here, we report the observation of coherent spin-changing collisions in a gas of spin-1 bosons Starting with condensates occupying two spin states, a condensate in the third spin state is coherently and reversibly created by atomic collisions The observed dynamics are analogous to Josephson oscillations in weakly connected superconductors and represent a type of matter–wave four-wave mixing The spin-dependent scattering length is determined from these oscillations to be −145(32) bohr Finally, we demonstrate coherent control of the evolution of the system by applying differential phase shifts to the spin states using magnetic fields

Transformation of spin information into large electrical signals via carbon nanotubes

Abstract: Spin electronics (spintronics) exploits the magnetic nature of the electron, and is commercially exploited in the spin valves of disc-drive read heads. There is currently widespread interest in using industrially relevant semiconductors in new types of spintronic devices based on the manipulation of spins injected into a semiconducting channel between a spin-polarized source and drain. However, the transformation of spin information into large electrical signals is limited by spin relaxation such that the magnetoresistive signals are below 1%. We overcome this long standing problem in spintronics by demonstrating large magnetoresistance effects of 61% at 5 K in devices where the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 micron gap between epitaxial electrodes of the highly spin polarized manganite La0.7Sr0.3MnO3. This improvement arises because the spin lifetime in nanotubes is long due the small spin-orbit coupling of carbon, because the high nanotube Fermi velocity permits the carrier dwell time to not significantly exceed this spin lifetime, because the manganite remains highly spin polarized up to the manganite-nanotube interface, and because the interfacial barrier is of an appropriate height. We support these latter statements regarding the interface using density functional theory calculations. The success of our experiments with such chemically and geometrically different materials should inspire adventure in materials selection for some future spintronics

Quantum criticality and universal scaling of a quantum antiferromagnet

Abstract: Quantum effects dominate the behaviour of many diverse materials. Of particular current interest are those systems in the vicinity of a quantum critical point (QCP). Their physical properties are predicted to reflect those of the nearby QCP with universal features independent of the microscopic details. The prototypical QCP is the Luttinger liquid (LL), which is of relevance to many quasi-one-dimensional materials. The magnetic material KCuF3 realizes an array of weakly coupled spin chains (or LLs) and thus lies close to but not exactly at the LL quantum critical point. By using inelastic neutron scattering we have collected a complete data set of the magnetic correlations of KCuF3 as a function of momentum, energy and temperature. The LL description is found to be valid over an extensive range of these parameters, and departures from this behaviour at high and low energies and temperatures are identified and explained.

Spin Relaxation and Decoherence of Holes in Quantum Dots

Abstract: We investigate heavy-hole spin relaxation and decoherence in quantum dots in perpendicular magnetic fields. We show that at low temperatures the spin decoherence time is 2 times longer than the spin relaxation time. We find that the spin relaxation time for heavy holes can be comparable to or even longer than that for electrons in strongly two-dimensional quantum dots. We discuss the difference in the magnetic-field dependence of the spin relaxation rate due to Rashba or Dresselhaus spin-orbit coupling for systems with positive (i.e., GaAs quantum dots) or negative (i.e., InAs quantum dots) g factor.

Current-spin coupling for ferromagnetic domain walls in fine wires.

Abstract: The coupling between a current and a domain wall is examined. In the presence of a finite current and in the absence of a potential which breaks the translational symmetry, there is a perfect transfer of angular momentum from the conduction electrons to the wall. As a result, the ground state is in uniform motion and this remains the case even when relaxation is included. This is described by, appropriately modified, Landau-Lifshitz-Gilbert equations. The results for a simple pinning model are compared with experiment.

Theory of Spin Hall Conductivity in n-Doped GaAs

Abstract: We develop a theory of extrinsic spin currents in semiconductors, resulting from spin-orbit coupling at charged scatterers, which leads to skew-scattering and side-jump contributions to the spin-Hall conductivity. Applying the theory to bulk $n$-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with experiments by Kato et al. [Science 306, 1910 (2004)].

Thermal entanglement in a two-qubit Heisenberg XXZ spin chain under an inhomogeneous magnetic field

Abstract: The thermal entanglement in a two-qubit Heisenberg XXZ spin chain is investigated under an inhomogeneous magnetic field b. We show that the ground-state entanglement is independent of the interaction of z-component J(z). The thermal entanglement at the fixed temperature can be enhanced when J(z) increases. We strictly show that for any temperature T and J(z), the entanglement is symmetric with respect to zero inhomogeneous magnetic field, and the critical inhomogeneous magnetic field b(c) is independent of J(z). The critical magnetic field B-c increases with the increasing parallel to b parallel to but the maximum entanglement value that the system can arrive at becomes smaller.

Thermopower of a Kondo spin-correlated quantum dot.

Abstract: The thermopower of a Kondo-correlated gate-defined quantum dot is studied using a current heating technique. In the presence of spin correlations, the thermopower shows a clear deviation from the semiclassical Mott relation between thermopower and conductivity. The strong thermopower signal indicates a significant asymmetry in the spectral density of states of the Kondo resonance with respect to the Fermi energies of the reservoirs. The observed behavior can be explained within the framework of an Anderson-impurity model.

Geometric phases and criticality in spin-chain systems.

Abstract: A relation between geometric phases and criticality of spin chains is established. As a result, we show how geometric phases can be exploited as a tool to detect regions of criticality without having to undergo a quantum phase transition. We analytically evaluate the geometric phase that corresponds to the ground and excited states of the anisotropic XY model in the presence of a transverse magnetic field when the direction of the anisotropy is adiabatically rotated. It is demonstrated that the resulting phase is resilient against the main sources of errors. A physical realization with ultracold atoms in optical lattices is presented.

Conclusive and arbitrarily perfect quantum-state transfer using parallel spin-chain channels

Abstract: We suggest a protocol for perfect quantum communication through spin-chain channels. By combining a dual-rail encoding with measurements only at the receiving end, we can get conclusively perfect state transfer, whose probability of success can be made arbitrarily close to unity. As an example of such an amplitude-delaying channel, we show how two parallel Heisenberg spin chains can be used as quantum wires. Perfect state transfer with a probability of failure lower than P in a Heisenberg chain of N spin-(1/2) particles can be achieved in a timescale of the order of (0.33({Dirac_h}/2{pi})/J)N{sup 1.7} vertical bar ln P vertical bar. We demonstrate that our scheme is more robust to decoherence and nonoptimal timing than any scheme using single spin chains.

Concepts for spin injection into semiconductors—a review

Abstract: Spintronics is a topic that has raised a lot of interest during the past years. The transport and manipulation of spin polarized electrons or holes in semiconductors offers a huge potential for novel devices that combine non-volatile information storage with high processing speed at low power, and which may even be useful for quantum computation. However, one major ingredient that has been missing for a long time is the transfer of spin polarized carriers from a magnetic contact into a non-magnetic semiconductor. Highly efficient electrical spin injection was realized for the first time in 1999 (Fiederling R et al Nature 402 787). Since then, several experiments have successfully demonstrated spin injection into semiconductors and additional concepts for spin filters and spin aligners have been proposed. Some of the experiments also yielded high spin injection efficiencies; however, in other experiments no unambiguous results could be obtained, and for many of the proposed concepts even a proof of principle is still missing. In this review, the suitability of different contact types for spin injection will be discussed together with a review of possible detection mechanisms that can be used in the experiment. By using a simple model based on load-lines we will show that for most spin aligners a first estimate of the injection efficiency can easily be obtained without complicated calculations.

Algebraic spin liquid as the mother of many competing orders

Abstract: We study the properties of a class of two-dimensional interacting critical states-dubbed algebraic spin liquids-that can arise in two-dimensional quantum magnets. A particular example that we focus on is the staggered flux spin liquid, which plays a key role in some theories of underdoped cup rate superconductors. We show that the low-energy theory of such states has much higher symmetry than the underlying microscopic spin system. This symmetry has remarkable consequences, leading in particular to the unification of a number of seemingly unrelated competing orders. The correlations of these orders-including, in the staggered flux state, the Neel vector, and the order parameter for the columnar and box valence-bond solid states-all exhibit the same slow power-law decay. Implications for experiments in the pseudogap regime of the cuprates and for numerical calculations on model systems are discussed.

Quantum helimagnetism of the frustrated spin-1/2 chain LiCuVO4

Abstract: Inelastic neutron scattering,susceptibility,and high-field magnetization identify LiCuVO4 as a nearest-neighbour ferromagnetic,next-nearest-neighbour frustrated,quasi-one- dimensional helimagnet,which is largely influenced by quantum fluctuations. Complementary band structure calculations provide a microscopic model with the correct sign and magnitude of the major exchange integrals.

Estimation of spin-diffusion length from the magnitude of spin-current absorption: Multiterminal ferromagnetic/nonferromagnetic hybrid structures

Abstract: We demonstrate the method to calculate the spatial distributions of the spin current and accumulation in the multiterminal ferromagnetic/nonmagnetic hybrid structure using an approximate electrotransmission line. The analyses based on the obtained equation yield the results, in good agreement with the experimental ones. This implies that the method allows us to determine the spin diffusion length of an additionally connected electrically floating wire from the reduction of the spin signal.

Quantum criticality and universal scaling of a quantum antiferromagnet

Abstract: Quantum effects dominate the behaviour of many diverse materials. Of particular current interest are those systems in the vicinity of a quantum critical point (QCP). Their physical properties are predicted to reflect those of the nearby QCP with universal features independent of the microscopic details. The prototypical QCP is the Luttinger liquid (LL) which is of relevance to many quasi-one-dimensional materials. The magnetic material KCuF3 realizes an array of weakly-coupled spin chains (or LLs) and thus lies close to but not exactly at the Luttinger liquid quantum critical point. By using inelastic neutron scattering we have collected a complete data set of the magnetic correlations of KCuF3 as a function of momentum, energy, and temperature. The LL description is found to be valid over an extensive range of these parameters, and departures from this behaviour at high and low energies and temperatures have been identified and explained.

Effects of spin-exchange collisions in a high-density alkali-metal vapor in low magnetic fields

Abstract: Spin-exchange collisions often play a dominant role in the broadening of Zeeman resonances in an alkali-metal vapor. Contrary to intuitive expectations, at high alkali-metal densities this broadening can be completely eliminated by operating in a low magnetic field, allowing construction of ultrasensitive atomic magnetometers. We describe a detailed study of the Zeeman resonance frequencies and linewidths as a function of the magnetic field, alkali-metal density, and the degree of spin polarization of the atoms. Due to the nonlinear nature of the density matrix equations describing the spin-exchange collisions both the gyromagnetic ratio and the linewidth change as a function of the polarization. The results of experimental measurements are in excellent agreement with analytical and numerical solutions of the density matrix equations.

Symmetry of spin transport in two-terminal waveguides with a spin-orbital interaction and magnetic field modulations

Abstract: We analyze symmetries of spin transport in two-terminal quantum waveguide structures with Rashba spin-orbit coupling and magnetic field modulations. Constraints, imposed by the device structure, on the spin polarization of the transmitted electron beam from the waveguide devices are derived. The results are expected to provide accuracy tests for experimental measurements and numerical calculations, as well as guidelines for spin-based device designs.

Multipartite entanglement in spin chains

Abstract: We investigate the presence of multipartite entanglement in macroscopic spin chains. We discuss the Heisenberg and the XY model and derive bounds on the internal energy for systems without multipartite entanglement. Based on this we show that in thermal equilibrium the above-mentioned spin systems contain genuine multipartite entanglement, even at finite modest temperatures.

Spin-torque switching: Fokker-Planck rate calculation

Abstract: We describe a general Fokker-Planck approach to understanding and calculating magnetization switching rates and noise in the recently observed phenomenon of spin-torque switching. In this phenomenon, which has possible applications to information storage, a large current passing from a pinned ferromagnetic (FM) layer to a free FM layer switches the free layer. Beginning with Brown [Phys. Rev. 130, 1677 (1963)], switching rates in magnetic systems have been calculated using the Fokker-Planck equation. In the small-oscillation limit, the equations have been solved analytically, giving a first-principles justification for phenomenological effective temperature theories: the spin-torque effect increases the Arrhenius factor $\mathrm{exp}(\ensuremath{-}E∕kT)$ in the switching rate by raising the effective spin temperature $T$. In the present Rapid Communication we generalize the nonlinear Fokker-Planck equation to the case of a Slonczewski spin torque. As an example, we use a linear approximation to calculate telegraph noise rates, leading to good qualitative agreement with recent experiments. However, our nonlinear formulation is also valid for large precessional oscillations. The method also allows the calculation of current-induced magnetic noise in current perpendicular to plane spin valve read heads.

Spinning strings in AdS5 × S5: one-loop correction to energy in SL(2) sector

Abstract: We consider a circular string with spin S in AdS5 wrapped around big circle of S5 and carrying also momentum J. The corresponding N = 4 SYM operator belongs to the SL(2) sector, i.e. has tr(DSZJ)+... structure. The leading large J term in its 1-loop anomalous dimension can be computed using Bethe ansatz for the SL(2) spin chain and was previously found to match the leading term in the classical string energy. The string solution is stable at large J, and the Lagrangian for string fluctuations has constant coefficients, so that the 1-loop string correction to the energy E1 is given simply by the sum of characteristic frequencies. Curiously, we find that the leading term in the zero-mode part of E1 is the same as a 1/J correction to the one-loop anomalous dimension on the gauge theory (spin chain) side that was found in [1]. However, the contribution of non-zero string modes does not vanish. We also discuss the ``fast string'' expansion of the classical string action which coincides with the coherent state action of the SL(2) spin chain at the first order in λ, and extend this expansion to higher orders clarifying the role of the S5 winding number.

Estimation of spin-diffusion length from the magnitude of spin current absorption

Abstract: We demonstrate the method to calculate the spatial distributions of the spin current and accumulation in the multi-terminal ferromagnetic/nonmagnetic hybrid structure using an approximate electro-transmission line. The analyses based on the obtained equation yield the results in good agreement with the experimental ones. This implies that the method allows us to determine the spin diffusion length of additionally connected electrically floating wire from the reduction of the spin signal.

Colloquium: Photons and electrons as emergent phenomena

Abstract: Recent advances in condensed-matter theory have revealed that new and exotic phases of matter can exist in spin models (or more precisely, local bosonic models) via a simple physical mechanism, known as ``string-net condensation.'' These new phases of matter have the unusual property that their collective excitations are gauge bosons and fermions. In some cases, the collective excitations can behave just like the photons, electrons, gluons, and quarks in our vacuum. This suggests that photons, electrons, and other elementary particles may have a unified origin---string-net condensation in our vacuum. In addition, the string-net picture indicates how to make artificial photons, artificial electrons, and artificial quarks and gluons in condensed-matter systems.

Spin injection from the Heusler alloy Co2MnGe into Al0.1Ga0.9As∕GaAs heterostructures

Abstract: Electrical spin injection from the Heusler alloy Co2MnGe into a p-i-nAl0.1Ga0.9As∕GaAs light emitting diode is demonstrated. A maximum steady-state spin polarization of approximately 13% at 2 K is measured in two types of heterostructures. The injected spin polarization at 2 K is calculated to be 27% based on a calibration of the spin detector using Hanle effect measurements. Although the dependence on electrical bias conditions is qualitatively similar to Fe-based spin injection devices of the same design, the spin polarization injected from Co2MnGe decays more rapidly with increasing temperature.

Spinning strings in AdS_5 x S^5: one-loop correction to energy in SL(2) sector

Abstract: We consider a circular string with spin $S$ in $AdS_5$ wrapped around big circle of $S^5$ and carrying also momentum $J$. The corresponding N=4 SYM operator belongs to the SL(2) sector, i.e. has tr$(D^S Z^J)+...$ structure. The leading large $J$ term in its 1-loop anomalous dimension can be computed using Bethe ansatz for the SL(2) spin chain and was previously found to match the leading term in the classical string energy. The string solution is stable at large $J$, and the Lagrangian for string fluctuations has constant coefficients, so that the 1-loop string correction to the energy $E_1$ is given simply by the sum of characteristic frequencies. Curiously, we find that the leading term in the zero-mode part of $E_1$ is the same as a 1/J correction to the one-loop anomalous dimension on the gauge theory (spin chain) side that was found in hep-th/0410105. However, the contribution of non-zero string modes does not vanish. We also discuss the ``fast string'' expansion of the classical string action which coincides with the coherent state action of the SL(2) spin chain at the first order in $\l$, and extend this expansion to higher orders clarifying the role of the $S^5$ winding number.

Information-capacity description of spin-chain correlations

Abstract: Information capacities achievable in the multi-parallel-use scenarios are employed to characterize the quantum correlations in unmodulated spin chains. By studying the qubit amplitude damping channel, we calculate the quantum capacity $Q$, the entanglement assisted capacity ${C}_{E}$, and the classical capacity ${C}_{1}$ of a spin chain with ferromagnetic Heisenberg interactions.