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Showing papers on "Spin-½ published in 2016"


Journal ArticleDOI
Abstract: "Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits The field is expected to provide new opportunities - especially with regard to high sensitivity and precision - in applied physics and other areas of science In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist

1,878 citations


Journal ArticleDOI
TL;DR: A magnetotransport study of zirconium pentatelluride, ZrTe5, has been carried out in this paper, which reveals evidence for a chiral magnetic effect, a striking macroscopic manifestation of the quantum and relativistic nature of Weyl semimetals.
Abstract: A magnetotransport study of zirconium pentatelluride now reveals evidence for a chiral magnetic effect, a striking macroscopic manifestation of the quantum and relativistic nature of Weyl semimetals The chiral magnetic effect is the generation of an electric current induced by chirality imbalance in the presence of a magnetic field It is a macroscopic manifestation of the quantum anomaly1,2 in relativistic field theory of chiral fermions (massless spin 1/2 particles with a definite projection of spin on momentum)—a remarkable phenomenon arising from a collective motion of particles and antiparticles in the Dirac sea The recent discovery3,4,5,6 of Dirac semimetals with chiral quasiparticles opens a fascinating possibility to study this phenomenon in condensed matter experiments Here we report on the measurement of magnetotransport in zirconium pentatelluride, ZrTe5, that provides strong evidence for the chiral magnetic effect Our angle-resolved photoemission spectroscopy experiments show that this material’s electronic structure is consistent with a three-dimensional Dirac semimetal We observe a large negative magnetoresistance when the magnetic field is parallel with the current The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect The observed phenomenon stems from the effective transmutation of a Dirac semimetal into a Weyl semimetal induced by parallel electric and magnetic fields that represent a topologically non-trivial gauge field background We expect that the chiral magnetic effect may emerge in a wide class of materials that are near the transition between the trivial and topological insulators

806 citations


Journal ArticleDOI
24 Nov 2016-Nature
TL;DR: This work discusses SOC as a means of producing such fundamentally new physical phenomena in thin films and heterostructures and puts into context the technological promise of these material classes for developing spin-based device applications at room temperature.
Abstract: Spin–orbit coupling (SOC) describes the relativistic interaction between the spin and momentum degrees of freedom of electrons, and is central to the rich phenomena observed in condensed matter systems. In recent years, new phases of matter have emerged from the interplay between SOC and low dimensionality, such as chiral spin textures and spin-polarized surface and interface states. These low-dimensional SOC-based realizations are typically robust and can be exploited at room temperature. Here we discuss SOC as a means of producing such fundamentally new physical phenomena in thin films and heterostructures. We put into context the technological promise of these material classes for developing spin-based device applications at room temperature. The interplay between spin–orbit coupling and two-dimensionality has led to the emergence of new phases of matter, such as spin-polarized surface states in topological insulators, interfacial chiral spin interactions, and magnetic skyrmions in thin films, with great potential for spin-based devices. Substantial progress in the past decade in the fabrication and modelling of atomically precise interfaces and surfaces has led to the discovery of many electronic effects that are of interest for practical devices with novel functionalities. Christos Panagopoulos and colleagues review various such effects—and their technological potential—with a focus on the role of spin–orbit coupling, the fundamental interaction between the spin and charge degrees of freedom of an electron. Spin–orbit coupling can affect the electronic properties of materials at reduced dimensions, and the authors discuss the basic principles for understanding and engineering interfaces and surfaces in which spin–orbit coupling is harnessed. Examples are structures based on topological insulators where spin currents are generated or converted and magnetic layers where spin–orbit coupling leads to spin textures that can be controlled.

775 citations


Journal ArticleDOI
30 Jun 2016-Nature
TL;DR: This work reports an alternative platform for the study of spin systems, using individual atoms trapped in tunable two-dimensional arrays of optical microtraps with arbitrary geometries, to establish arrays of single Rydberg atoms as a versatile platform forThe study of quantum magnetism.
Abstract: Spin models are the prime example of simplified many-body Hamiltonians used to model complex, strongly correlated real-world materials. However, despite the simplified character of such models, their dynamics often cannot be simulated exactly on classical computers when the number of particles exceeds a few tens. For this reason, quantum simulation of spin Hamiltonians using the tools of atomic and molecular physics has become a very active field over the past years, using ultracold atoms or molecules in optical lattices, or trapped ions. All of these approaches have their own strengths and limitations. Here we report an alternative platform for the study of spin systems, using individual atoms trapped in tunable two-dimensional arrays of optical microtraps with arbitrary geometries, where filling fractions range from 60 to 100 per cent. When excited to high-energy Rydberg D states, the atoms undergo strong interactions whose anisotropic character opens the way to simulating exotic matter. We illustrate the versatility of our system by studying the dynamics of a quantum Ising-like spin-1/2 system in a transverse field with up to 30 spins, for a variety of geometries in one and two dimensions, and for a wide range of interaction strengths. For geometries where the anisotropy is expected to have small effects on the dynamics, we find excellent agreement with ab initio simulations of the spin-1/2 system, while for strongly anisotropic situations the multilevel structure of the D states has a measurable influence. Our findings establish arrays of single Rydberg atoms as a versatile platform for the study of quantum magnetism.

593 citations


Book
23 Aug 2016
TL;DR: In this article, reduced density operators were used to estimate the mean field and the mean matrix of an EM field, and the spin statistics of the spin statistic was used to determine the distance to the ground truth.
Abstract: Introduction.- Reduced Density Operators.- Correlations due to the Spin Statistics.- Mean-Field Approximation.- Correlations and their Dynamics.- Non-Markovian Effects.- Kinetic Equations with Selfenergy.- Properties of the Kinetic Equation.- T-Matrix Approximation.- Random Phase Approximation.- Screened Ladder Approximation.- Charged Carriers in EM Fields.- Non-Equilibrium Green's Functions.- Kinetics vs. Molecular Dynamics.- Conclusion.

362 citations


Journal ArticleDOI
TL;DR: In this paper, the spin-momentum locking of Dirac surface states offers intriguing possibilities for converting between charge and spin currents, and experiments show that fine tuning of the Fermi level is critical for maximizing the efficiency of such conversions.
Abstract: The spin–momentum locking of Dirac surface states offers intriguing possibilities for converting between charge and spin currents. Experiments show that fine tuning of the Fermi level is critical for maximizing the efficiency of such conversions.

326 citations


Journal ArticleDOI
TL;DR: The observation of an inverse Edelstein effect length much longer than those generally found for Rashba interfaces demonstrates the potential of TIs for the conversion between spin and charge in spintronic devices.
Abstract: We present results on spin to charge current conversion in experiments of resonant spin pumping into the Dirac cone with helical spin polarization of the elemental topological insulator (TI) α-Sn. By angle-resolved photoelectron spectroscopy (ARPES), we first check that the Dirac cone (DC) at the α-Sn (0 0 1) surface subsists after covering Sn with Ag. Then we show that resonant spin pumping at room temperature from Fe through Ag into α-Sn layers induces a lateral charge current that can be ascribed to the inverse Edelstein effect by the DC states. Our observation of an inverse Edelstein effect length much longer than those generally found for Rashba interfaces demonstrates the potential of TIs for the conversion between spin and charge in spintronic devices. By comparing our results with data on the relaxation time of TI free surface states from time-resolved ARPES, we can anticipate the ultimate potential of the TI for spin to charge conversion and the conditions to reach it.

315 citations


Journal ArticleDOI
TL;DR: In this paper, the authors make use of an interface-driven spin-orbit coupling mechanism in the oxide two-dimensional electron system (2DES) LaAlO3/SrTiO3 to achieve spin-to-charge conversion with unprecedented efficiency.
Abstract: The spin-orbit interaction couples the electrons' motion to their spin. Accordingly, passing a current in a material with strong spin-orbit coupling generates a transverse spin current (spin Hall effect, SHE) and vice-versa (inverse spin Hall effect, ISHE). The emergence of SHE and ISHE as charge-to-spin interconversion mechanisms offers a variety of novel spintronics functionalities and devices, some of which do not require any ferromagnetic material. However, the interconversion efficiency of SHE and ISHE (spin Hall angle) is a bulk property that rarely exceeds ten percent, and does not take advantage of interfacial and low-dimensional effects otherwise ubiquitous in spintronics hetero- and mesostructures. Here, we make use of an interface-driven spin-orbit coupling mechanism - the Rashba effect - in the oxide two-dimensional electron system (2DES) LaAlO3/SrTiO3 to achieve spin-to-charge conversion with unprecedented efficiency. Through spin-pumping, we inject a spin current from a NiFe film into the oxide 2DES and detect the resulting charge current, which can be strongly modulated by a gate voltage. We discuss the amplitude of the effect and its gate dependence on the basis of the electronic structure of the 2DES.

274 citations


Journal ArticleDOI
TL;DR: A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields are applied parallel to theeasy axis of the MnF_{2} thin film.
Abstract: We report on the observation of the spin Seebeck effect in antiferromagnetic MnF_{2}. A device scale on-chip heater is deposited on a bilayer of MnF_{2} (110) (30 nm)/Pt (4 nm) grown by molecular beam epitaxy on a MgF_{2} (110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF_{2} through the inverse spin Hall effect. The low temperature (2-80 K) and high magnetic field (up to 140 kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel to the easy axis of the MnF_{2} thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

262 citations


Journal ArticleDOI
TL;DR: An architecture for a silicon-based quantum computer processor based on complementary metal-oxide-semiconductor (CMOS) technology featuring a spin qubit surface code and individual qubit control via floating memory gate electrodes is proposed.
Abstract: Recent advances in quantum error correction (QEC) codes for fault-tolerant quantum computing \cite{Terhal2015} and physical realizations of high-fidelity qubits in a broad range of platforms \cite{Kok2007, Brown2011, Barends2014, Waldherr2014, Dolde2014, Muhonen2014, Veldhorst2014} give promise for the construction of a quantum computer based on millions of interacting qubits. However, the classical-quantum interface remains a nascent field of exploration. Here, we propose an architecture for a silicon-based quantum computer processor based entirely on complementary metal-oxide-semiconductor (CMOS) technology, which is the basis for all modern processor chips. We show how a transistor-based control circuit together with charge-storage electrodes can be used to operate a dense and scalable two-dimensional qubit system. The qubits are defined by the spin states of a single electron confined in a quantum dot, coupled via exchange interactions, controlled using a microwave cavity, and measured via gate-based dispersive readout \cite{Colless2013}. This system, based entirely on available technology and existing components, is compatible with general surface code quantum error correction \cite{Terhal2015}, enabling large-scale universal quantum computation.

242 citations


Journal ArticleDOI
TL;DR: The results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.
Abstract: In iron-based superconductors the interactions driving the nematic order (that breaks four-fold rotational symmetry in the iron plane) may also mediate the Cooper pairing. The experimental determination of these interactions, which are believed to depend on the orbital or the spin degrees of freedom, is challenging because nematic order occurs at, or slightly above, the ordering temperature of a stripe magnetic phase. Here, we study FeSe (ref. )-which exhibits a nematic (orthorhombic) phase transition at Ts = 90 K without antiferromagnetic ordering-by neutron scattering, finding substantial stripe spin fluctuations coupled with the nematicity that are enhanced abruptly on cooling through Ts. A sharp spin resonance develops in the superconducting state, whose energy (∼4 meV) is consistent with an electron-boson coupling mode revealed by scanning tunnelling spectroscopy. The magnetic spectral weight in FeSe is found to be comparable to that of the iron arsenides. Our results support recent theoretical proposals that both nematicity and superconductivity are driven by spin fluctuations.

Journal ArticleDOI
TL;DR: In this article, the authors showed that the spin memory formula is a Fourier transform in time of the recently-discovered sub-leading soft graviton theorem, which is a new type of gravitational "spin memory".
Abstract: The conventional gravitational memory effect is a relative displacement in the position of two detectors induced by radiative energy flux. We find a new type of gravitational ‘spin memory’ in which beams on clockwise and counterclockwise orbits acquire a relative delay induced by radiative angular momentum flux. It has recently been shown that the displacement memory formula is a Fourier transform in time of Weinberg’s soft graviton theorem. Here we see that the spin memory formula is a Fourier transform in time of the recently-discovered subleading soft graviton theorem.

Journal ArticleDOI
TL;DR: It is demonstrated that the spin lifetime does not depend on the contact resistance area products when comparing all bottom-up devices indicating that spin absorption at the contacts is not the predominant source for spin dephasing.
Abstract: We show spin lifetimes of 12.6 ns and spin diffusion lengths as long as 30.5 μm in single layer graphene nonlocal spin transport devices at room temperature. This is accomplished by the fabrication of Co/MgO-electrodes on a Si/SiO2 substrate and the subsequent dry transfer of a graphene-hBN-stack on top of this electrode structure where a large hBN flake is needed in order to diminish the ingress of solvents along the hBN-to-substrate interface. Interestingly, long spin lifetimes are observed despite the fact that both conductive scanning force microscopy and contact resistance measurements reveal the existence of conducting pinholes throughout the MgO spin injection/detection barriers. Compared to previous devices, we observe an enhancement of the spin lifetime in single layer graphene by a factor of 6. We demonstrate that the spin lifetime does not depend on the contact resistance area products when comparing all bottom-up devices indicating that spin absorption at the contacts is not the predominant so...

Journal ArticleDOI
12 May 2016-Nature
TL;DR: Lightwave-driven charge transport is exploited, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain and suggests a promising new way of generating sub-femtosecond pulses.
Abstract: Ever since Ernest Rutherford scattered α-particles from gold foils1, collision experiments have revealed insights into atoms, nuclei and elementary particles2 In solids, many-body correlations lead to characteristic resonances3—called quasiparticles—such as excitons, dropletons4, polarons and Cooper pairs The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity5 However, the extremely short lifetimes of these entities6 make practical implementations of a suitable collider challenging Here we exploit lightwave-driven charge transport7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, the foundation of attosecond science9, 10, 11, 12, 13, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron–hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands17, 18, 19 of the optical excitation A full quantum theory explains our observations microscopically This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses

Journal ArticleDOI
TL;DR: In this article, the authors reported neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in an extremely clean antiferromagnet YbMgGaO4.
Abstract: Quantum spin liquid (QSL) is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. In addition to its relevance to high-temperature superconductivity and quantum-information applications, experimental identification of this new state of matter in its own right is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various QSL ground states, most of which are characterized by exotic spin excitations with fractional quantum numbers (termed `spinon'). However, a conclusive experimental confirmation of any QSL and spinon excitations remains outstanding. Here, we report neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in an extremely clean antiferromagnet YbMgGaO4. The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, which is naturally accounted by the particle-hole excitation of a spinon Fermi surface. Combining this with the low-temperature heat capacity results we propose that YbMgGaO4 is a gapless U(1) QSL with a spinon Fermi surface. Our results therefore identify a QSL in a perfect spin-1/2 triangular lattice occurring in the original proposal by Anderson.

Journal ArticleDOI
TL;DR: The promise for spin-liquid behavior in the 4d5 honeycomb halide α-RuCl3 is discussed, and indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field are found.
Abstract: Using quantum chemistry calculations we shed fresh light on the electronic structure and magnetic properties of RuCl3, a proposed realization of the honeycomb Kitaev spin model. It is found that the nearest-neighbor Kitaev exchange K is weaker than in 5d5 Ir oxides but still larger than other effective spin couplings. The electronic-structure computations also indicate a ferromagnetic K in the halide, which is supported by a detailed analysis of the field-dependent magnetization. From exact-diagonalization calculations for extended Kitaev-Heisenberg Hamiltonians we additionally find that a transition from zigzag order to a spin-liquid ground state can be induced in RuCl3 with external magnetic field.


Journal ArticleDOI
TL;DR: A bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions, will pave a route to the engineering of magnetically functionalized Dirac materials.
Abstract: For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm(2)/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials.

Journal ArticleDOI
TL;DR: In this article, a novel electric dipole-mediated mechanism of nonlinear terahertz-spin coupling that is much stronger than linear Zeeman coupling to the teraherstz magnetic field was explored.
Abstract: Future information technologies, such as ultrafast data recording, quantum computation or spintronics, call for ever faster spin control by light1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16. Intense terahertz pulses can couple to spins on the intrinsic energy scale of magnetic excitations5, 11. Here, we explore a novel electric dipole-mediated mechanism of nonlinear terahertz-spin coupling that is much stronger than linear Zeeman coupling to the terahertz magnetic field5, 10. Using the prototypical antiferromagnet thulium orthoferrite (TmFeO3), we demonstrate that resonant terahertz pumping of electronic orbital transitions modifies the magnetic anisotropy for ordered Fe3+ spins and triggers large-amplitude coherent spin oscillations. This mechanism is inherently nonlinear, it can be tailored by spectral shaping of the terahertz waveforms and its efficiency outperforms the Zeeman torque by an order of magnitude. Because orbital states govern the magnetic anisotropy in all transition-metal oxides, the demonstrated control scheme is expected to be applicable to many magnetic materials.

Journal ArticleDOI
TL;DR: By decomposing the room temperature SHE and inverse SHE currents into bulk and interface terms, this work discovers a giant interface SHA that dominates the total inverse SHE current with potentially major consequences for applications.
Abstract: The spin Hall angle (SHA) is a measure of the efficiency with which a transverse spin current is generated from a charge current by the spin-orbit coupling and disorder in the spin Hall effect (SHE). In a study of the SHE for a Pt|Py (Py=Ni 80 Fe 20 ) bilayer using a first-principles scattering approach, we find a SHA that increases monotonically with temperature and is proportional to the resistivity for bulk Pt. By decomposing the room temperature SHE and inverse SHE currents into bulk and interface terms, we discover a giant interface SHA that dominates the total inverse SHE current with potentially major consequences for applications.

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate a 12 quantum dot device fabricated on an undoped Si/SiGe heterostructure as a proof-of-concept for a scalable, linear gate architecture for semiconductor quantum dots.
Abstract: We demonstrate a 12 quantum dot device fabricated on an undoped Si/SiGe heterostructure as a proof-of-concept for a scalable, linear gate architecture for semiconductor quantum dots. The device consists of 9 quantum dots in a linear array and 3 single quantum dot charge sensors. We show reproducible single quantum dot charging and orbital energies, with standard deviations less than 20% relative to the mean across the 9 dot array. The single quantum dot charge sensors have a charge sensitivity of 8.2 x 10^{-4} e/root(Hz) and allow the investigation of real-time charge dynamics. As a demonstration of the versatility of this device, we use single-shot readout to measure a spin relaxation time T1 = 170 ms at a magnetic field B = 1 T. By reconfiguring the device, we form two capacitively coupled double quantum dots and extract a mutual charging energy of 200 microeV, which indicates that 50 GHz two-qubit gate operation speeds are feasible.

Journal ArticleDOI
TL;DR: This work predicts that a newly discovered family of topological materials, the Weyl semimetals, exhibits a large intrinsicspin Hall effect that can be utilized to generate and detect spin currents, and proposes a new guideline for searching for the spin Hall effect in various topologicalmaterials.
Abstract: Since their discovery, topological insulators are expected to be ideal spintronic materials owing to the spin currents carried by surface states with spin-momentum locking. However, the bulk doping problem remains an obstacle that hinders such an application. In this work, we predict that a newly discovered family of topological materials, the Weyl semimetals, exhibits a large intrinsic spin Hall effect that can be utilized to generate and detect spin currents. Our ab initio calculations reveal a large spin Hall conductivity in the TaAs family of Weyl materials. Considering the low charge conductivity of semimetals, Weyl semimetals are believed to present a larger spin Hall angle (the ratio of the spin Hall conductivity over the charge conductivity) than that of conventional spin Hall systems such as the $4d$ and $5d$ transition metals. The spin Hall effect originates intrinsically from the bulk band structure of Weyl semimetals, which exhibit a large Berry curvature and spin-orbit coupling, so the bulk carrier problem in the topological insulators is naturally avoided. Our work not only paves the way for employing Weyl semimetals in spintronics, but also proposes a new guideline for searching for the spin Hall effect in various topological materials.

Journal ArticleDOI
TL;DR: In this paper, a van de Waals heterostructure of atomically thin graphene and semiconducting MoS2 is proposed to switch the spin current in the channel between ON and OFF states by tuning the spin absorption into the semiconductor with a gate electrode.
Abstract: Future development in spintronic devices will require an advanced control of spin currents, for example by an electric field. Here we demonstrate an approach that differs from previous proposals such as the Datta and Das modulator, and that is based on a van de Waals heterostructure of atomically thin graphene and semiconducting MoS2. Our device combines the superior spin transport properties of graphene with the strong spin–orbit coupling of MoS2 and allows switching of the spin current in the graphene channel between ON and OFF states by tuning the spin absorption into the MoS2 with a gate electrode. Our proposal holds potential for technologically relevant applications such as search engines or pattern recognition circuits, and opens possibilities towards electrical injection of spins into transition metal dichalcogenides and alike materials. By forming heterostructures of different layered two-dimensional materials, functional spintronic devices may be built by exploiting the materials’ different spin-orbit coupling and spin transport properties. Here, the authors demonstrate a spin switch in a gated structure of graphene and MoS2.

Journal ArticleDOI
TL;DR: In this paper, it was shown that unitary, sparse two-dimensional CFTs with large central charge and a set of higher spin currents of bounded spin do not violate the chaos bound, instead exhibiting no chaos.
Abstract: Thermal states of quantum systems with many degrees of freedom are subject to a bound on the rate of onset of chaos, including a bound on the Lyapunov exponent, λ L ≤ 2π/β. We harness this bound to constrain the space of putative holographic CFTs and their would-be dual theories of AdS gravity. First, by studying out-of-time-order four-point functions, we discuss how λ L = 2π/β in ordinary two-dimensional holographic CFTs is related to properties of the OPE at strong coupling. We then rule out the existence of unitary, sparse two-dimensional CFTs with large central charge and a set of higher spin currents of bounded spin; this implies the inconsistency of weakly coupled AdS3 higher spin gravities without infinite towers of gauge fields, such as the SL(N) theories. This fits naturally with the structure of higher-dimensional gravity, where finite towers of higher spin fields lead to acausality. On the other hand, unitary CFTs with classical W ∞ [λ] symmetry, dual to 3D Vasiliev or hs[λ] higher spin gravities, do not violate the chaos bound, instead exhibiting no chaos: λ L = 0. Independently, we show that such theories violate unitarity for |λ| > 2. These results encourage a tensionless string theory interpretation of the 3D Vasiliev theory.

Journal ArticleDOI
TL;DR: In this article, the spin magnitude and direction of a black hole resulting from the merger of two black holes with arbitrary masses and spins inspiralling in quasi-circular orbits is predicted.
Abstract: We revisit the problem of predicting the spin magnitude and direction of the black hole resulting from the merger of two black holes with arbitrary masses and spins inspiralling in quasi-circular orbits. We do this by analyzing a catalog of 619 recent numerical-relativity simulations collected from the literature and spanning a large variety of initial conditions. By combining information from the post-Newtonian approximation, the extreme mass-ratio limit and perturbative calculations, we improve our previously proposed phenomenological formulae for the final remnant spin. In contrast with alternative suggestions in the literature, and in analogy with our previous expressions, the new formula is a simple algebraic function of the initial system parameters and is not restricted to binaries with spins aligned/anti-aligned with the orbital angular momentum, but can be employed for fully generic binaries. The accuracy of the new expression is significantly improved, especially for almost extremal progenitor spins and for small mass ratios, yielding a root-mean-square error $\sigma\approx0.002$ for aligned/anti-aligned binaries and $\sigma\approx0.006$ for generic binaries. Our new formula is suitable for cosmological applications and can be employed robustly in the analysis of the gravitational waveforms from advanced interferometric detectors.

Journal ArticleDOI
TL;DR: It is demonstrated that higher-energy exchange magnons play a key role in the SSE, which is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact.
Abstract: Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE.

Posted Content
TL;DR: In this article, the standard boundary state of a topological insulator in 3+1 dimensions has gapless charged fermions, and the authors present model systems that reproduce this standard gapless boundary state in one phase, but also have gapped phases with topological order.
Abstract: The standard boundary state of a topological insulator in 3+1 dimensions has gapless charged fermions. We present model systems that reproduce this standard gapless boundary state in one phase, but also have gapped phases with topological order. Our models are weakly coupled and all the dynamics is explicit. We rederive some known boundary states of topological insulators and construct new ones. Consistency with the standard spin/charge relation of condensed matter physics places a nontrivial constraint on models.

Journal ArticleDOI
TL;DR: In this article, the authors investigate the dynamics following a global parameter quench for two one-dimensional models with variable-range power-law interactions: a long-range transverse Ising model, which has recently been realized in chains of trapped ions, and a longrange lattice model for spinless fermions with long-term tunneling.
Abstract: We investigate the dynamics following a global parameter quench for two one-dimensional models with variable-range power-law interactions: a long-range transverse Ising model, which has recently been realized in chains of trapped ions, and a long-range lattice model for spinless fermions with long-range tunneling. For the transverse Ising model, the spreading of correlations and growth of entanglement are computed using numerical matrix product state techniques, and are compared with exact solutions for the fermionic tunneling model. We identify transitions between regimes with and without an apparent linear light cone for correlations, which correspond closely between the two models. For long-range interactions, we find that despite the lack of a light cone, correlations grow slowly as a power law at short times, and that—depending on the structure of the initial state—the growth of entanglement can also be sublinear. These results are understood through analytical calculations, and should be measurable in experiments with trapped ions.

Journal ArticleDOI
TL;DR: In this paper, the authors consider conformal field theories with slightly broken higher spin symmetry in arbitrary spacetime dimensions and make a prediction for the anomalous dimensions of higher spin currents in the 3D Ising model.
Abstract: We consider conformal field theories with slightly broken higher spin symmetry in arbitrary spacetime dimensions. We analyze the crossing equation in the double light-cone limit and solve for the anomalous dimensions of higher spin currents γ s with large spin s. The result depends on the symmetries and the spectrum of the unperturbed conformal field theory. We reproduce all known results and make further predictions. In particular we make a prediction for the anomalous dimensions of higher spin currents in the 3d Ising model.

Journal ArticleDOI
04 Nov 2016
TL;DR: In this article, the authors present a direct measurement of the exchange interaction in room temperature ferromagnetic graphene and demonstrate that a proximity induced 2D ferromagnet can efficiently modulate a spin current by controlling the direction of exchange field.
Abstract: We present a direct measurement of the exchange interaction in room temperature ferromagnetic graphene. We study the spin transport in exfoliated graphene on an yttrium-iron-garnet substrate where the observed spin precession clearly indicates the presence and strength of an exchange field that is an unambiguous evidence of induced ferromagnetism. We describe the results with a modified Bloch diffusion equation and extract an average exchange field of the order of 0.2 T. Further, we demonstrate that a proximity induced 2D ferromagnet can efficiently modulate a spin current by controlling the direction of the exchange field. These findings can create a building block for magnetic-gate tuneable spin transport in one-atom-thick spintronic devices.