Showing papers on "Spin-½ published in 2007"
15 Dec 2007
TL;DR: In this paper, a review of quantumphase transition from one state to another is provided by adjusting a tuning parameter other than temperature, and their consequences for finite temperature experiments are discussed.
Abstract: Thermal fluctuations induced by increasing temperature can change the state of matter, for example, when water boils to steam. It also is possible to change the state of matter at absolute zero temperature by quantum fluctuations demanded by Heisenberg's uncertainty principle. In this case, the quantumphase transition from one state to another is provided by adjusting a tuning parameter other than temperature. A few characteristic examples of quantumphase transitions are reviewed, and their consequences for finite temperature experiments are discussed.
Keywords:
quantum phase transitions;
broken symmetry;
Landau theory;
Berry phases;
confinement;
quantum criticality;
deconfined criticality;
spin gap;
monopole;
valence bond solid
1,270 citations
TL;DR: The experimentally realized coherent control of a single-electron spin in a quantum dot using an oscillating electric field generated by a local gate to establish the feasibility of fully electrical manipulation of spin qubits.
Abstract: Manipulation of single spins is essential for spin-based quantum information processing Electrical control instead of magnetic control is particularly appealing for this purpose, because electric fields are easy to generate locally on-chip We experimentally realized coherent control of a single-electron spin in a quantum dot using an oscillating electric field generated by a local gate The electric field induced coherent transitions (Rabi oscillations) between spin-up and spin-down with 90 degrees rotations as fast as approximately 55 nanoseconds Our analysis indicated that the electrically induced spin transitions were mediated by the spin-orbit interaction Taken together with the recently demonstrated coherent exchange of two neighboring spins, our results establish the feasibility of fully electrical manipulation of spin qubits
852 citations
Journal Article•
628 citations
01 Jan 2007
TL;DR: The new measurement has an uncertainty that is about six times smaller, and it shifts the values by 1.7 standard deviations, so the lowest quantum levels of the spin and cyclotron motion are resolved, and the cycle frequencies as well as spin frequencies are determined using quantum jump spectroscopy.
Abstract: The new measurement has an uncertainty that is about six times smaller, and it shifts the values by 1.7 standard deviations. One electron suspended in a Penning trap is used for the new measurement, like in the old measurement. What is different is that the lowest quantum levels of the spin and cyclotron motion are resolved, and the cyclotron as well as spin frequencies are determined using quantum jump spectroscopy.
616 citations
TL;DR: A spin chain is a permanently coupled 1D system of spins as discussed by the authors, which can be used to connect quantum registers without resorting to optics, and it has been shown that it is possible to achieve perfect quantum state transfer through spin chains.
Abstract: We present an introductory overview of the use of spin chains as quantum wires, which has recently developed into a topic of lively interest. The principal motivation is in connecting quantum registers without resorting to optics. A spin chain is a permanently coupled 1D system of spins. When one places a quantum state on one end of it, the state will be dynamically transmitted to the other end with some efficiency if the spins are coupled by an exchange interaction. No external modulations or measurements on the body of the chain, except perhaps at the very ends, is required for this purpose. For the simplest (uniformly coupled) chain and the simplest encoding (single qubit encoding), however, dispersion reduces the quality of transfer. We present a variety of alternatives proposed by various groups to achieve perfect quantum state transfer through spin chains. We conclude with a brief discussion of the various directions in which the topic is developing.
439 citations
TL;DR: Based on the observations that their in-plane lattice constants match almost perfectly and their electronic structures overlap in reciprocal space for one spin direction only, the authors predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co.
Abstract: Based upon the observations (i) that their in-plane lattice constants match almost perfectly and (ii) that their electronic structures overlap in reciprocal space for one spin direction only, we predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co The spin filtering is quite insensitive to roughness and disorder The formation of a chemical bond between graphite and the open d-shell transition metals that might complicate or even prevent spin injection into a single graphene sheet can be simply prevented by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin-injection property
418 citations
TL;DR: In this article, the authors consider high spin operators and give a general argument for the logarithmic scaling of their anomalous dimensions which is based on the symmetries of the problem.
Abstract: We consider high spin operators. We give a general argument for the logarithmic scaling of their anomalous dimensions which is based on the symmetries of the problem. By an analytic continuation we can also see the origin of the double logarithmic divergence in the Sudakov factor. We show that the cusp anomalous dimension is the energy density for a flux configuration of the gauge theory on AdS3 × S1. We then focus on operators in = 4 super Yang Mills which carry large spin and SO(6) charge and show that in a particular limit their properties are described in terms of a bosonic O(6) sigma model. This can be used to make certain all loop computations in the string theory.
395 citations
TL;DR: This work proposes to use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins with S = 1/2 can be coupled through the electrons of the central core, and two-qubit gates and qubit readout can be implemented.
Abstract: Spin qubits offer one of the most promising routes to the implementation of quantum computers. Very recent results in semiconductor quantum dots show that electrically-controlled gating schemes are particularly well-suited for the realization of a universal set of quantum logical gates. Scalability to a larger number of qubits, however, remains an issue for such semiconductor quantum dots. In contrast, a chemical bottom-up approach allows one to produce identical units in which localized spins represent the qubits. Molecular magnetism has produced a wide range of systems with properties that can be tailored, but so far, there have been no molecules in which the spin state can be controlled by an electrical gate. Here we propose to use the polyoxometalate [PMo12O40(VO)2]q-, where two localized spins with S = 1/2 can be coupled through the electrons of the central core. Through electrical manipulation of the molecular redox potential, the charge of the core can be changed. With this setup, two-qubit gates and qubit readout can be implemented.
365 citations
TL;DR: In this article, precise measurements of the spin structure functions of the proton g1p(x,Q2) and deuteron g1d(x and Q2) were presented over the kinematic range 0.0041≤x≤0.9 and 0.18
Abstract: Precise measurements of the spin structure functions of the proton g1p(x,Q2) and deuteron g1d(x,Q2) are presented over the kinematic range 0.0041≤x≤0.9 and 0.18 GeV2≤Q2≤20 GeV2. The data were collected at the HERMES experiment at DESY, in deep-inelastic scattering of 27.6 GeV longitudinally polarized positrons off longitudinally polarized hydrogen and deuterium gas targets internal to the HERA storage ring. The neutron spin structure function g1n is extracted by combining proton and deuteron data. The integrals of g1p,d at Q2=5 GeV2 are evaluated over the measured x range. Neglecting any possible contribution to the g1d integral from the region x≤0.021, a value of 0.330±0.011(theo)±0.025(exp)±0.028(evol) is obtained for the flavor-singlet axial charge a0 in a leading-twist next-to-next-to-leading-order analysis.
301 citations
TL;DR: In this paper, the authors consider high spin operators and give a general argument for the logarithmic scaling of their anomalous dimensions which is based on the symmetries of the problem.
Abstract: We consider high spin operators. We give a general argument for the logarithmic scaling of their anomalous dimensions which is based on the symmetries of the problem. By an analytic continuation we can also see the origin of the double logarithmic divergence in the Sudakov factor. We show that the cusp anomalous dimension is the energy density for a flux configuration of the gauge theory on $AdS_3 \times S^1$. We then focus on operators in ${\cal N}=4$ super Yang Mills which carry large spin and SO(6) charge and show that in a particular limit their properties are described in terms of a bosonic O(6) sigma model. This can be used to make certain all loop computations in the string theory.
297 citations
Posted Content•
TL;DR: In this article, the authors introduce a set of consistency conditions on the S-matrix of theories of massless particles of arbitrary spin in four-dimensional Minkowski space-time.
Abstract: We introduce a set of consistency conditions on the S-matrix of theories of massless particles of arbitrary spin in four-dimensional Minkowski space-time. We find that in most cases the constraints, derived from the conditions, can only be satisfied if the S-matrix is trivial. Our conditions apply to theories where four-particle scattering amplitudes can be obtained from three-particle ones via a recent technique called BCFW construction. We call theories in this class constructible. We propose a program for performing a systematic search of constructible theories that can have non-trivial S-matrices. As illustrations, we provide simple proofs of already known facts like the impossibility of spin $s > 2$ non-trivial S-matrices, the impossibility of several spin 2 interacting particles and the uniqueness of a theory with spin 2 and spin 3/2 particles.
TL;DR: In this article, electrical injection, detection, and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts is presented, where Fe∕Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel.
Abstract: We present the electrical injection, detection, and magnetic field modulation of lateral diffusive spin transport through silicon using surface contacts Fe∕Al2O3 tunnel barrier contacts are used to create and analyze the flow of pure spin current in a silicon transport channel Nonlocal detection techniques show that the spin current detected after transport through the silicon is sensitive to the relative orientation of the magnetization of the injecting and detecting contacts Hanle effect measurements demonstrate that the spin current can be modulated by a perpendicular magnetic field, which causes the spin to precess and dephase in the transport channel
TL;DR: It is proposed that the nu=0 and +/-1 states arise from the lifting of the spin and sublattice degeneracy of the n=0 Landau level, respectively.
Abstract: We investigate the quantum Hall (QH) states near the charge-neutral Dirac point of a high mobility graphene sample in high magnetic fields. We find that the QH states at filling factors $\ensuremath{
u}=\ifmmode\pm\else\textpm\fi{}1$ depend only on the perpendicular component of the field with respect to the graphene plane, indicating that they are not spin related. A nonlinear magnetic field dependence of the activation energy gap at filling factor $\ensuremath{
u}=1$ suggests a many-body origin. We therefore propose that the $\ensuremath{
u}=0$ and $\ifmmode\pm\else\textpm\fi{}1$ states arise from the lifting of the spin and sublattice degeneracy of the $n=0$ Landau level, respectively.
TL;DR: The experimental observations strongly suggest that the primary spin relaxation mechanism in the organic is the Elliott-Yafet mode, in which the spin relaxes whenever a carrier scatters and its velocity changes.
Abstract: Organic semiconductors that are π-conjugated are emerging as an important platform for ‘spintronics’, which purports to harness the spin degree of freedom of a charge carrier to store, process and/or communicate information1. Here, we report the study of an organic nanowire spin valve device, 50 nm in diameter, consisting of a trilayer of ferromagnetic cobalt, an organic, Alq3, and ferromagnetic nickel. The measured spin relaxation time in the organic is found to be exceptionally long—between a few milliseconds and a second—and it is relatively temperature independent up to 100 K. Our experimental observations strongly suggest that the primary spin relaxation mechanism in the organic is the Elliott–Yafet mode, in which the spin relaxes whenever a carrier scatters and its velocity changes.
22 Jan 2007
TL;DR: In this article, a new SU(2) spin rotation symmetry was discovered for spin-orbit coupled systems, which is robust against spin-independent disorder and interactions, and is generated by operators whose wavevector depends on the coupling strength.
Abstract: Spin-orbit coupled systems generally break the spin rotation symmetry. However, for a model with equal Rashba and Dresselhauss coupling constant (the ReD model), and for the [110] Dresselhauss model, a new type of SU(2) spin rotation symmetry is discovered. This symmetry is robust against spin-independent disorder and interactions, and is generated by operators whose wavevector depends on the coupling strength. It renders the spin lifetime infinite at this wavevector, giving rise to a Persistent Spin Helix (PSH). We obtain the spin fluctuation dynamics at, and away, from the symmetry point, and suggest experiments to observe the PSH.
TL;DR: In this article, X-ray structural and magnetic studies on ferric SCO complexes are presented and the possible origin of the rapid spin interconversion is discussed and the transition rate of some SE-type SCO is shown to be 10 7 ǫ s −1.
TL;DR: In this article, an eikonal approximation to high energy interactions in anti-de Sitter spacetime was derived by generalizing a position space derivation of the amplitude in flat space.
Abstract: We derive an eikonal approximation to high energy interactions in Anti-de Sitter spacetime, by generalizing a position space derivation of the eikonal amplitude in flat space We are able to resum, in terms of a generalized phase shift, ladder and cross ladder graphs associated to the exchange of a spin j field, to all orders in the coupling constant Using the AdS/CFT correspondence, the resulting amplitude determines the behavior of the dual conformal field theory four-point function for small values of the cross ratios, in a Lorentzian regime Finally we show that the phase shift is dominated by graviton exchange and computes, in the dual CFT, the anomalous dimension of the double trace primary operators O-1 partial derivativepartial derivative O-2 of large dimension and spin, corresponding to the relative motion of the two interacting particles The results are valid at strong t'Hooft coupling and are exact in the 1/N expansion
TL;DR: In this article, it was shown that polar molecules in optical lattices driven by microwave fields naturally give rise to Hubbard models with strong nearest-neighbour three-body interactions, whereas the two-body terms can be tuned with external fields.
Abstract: Fundamental interactions between particles, such as the Coulomb law, involve pairs of particles, and our understanding of the plethora of phenomena in condensed-matter physics rests on models involving effective two-body interactions. On the other hand, exotic quantum phases, such as topological phases or spin liquids, are often identified as ground states of hamiltonians with three- or more-body terms. Although the study of these phases and the properties of their excitations is currently one of the most exciting developments in theoretical condensed-matter physics, it is difficult to identify real physical systems exhibiting such properties. Here, we show that polar molecules in optical lattices driven by microwave fields naturally give rise to Hubbard models with strong nearest-neighbour three-body interactions, whereas the two-body terms can be tuned with external fields. This may open a new route for an experimental study of exotic quantum phases with quantum degenerate molecular gases.
TL;DR: In this paper, a measurement of the deuteron spin-dependent structure function g 1 d based on the data collected by the COMPASS experiment at CERN during the years 2002-2004 is presented.
TL;DR: In this paper, it is argued that the resonating valence bond (RVB) picture, as formulated using gauge theory with fermionic and bosonic matter fields, gives an adequate physical understanding, even though many details are beyond the powers of current calculational tools.
Abstract: This review gives a rather general discussion of high temperature superconductors as an example of a strongly correlated material. The argument is made that in view of the many examples of unconventional superconductors discovered in the past twenty years, we should no longer be surprised that superconductivity emerges as a highly competitive ground state in systems where Coulomb repulsion plays a dominant role. The physics of the cuprates is discussed, emphasizing the unusual pseudogap phase in the underdoped region. It is argued that the resonating valence bond (RVB) picture, as formulated using gauge theory with fermionic and bosonic matter fields, gives an adequate physical understanding, even though many details are beyond the powers of current calculational tools. The recent discovery of quantum oscillations in a high magnetic field is discussed in this context. Meanwhile, the problem of the quantum spin liquid (a spin system with antiferromagnetic coupling which refuses to order even at zero temperature) is a somewhat simpler version of the high $T_c$ problem where significant progress has been made recently. It is understood that the existence of matter fields can lead to de-confinement of the U(1) gauge theory in 2+1 dimensions, and novel new particles (called fractionalized particles), such as fermionic spinons which carry spin ${1\over 2}$ and no charge, and gapless gauge bosons can emerge to create a new critical state at low energies. We even have a couple of real materials where such a scenario may be realized experimentally. The article ends with answers to questions such as: what limits $T_c$ if pairing is driven by an electronic energy scale? why is the high $T_c$ problem hard? why is there no consensus? and why is the high $T_c$ problem important?
TL;DR: Measurements reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases, and demonstrate that the pseudogap phenomenon in the electron-doped materials arises from a build-up of spin correlations, in agreement with recent theoretical proposals.
Abstract: High-transition-temperature (high-T(c)) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd2-xCexCuO4+/-delta (NCCO) shows bulk superconductivity above x approximately 0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x approximately 0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies, arises from a build-up of spin correlations, in agreement with recent theoretical proposals.
TL;DR: The complete set of generalized spin squeezing inequalities are determined, which can be used for the experimental detection of entanglement in a system of spin-1/2 particles in which the spins cannot be individually addressed.
Abstract: We determine the complete set of generalized spin squeezing inequalities. These are entanglement criteria that can be used for the experimental detection of entanglement in a system of spin-$\frac{1}{2}$ particles in which the spins cannot be individually addressed. They can also be used to show the presence of bound entanglement in the thermal states of several spin models.
TL;DR: Two-component attractive Fermi gases with imbalanced spin populations in trapped one-dimensional configurations with ground state properties determined with the local density approximation starting from the exact Bethe-ansatz equations for the homogeneous case.
Abstract: We investigate two-component attractive Fermi gases with imbalanced spin populations in trapped one-dimensional configurations. The ground state properties are determined with the local density approximation, starting from the exact Bethe-ansatz equations for the homogeneous case. We predict that the atoms are distributed according to a two-shell structure: a partially polarized phase in the center of the trap and either a fully paired or a fully polarized phase in the wings. The partially polarized core is expected to be a superfluid of the Fulde-Ferrell-Larkin-Ovchinnikov type. The size of the cloud as well as the critical spin polarization needed to suppress the fully paired shell are calculated as a function of the coupling strength.
Journal Article•
TL;DR: In this article, the half-integer quantum Hall (QH) effect in two-dimensional electron and hole gas is studied in high quality graphene samples and a distinctive halfinteger QH effect is discovered with QH plateaus appearing at a filling factor sequence, ν = 4 (n + 1 / 2 ), where n is the Landau level (LL) index.
TL;DR: In this article, the authors used inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La2−xSrxCuO4 are more structured than previously thought.
Abstract: The excitations responsible for producing high-temperature superconductivity in the copper oxides have yet to be identified. Two promising candidates are collective spin excitations and phonons1. A recent argument against spin excitations is based on their inability to explain structures observed in electronic spectroscopies such as photoemission2,3,4,5 and optical conductivity6,7. Here, we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La2−xSrxCuO4 are more structured than previously thought. The excitations have a two-component structure with a low-frequency component strongest around 18 meV and a broader component peaking near 40–70 meV. The second component carries most of the spectral weight and its energy matches structures observed in photoemission2,3,4,5 in the range 50–90 meV. Our results demonstrate that collective spin excitations can explain features of electronic spectroscopies and are therefore likely to be strongly coupled to the electron quasiparticles.
TL;DR: This work identifies the polarized superfluid as having an Fulde-Ferrell-Larkin-Ovchinnikov structure, and predicts the resulting mode frequency as a function of the spin polarization, and identifies the ground state of a one-dimensional trapped Fermi gas with imbalanced spin population.
Abstract: Based on the integrable Gaudin model and local density approximation, we discuss the ground state of a one-dimensional trapped Fermi gas with imbalanced spin population, for an arbitrary attractive interaction. A phase separation state, with a polarized superfluid core immersed in an unpolarized superfluid shell, emerges below a critical spin polarization. Above it, coexistence of polarized superfluid matter and a fully polarized normal gas is favored. These two exotic states could be realized experimentally in highly elongated atomic traps, and diagnosed by measuring the lowest density compressional mode. We identify the polarized superfluid as having an Fulde-Ferrell-Larkin-Ovchinnikov structure, and predict the resulting mode frequency as a function of the spin polarization.
TL;DR: In this paper, the spin polarization of Co2FeAlxSi1−x (x=00, 03, 05, 07) bulk alloys measured by the point contact Andreev reflection method was reported.
Abstract: We report the spin polarization of Co2FeAlxSi1−x (x=00, 03, 05, 07) bulk alloys measured by the point contact Andreev reflection method All the Co2FeAlxSi1−x alloys had an L21 structure along with A2- and B2-type disorder Several off-stoichiometric alloys (CoxFeyAl05Si05) were prepared to understand the effect of the compositional deviation from the stoichiometry on the spin polarization By substituting Al for Si, the spin polarization changed from 057±001 for x=00 to a maximum value of 060±001 for x=05 The off-stoichiometric alloys had spin polarizations of 057−060±001 Ab initio calculations were performed to interpret the effect of Al addition as well as the effect of disorder on the magnetic properties and on the electronic structure
TL;DR: Graphene's exchange and random-phase-approximation correlation energies are evaluated to show that the tendency of Coulomb interactions in lightly doped graphene to favor states with larger net chirality leads to suppressed spin and charge susceptibilities.
Abstract: Graphene is described at low energy by a massless Dirac equation whose eigenstates have definite chirality. We show that the tendency of Coulomb interactions in lightly doped graphene to favor states with larger net chirality leads to suppressed spin and charge susceptibilities. Our conclusions are based on an evaluation of graphene's exchange and random-phase-approximation correlation energies. The suppression is a consequence of the quasiparticle chirality switch which enhances quasiparticle velocities near the Dirac point.
TL;DR: The experimental results are confirmed by first-principles calculations, and it is concluded that the thermally excited spin-state transition occurs from a low to an intermediate spin state, which can be distinguished from the high-spin state.
Abstract: LaCoO3 exhibits an anomaly in its magnetic susceptibility around 80 K associated with a thermally excited transition of the Co 3� -ion spin. We show that electron energy-loss spectroscopy is sensitive to this Co 3� -ion spin-state transition, and that the O K edge prepeak provides a direct measure of the Co 3� spin state in LaCoO3 as a function of temperature. Our experimental results are confirmed by first-principles calculations, and we conclude that the thermally excited spin-state transition occurs from a low to an intermediate spin state, which can be distinguished from the high-spin state.