Showing papers on "Spin-½ published in 2000"
TL;DR: In this article, a condition for boundary Majorana fermions is expressed as a condition on the bulk electron spectrum, which is satisfied in the presence of an arbitrary small energy gap induced by proximity of a 3-dimensional p-wave superconductor, provided that the normal spectrum has an odd number of Fermi points in each half of the Brillouin zone.
Abstract: Certain one-dimensional Fermi systems have an energy gap in the bulk spectrum while boundary states are described by one Majorana operator per boundary point. A finite system of length $L$ possesses two ground states with an energy difference proportional to $\exp(-L/l_0)$ and different fermionic parities. Such systems can be used as qubits since they are intrinsically immune to decoherence. The property of a system to have boundary Majorana fermions is expressed as a condition on the bulk electron spectrum. The condition is satisfied in the presence of an arbitrary small energy gap induced by proximity of a 3-dimensional p-wave superconductor, provided that the normal spectrum has an odd number of Fermi points in each half of the Brillouin zone (each spin component counts separately).
2,986 citations
TL;DR: In this paper, the Coulomb blockade for electron tunneling is overcome completely by the Kondo effect, and the conductance reaches the unitary limit value in a semiconductor quantum dot when a small magnetic field is applied.
Abstract: We observe a strong Kondo effect in a semiconductor quantum dot when a small magnetic field is applied. The Coulomb blockade for electron tunneling is overcome completely by the Kondo effect, and the conductance reaches the unitary limit value. We compare the experimental Kondo temperature with the theoretical predictions for the spin- 12 Anderson impurity model. Excellent agreement is found throughout the Kondo regime. Phase coherence is preserved when a Kondo quantum dot is included in one of the arms of an Aharonov-Bohm ring structure, and the phase behavior differs from previous results on a non-Kondo dot.
614 citations
TL;DR: An unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field is reported.
Abstract: The Kondo effect—a many-body phenomenon in condensed-matter physics involving the interaction between a localized spin and free electrons—was discovered in metals containing small amounts of magnetic impurities, although it is now recognized to be of fundamental importance in a wide class of correlated electron systems1,2 In fabricated structures, the control of single, localized spins is of technological relevance for nanoscale electronics3,4 Experiments have already demonstrated artificial realizations of isolated magnetic impurities at metallic surfaces5,6, nanoscale magnets7, controlled transitions between two-electron singlet and triplet states8, and a tunable Kondo effect in semiconductor quantum dots9,10,11,12 Here we report an unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field We observe the effect for an even number of electrons, when the singlet and triplet states are degenerate The characteristic energy scale is much larger than in the ordinary spin-1/2 case
388 citations
TL;DR: A theory of spin dependent transport in an electronic circuit involving ferromagnetic elements with noncollinear magnetization which is based on the conservation of spin and charge current is formulated.
Abstract: We formulate a theory of spin dependent transport in an electronic circuit involving ferromagnetic elements with noncollinear magnetization which is based on the conservation of spin and charge current. The theory considerably simplifies the calculation of the transport properties of complicated ferromagnet--normal metal systems. We illustrate the theory by considering a novel three-terminal device.
388 citations
TL;DR: Using the algebraic Bethe ansatz method and the solution of the quantum inverse scattering problem for local spins, this article obtained multiple integral representations of the n-point correlation functions of the XXZ Heisenberg spin-1 2 chain in a constant magnetic field.
377 citations
TL;DR: A quantum dot in the Coulomb blockade regime weakly coupled to current leads is considered and it is shown that in the presence of a magnetic field it acts as an efficient spin filter (at the single-spin level), producing a spin-polarized current.
Abstract: We consider a quantum dot in the Coulomb blockade regime weakly coupled to current leads and show that in the presence of a magnetic field it acts as an efficient spin filter (at the single-spin level), producing a spin-polarized current. Conversely, if the leads are fully spin polarized the up or the down state of the spin on the dot results in a large sequential or a small cotunneling current, and, thus, together with ESR techniques, the setup can be operated as a single-spin memory.
247 citations
TL;DR: In this article, a comprehensive inelastic neutron scattering study of magnetic excitations in the near optimally doped high-temperature superconductor YBa2Cu3O6.85 is presented.
Abstract: A comprehensive inelastic neutron scattering study of magnetic excitations in the near optimally doped high-temperature superconductor YBa2Cu3O6.85 is presented. The spin correlations in the normal state are commensurate with the crystal lattice, and the intensity is peaked around the wave vector characterizing the antiferromagnetic state of the insulating precursor, YBa2Cu3O6. Profound modifications of the spin excitation spectrum appear abruptly below the superconducting transition temperature Tc, where a commensurate resonant mode and a set of weaker incommensurate peaks develop. The data are consistent with models that are based on an underlying two-dimensional Fermi surface, predicting a continuous, downward dispersion relation connecting the resonant mode and the incommensurate excitations. The magnetic incommensurability in the YBa2Cu3O6+xsystem is thus not simply related to that of another high-temperature superconductor, La2–xSrxCuO4, where incommensurate peaks persist well aboveTc. The temperature-dependent incommensurability is difficult to reconcile with interpretations based on charge stripe formation in YBa2Cu3O6+xnear optimum doping.
189 citations
TL;DR: In this article, the authors studied the localization of various spin fields on a string-like defect in a general space-time dimension from the viewpoint of field theory, and showed that spin 0 and 2 fields are localized on a defect with the exponentially decreasing warp factor.
187 citations
TL;DR: The phase evolution of electrons as they traverse a quantum dot formed in a two-dimensional electron gas that serves as a localized spin is measured, with a range twice as large as theoretically predicted.
Abstract: We measured the phase evolution of electrons as they traverse a quantum dot (QD) formed in a two-dimensional electron gas that serves as a localized spin. The traversal phase, determined by embedding the QD in a double path electron interferometer and measuring the quantum interference of the electron wave functions manifested by conductance oscillation as a function of a weak magnetic field, evolved by π radians, a range twice as large as theoretically predicted. As the correlation weakened, a gradual transition to the familiar phase evolution of a QD was observed. The specific phase evolution observed is highly sensitive to the onset of Kondo correlation, possibly serving as an alternative fingerprint of the Kondo effect.
180 citations
TL;DR: In this article, the feasibility of quantum communication with entangled electrons in an interacting many-body environment was investigated using an interference experiment using a scattering setup with an entangler and a beam splitter, and it was shown that due to electron-electron interaction, the spin correlation of the entangled singlet and triplet states is reduced by z(F 2 ) in a conductor described by Fermi liquid theory.
Abstract: Addressing the feasibility of quantum communication with entangled electrons in an interacting many-body environment, we propose an interference experiment using a scattering setup with an entangler and a beam splitter. It is shown that, due to electron-electron interaction, the spin correlation of the entangled singlet and triplet states is reduced by z(F)(2) in a conductor described by Fermi liquid theory. We calculate the quasiparticle weight factor z(F) for a two-dimensional electron system. The current noise for electronic singlet states turns out to be enhanced (bunching behavior), while it is reduced for triplet states (antibunching). Within standard scattering theory, we find that the Fano factor (noise-to-current ratio) for singlets is twice as large as for independent classical particles and is reduced to zero for triplets.
161 citations
TL;DR: The exact eigenspectra and eigenstates of spin-1 and spin-2 Bose-Einstein condensates (BECs) are found, and their response to a weak magnetic field is studied and compared with their mean-field counterparts.
Abstract: The exact eigenspectra and eigenstates of spin-1 and spin-2 Bose-Einstein condensates (BECs) are found, and their response to a weak magnetic field is studied and compared with their mean-field counterparts. Whereas mean-field theory predicts the vanishing population of the zero magnetic-quantum-number component of a spin-1 antiferromagnetic BEC, the component is found to become populated as the magnetic field decreases. The spin-2 BEC exhibits an even richer magnetic response due to quantum correlations among three bosons.
TL;DR: Theoretical and experimental studies of the optical injection in semiconductors of spin-polarized currents, and pure spin currents, by one and two-photon absorption across the band gap are reviewed.
Abstract: We show that quantum interference of one and two photon absorption from a two color field allows one to optically inject ballistic spin currents in unbiased semiconductors. The spin currents can be generated with or without an accompanying electrical current and can be controlled using the relative phase of the two colors. We characterize the injected spin currents using symmetry arguments and an eight-band Kane model.
TL;DR: In this paper, the authors studied submicron lateral spin valve junctions, based on highmobility InAs/AlSb two-dimensional electron gas, with Ni, Co, and permalloy as ferromagnetic electrodes.
Abstract: The electrical injection of spin-polarized electrons in a semiconductor can be achieved in principle by driving a current from a ferromagnetic metal, where current is known to be significantly spin polarized, into the semiconductor via Ohmic conduction. For detection a second ferromagnet can be used as a drain. We studied submicron lateral spin valve junctions, based on high-mobility InAs/AlSb two-dimensional electron gas, with Ni, Co, and permalloy as ferromagnetic electrodes. In the standard geometry it is very difficult to separate true spin injection from other effects, including local Hall effect, anomalous magnetoresistance contribution from the ferromagnetic electrodes and weak localization/antilocalization corrections, which can closely mimic the signal expected from spin valve effect. The reduction in size, and the use of a multiterminal nonlocal geometry allowed us to reduce the unwanted effects to a minimum. Despite all our efforts, we have not been able to observe spin injection. However, we find that this ‘‘negative’’ result in these systems is actually consistent with theoretical predictions for spin transport in diffusive systems.
01 Apr 2000
TL;DR: For heavy-light mesons, a relativistic symmetry that suppresses splittings in hadrons is identified and it is argued that the dynamics necessary for this symmetry are possible in QCD.
Abstract: Experimental data indicate small spin-orbit splittings in hadrons. For heavy-light mesons we identify a relativistic symmetry that suppresses these splittings. We suggest an experimental test in electron-positron annihilation. Furthermore, we argue that the dynamics necessary for this symmetry are possible in QCD.
TL;DR: A genuine and somehow unexpected "plaquette resonating valence bond," with spontaneously broken translation symmetry and no broken rotation symmetry, comes out from the numerical simulations as the most plausible ground state for J(2)/J(1) approximately 0.5.
Abstract: We investigate the nonmagnetic phase of the spin-half frustrated Heisenberg antiferromagnet on the square lattice using exact diagonalization (up to 36 sites) and quantum Monte Carlo techniques (up to 144 sites). The spin gap and the susceptibilities for the most important crystal symmetry breaking operators are computed. A genuine and somehow unexpected "plaquette resonating valence bond," with spontaneously broken translation symmetry and no broken rotation symmetry, comes out from our numerical simulations as the most plausible ground state for J(2)/J(1) approximately 0.5.
TL;DR: In this article, a generalised Landau-Zener transition rate theory was proposed to explain the topological quantum interference of two tunnel paths of opposite windings in molecular clusters.
Abstract: The Landau-Zener method allows to measure very small tunnel splittings Δ in molecular clusters Fe8. The observed oscillations of Δ as a function of the magnetic field applied along the hard anisotropy axis are explained in terms of topological quantum interference of two tunnel paths of opposite windings. Studies of the temperature dependence of the Landau-Zener transition rate P gives access to the topological quantum interference between excited spin levels. The influence of nuclear spins is demonstrated by comparing P of the standard Fe8 sample with two isotopically substituted samples. The need of a generalised Landau-Zener transition rate theory is shown.
TL;DR: This work uses a magnetic field to adjust the single-particle-state degeneracy, and finds that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.
Abstract: We determine contributions from the direct Coulomb and exchange interactions to the total interaction in artificial semiconductor atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. The electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle-state degeneracy, and find that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.
TL;DR: In this article, the authors report a comprehensive polarized and unpolarized neutron scattering study of the evolution of the dynamical spin susceptibility with temperature and doping in three underdoped single crystals of the YBCO{6+x} high temperature superconductor.
Abstract: We report a comprehensive polarized and unpolarized neutron scattering study of the evolution of the dynamical spin susceptibility with temperature and doping in three underdoped single crystals of the \YBCO{6+x} high temperature superconductor: \YBCO{6.5} (Tc = 52 K), \YBCO{6.7} (Tc = 67 K), and \YBCO{6.85} (T_c = 87 K). Theoretical implications of these data are discussed, and a critique of recent attempts to relate the spin excitations to the thermodynamics of high temperature superconductors is given.
TL;DR: In this article, the magnetic ground-state properties of a large set of spiral vectors and lattice constants were calculated by means of an advanced version of the augmented spherical waves method which takes into account the full-shape potential and the intra-atomic noncollinearity of the magnetization.
Abstract: Using density-functional theory we calculate the magnetic ground-state properties of $\ensuremath{\gamma}\ensuremath{-}\mathrm{Fe}$ for a large set of spiral vectors and lattice constants. The effective single-particle equations are solved by means of an advanced version of the augmented spherical waves method which takes into account the full-shape potential and the intra-atomic noncollinearity of the magnetization. Together with the generalized gradient approximation the experimentally determined spiral magnetic ground state is reproduced successfully. Symmetry properties of the intra-atomic noncollinearity of the magnetization are analyzed and illustrated for spiral magnetic structures. We conclude that $\ensuremath{\gamma}\ensuremath{-}\mathrm{Fe}$ is an itinerant electron system possessing well-defined atomic moments.
TL;DR: In this paper, a theory of quasiparticle localization in superconductors in situations without spin rotation invariance is developed, and the existence and properties of superconducting phases with localized/delocalized quasIParticle excitations in such systems in various dimensionalities are discussed.
Abstract: We develop a theory of quasiparticle localization in superconductors in situations without spin rotation invariance. We discuss the existence and properties of superconducting phases with localized/delocalized quasiparticle excitations in such systems in various dimensionalities. Implications for a variety of experimental systems, and to the properties of random Ising models in two dimensions, are briefly discussed.
TL;DR: In this paper, the local spin and field operators of fundamental graded models were derived in terms of the elements of the monodromy matrix, which is a quantum analogue of the classical inverse scattering transform.
Abstract: We derive a formula that expresses the local spin and field operators of fundamental graded models in terms of the elements of the monodromy matrix. This formula is a quantum analogue of the classical inverse scattering transform. It applies to fundamental spin chains, such as the XYZ chain, and to a number of important exactly solvable models of strongly correlated electrons, such as the supersymmetric t-J model or the the EKS model.
TL;DR: In this paper, the spin-exchange coupling between two electrons located in two vertically tunnel-coupled quantum dots, and its variation when magnetic (B) and electric (E) fields (both in-plane and perpendicular) are applied.
Abstract: We determine the spin-exchange coupling J between two electrons located in two vertically tunnel-coupled quantum dots, and its variation when magnetic (B) and electric (E) fields (both in-plane and perpendicular) are applied. We predict a strong decrease of J as the in-plane B field is increased, mainly due to orbital compression. Combined with the Zeeman splitting, this leads to a singlet-triplet crossing, which can be observed as a pronounced jump in the magnetization at in-plane fields of a few T, and perpendicular fields of the order of 10 T for typical self-assembled dots. We use harmonic potentials to model the confining of electrons, and calculate the exchange J using the Heitler-London and Hund-Mulliken techniques, including the long-range Coulomb interaction. With our results we provide experimental criteria for the distinction of singlet and triplet states, and therefore for microscopic spin measurements. In the case where dots of different sizes are coupled, we present a simple method to switch the spin coupling on and off with exponential sensitivity using an in-plane electric field. Switching the spin coupling is essential for quantum computation using electronic spins as qubits.
TL;DR: In this paper, a formula for the single transverse spin asymmetry in the large p T pion production in the nucleon-nucleon collision is derived, where the chiral-odd contribution where the transversity distribution and the spin-independent twist-3 distribution contributes.
TL;DR: In this article, the effects of Kondo correlations on the transmission phase shift of a quantum dot in an Aharonov-Bohm ring were studied, and it was shown that the development of a Kondo resonance should affect the dependence of the phase shift on transport voltage, gate voltage, and temperature.
Abstract: We study the effects of Kondo correlations on the transmission phase shift of a quantum dot in an Aharonov-Bohm ring. We predict in detail how the development of a Kondo resonance should affect the dependence of the phase shift on transport voltage, gate voltage, and temperature. This system should allow the first direct observation of the well-known scattering phase shift of pi/2 expected (but not directly measurable in bulk systems) at zero temperature for an electron scattering off a spin- 1 / 2 impurity that is screened into a singlet.
TL;DR: In this article, a consequence of the Dirac equation in the exterior gravitational field of a rotating mass is considered in detail, namely, the difference in the energy of a spin-½ particle polarized vertically up and down near the surface of the rotating body is Ωsin θ.
Abstract: The gravitational couplings of intrinsic spin are briefly reviewed. A consequence of the Dirac equation in the exterior gravitational field of a rotating mass is considered in detail, namely, the difference in the energy of a spin-½ particle polarized vertically up and down near the surface of a rotating body is Ωsin θ. Here θ is the latitude and Ω = 2GJ/(c2R3), where J and R are, respectively, the angular momentum and radius of the body. It seems that this relativistic quantum gravitational effect could be measurable in the foreseeable future.
TL;DR: In this paper, the spin Coulomb drag is introduced, which is an intrinsic source of friction for spin currents due to the Coulomb interaction between spin ''up'' and ''down'' electrons.
Abstract: We introduce a distinctive feature of spin-polarized transport, the spin Coulomb drag: there is an intrinsic source of friction for spin currents due to the Coulomb interaction between spin ``up'' and spin ``down'' electrons. We calculate the associated ``spin trans-resistivity'' in a generalized random-phase approximation and show that, to the leading order in the interactions, it has no contribution from correlated impurity scattering. We show that, in an appropriate range of parameters, such resistivity is measurable, and we propose an experiment to measure it.
TL;DR: In this article, the gap equations for a color-superconducting condensate with total spin J = 0 in dense QCD were derived perturbatively and the prefactor was computed to leading logarithmic accuracy.
Abstract: We derive perturbatively the gap equations for a color-superconducting condensate with total spin J=0 in dense QCD. At zero temperature, we confirm the results of Son for the dependence of the condensate on the coupling constant, and compute the prefactor to leading logarithmic accuracy. At nonzero temperature, we find that to leading order in weak coupling, the temperature dependence of the condensate is identical to that in BCS-like theories. The condensates for total spin J=1 are classified; to leading logarithmic accuracy these condensates are of the same order as those of spin J=0. (c) 2000 The American Physical Society.
TL;DR: The behavior of spin diffusion in doped semiconductors is shown to be qualitatively different than in undoped (intrinsic) ones, and the motion of spin packets polarized antiparallel to the equilibrium carrier spin polarization is predicted to be an order of magnitude faster than for parallel polarized spin packets.
Abstract: The behavior of spin diffusion in doped semiconductors is shown to be qualitatively different than in undoped (intrinsic) ones. Whereas a spin packet in an intrinsic semiconductor must be a multiple-band disturbance, involving inhomogeneous distributions of both electrons and holes, in a doped semiconductor a single-band disturbance is possible. For $n$-doped nonmagnetic semiconductors the enhancement of diffusion due to a degenerate electron sea in the conduction band is much larger for these single-band spin packets than for charge packets---this explains the anomalously large spin diffusion recently observed in $n$-doped GaAs at 1.6 K. In $n$-doped ferromagnetic and semimagnetic semiconductors the motion of spin packets polarized antiparallel to the equilibrium carrier spin polarization is predicted to be an order of magnitude faster than for parallel polarized spin packets. These results are reversed for $p$-doped semiconductors.
TL;DR: In this paper, the scale dependence of the twist-3 quark-gluon parton distributions was studied using the observation that in the multi-color limit the corresponding QCD evolution equations possess an additional integral of motion and turn out to be effectively equivalent to the Schrodinger equation.
TL;DR: In this article, the authors investigated the spin dynamics of photoexcited carriers in GaAs/AlxGa1-xAs quantum wells, allowing full investigation of well-width and temperature dependences with minimal accidental variations due to growth conditions.
Abstract: Spin dynamics of photoexcited carriers in GaAs/AlxGa1-xAs quantum wells have been investigated in a wafer containing twelve different single quantum wells, allowing full investigation of well-width and temperature dependences with minimal accidental variations due to growth conditions. The behavior at low temperatures is dominated by excitonic effects, confirming results described in detail by others. Between 50 and 90 K there is a transition from excitonic to free-carrier-dominated behavior; both the temperature and time scale of the transition are in excellent agreement with a theoretical model for exciton dissociation. Above 90 K we find two-component spin decays consisting of an unresolved component (faster than 2 ps) associated with exciton dissociation and hole spin-relaxation and a longer-lived component that yields the electron spin-relaxation time. In the free-carrier regime, the electron spin-relaxation rate in wide wells follows that for bulk GaAs, which varies approximately as T2. For narrow wells the rate is approximately independent of temperature and varies quadratically with confinement energy. This behavior is consistent with dominance of the D'yakonov-Perel mechanism of electron-spin relaxation and the expected behavior of the electron mobility. The data show evidence of the influence of electron scattering by interface roughness.