Electronic Spin Degrees Of Freedom Research Articles

Total angular momentum conservation in Ehrenfest dynamics with a truncated basis of adiabatic states.

We show that standard Ehrenfest dynamics does not conserve linear and angular momentum when using a basis of truncated adiabatic states. However, we also show that previously proposed effective Ehrenfest equations of motion [M. Amano and K. Takatsuka, "Quantum fluctuation of electronic wave-packet dynamics coupled with classical nuclear motions," J. Chem. Phys. 122, 084113 (2005) and V. Krishna, "Path integral formulation for quantum nonadiabatic dynamics and the mixed quantum classical limit," J. Chem. Phys. 126, 134107 (2007)] involving the non-Abelian Berry force do maintain momentum conservation. As a numerical example, we investigate the Kramers doublet of the methoxy radical using generalized Hartree-Fock with spin-orbit coupling and confirm that angular momentum is conserved with the proper equations of motion. Our work makes clear some of the limitations of the Born-Oppenheimer approximation when using abinitio electronic structure theory to treat systems with unpaired electronic spin degrees of freedom, and we demonstrate that Ehrenfest dynamics can offer much improved, qualitatively correct results.

Diffusion and magnetization of metal adatoms on single-layer molybdenum disulfide at elevated temperatures

The present work models temperature-dependent (500−1300K) diffusion dynamics of Ag, Au, and Cu adatoms on MoS2 as well as electronic and magnetic properties of adatom (Ag, Au, and Cu)/MoS2 systems. Modeling is done by means of ab initio molecular dynamics (AIMD) simulations that account for van der Waals corrections and electronic spin degrees of freedom in the framework of density functional theory. It is found that Ag and Au adatoms exhibit super-diffusive motion on MoS2 at all temperatures, while Cu adatoms follow a random walk pattern of uncorrelated surface jumps. The observed behavior is consistent with AIMD-calculated effective migration barriers Ea (EaAg=190±50meV, EaAu=67±7meV, and EaCu=300±100meV) and can be understood on the basis of the considerably flatter potential energy landscapes encountered by Ag and Au adatoms on the MoS2 surface (corrugation of the order of tens of meV), as compared to Cu adatoms (corrugation >100meV). Moreover, evaluation of the electronic and magnetic properties of AIMD configurations suggest that Ag, Au, and Cu monomer adsorption induces semimetallic features in at least one spin channel of the adatom/MoS2 electronic structure at elevated temperatures. The overall results presented herein may provide insights into fabricating 2D-material-based heterostructure devices beyond graphene.

Mutual spin-phonon driving effects and phonon eigenvector renormalization in nickel (II) oxide

The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interests for decades. Despite its pivotal role in transport processes, the effect of spin-phonon coupling on the phonon system, especially acoustic phonon properties, has so far been elusive. By means of inelastic neutron scattering and first-principles calculations, anomalous scattering spectral intensity from acoustic phonons was identified in the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong spin-lattice correlations that renormalize the polarization of acoustic phonon. In particular, a clear magnetic scattering signature of the measured neutron scattering intensity from acoustic phonons is demonstrated by its momentum transfer and temperature dependences. The anomalous scattering intensity is successfully modeled with a modified magneto-vibrational scattering cross-section, suggesting the presence of spin precession driven by phonon. The renormalization of phonon eigenvector is indicated by the observed "geometry-forbidden" neutron scattering intensity from transverse acoustic phonon. Importantly, the eigenvector renormalization cannot be explained by magnetostriction but instead, it could result from the coupling between phonon and local magnetization of ions.

Constraints on the superconducting order parameter in Sr2RuO4 from oxygen-17 nuclear magnetic resonance.

Phases of matter are usually identified through spontaneous symmetry breaking, especially regarding unconventional superconductivity and the interactions from which it originates. In that context, the superconducting state of the quasi-two-dimensional and strongly correlated perovskite Sr2RuO4 is considered to be the only solid-state analogue to the superfluid 3He-A phase1,2, with an odd-parity order parameter that is unidirectional in spin space for all electron momenta and breaks time-reversal symmetry. This characterization was recently called into question by a search for an expected 'split' transition in a Sr2RuO4 crystal under in-plane uniaxial pressure, which failed to find any such evidence; instead, a dramatic rise and a peak in a single-transition temperature were observed3,4. Here we use nuclear magnetic resonance (NMR) spectroscopy of oxygen-17, which is directly sensitive to the order parameter via hyperfine coupling to the electronic spin degrees of freedom, to probe the nature of superconductivity in Sr2RuO4 and its evolution under strain. A reduction of the Knight shift is observed for all strain values and at temperatures below the critical temperature, consistent with a drop in spin polarization in the superconducting state. In unstrained samples, our results contradict a body of previous NMR work reporting no change in the Knight shift5 and the most prevalent theoretical interpretation of the order parameter as a chiral p-wave state. Sr2RuO4 is an extremely clean layered perovskite and its superconductivity emerges from a strongly correlated Fermi liquid, and our work imposes tight constraints on the order parameter symmetry of this archetypal system.

Intrinsic Negative Magnetoresistance in Van Der Waals FeNbTe2 Single Crystals.

Magnetoresistance, the dependence of resistivity on the applied magnetic field, provides the opportunity to manipulate and utilize the electronic spin degree of freedom, which is not only a long-term frontier field of solid-state physics but also the cornerstone of information storage technology. However, the negative magnetoresistance (nMR) is a relatively rare case of magnetoresistance in which the microscopic origin is still elusive and for which it is difficult to define a general interpretation. Herein, an experimental case of an intrinsic unsaturated nMR is demonstrated in van der Waals FeNbTe2 single crystal. The clear-cut evidence in angle-resolved photoemission spectroscopy (ARPES), the electronic transport measurement, and DC/AC magnetic susceptibility confirms that the intrinsic unsaturated nMR is derived from the comprehensive effect of Anderson localization and a spin glass state. Taking into consideration that intrinsic unsaturated nMR has so far been rarely reported, especially in van der Waals structures, it is anticipated that this work will not only lead to a deep understanding of the inherent microcosmic mechanism but will also serve as a guide to broaden the research of spintronics and information storage based on magnetoresistance.

Identifying and Mitigating Charge Instabilities in Shallow Diamond Nitrogen-Vacancy Centers

The charge degree of freedom in solid-state defects fundamentally underpins the electronic spin degree of freedom, a workhorse of quantum technologies. Here we measure, analyze, and control charge-state behavior in individual near-surface nitrogen-vacancy (NV) centers in diamond, where NV^{-} hosts the metrologically relevant electron spin. We find that NV^{-} initialization fidelity varies between individual centers and over time; we alleviate the deleterious effects of reduced NV^{-} initialization fidelity via logic-based initialization. Importantly, we also show that NV^{-} can ionize in the dark on experimentally relevant timescales, and we introduce measurement protocols that mitigate the compromising effects of charge conversion on spin measurements. We identify tunneling to a single local electron trap as the mechanism for ionization in the dark, and we develop novel NV-assisted techniques to control and read out the trap charge state. Our understanding and command of the NV's local electrostatic environment will simultaneously guide materials design and provide unique functionalities with NV centers.

Ultra-low magnetic damping of a metallic ferromagnet

Materials with low magnetic damping are important for a range of applications but are typically insulating, which limits their use. Thanks to a unique feature of the band structure, similar levels of damping can now be achieved in a metallic alloy. Magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom, as damping strongly affects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferromagnetic materials1,2,3. This presents a challenge for a broad range of applications in spintronics4 and spin-orbitronics that depend on materials and structures with ultra-low damping5,6. It is believed that achieving ultra-low damping in metallic ferromagnets is limited by the scattering of magnons by the conduction electrons. However, we report on a binary alloy of cobalt and iron that overcomes this obstacle and exhibits a damping parameter approaching 10−4, which is comparable to values reported only for ferrimagnetic insulators7,8. We explain this phenomenon by a unique feature of the band structure in this system: the density of states exhibits a sharp minimum at the Fermi level at the same alloy concentration at which the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms and a test of the theoretical predictions proposed by Mankovsky and colleagues3.

Long-Range Magnetic Ordering and Switching of Magnetic State by Electric Field in Porous Phosphorene.

We explored the possibility of long-range magnetic ordering in two-dimensional porous phosphorene (PP) layer by means of ab-initio calculations. The self-passivated pore geometry showed a nonmagnetic state while the pore geometry with dangling bond at two zigzag edges with a distance of 7.7 Å preferred an antiferromagnetic ordering (AFM). Pore to pore magnetic interaction with a distance of 13.5 Å between two pores was found to be remarkably long ranged, and this emerges from the interactions between the magnetic tails of the edge states in the armchair direction. The AFM state was persisted by the oxidation of the edge. Interestingly, the long-range AFM ordering changed to long-range ferromagnetic (FM) ordering by external electric field. The results are noteworthy in the interplay between electric field and electronic spin degree of freedom in phosphorene studies and may also open a promising way to explore phosphorene-based spintronics devices.

Anisotropy induced Feshbach resonances in mixture of

We theoretically explore the scattering properties in collisions between in the ground state and fermionic in the metastable state. Unlike the role of the electronic spin degree of freedom in collisions of two alkali-metal atoms, the orbital degrees of freedom from the p electron of the metastable atoms, which induce not only mixings between different partial waves but also couplings between channels in different fine-structure manifolds, introduce the anisotropic interactions for producing broad-enough Feshbach resonances. Our closed-coupling calculation shows that this mechanism is similar to that in highly magnetic atoms, but the resonances also suffer from a large inelastic rate at the magnitude of to lower-lying fine-structure states or Zeeman sublevels, which will be an obstacle for associating ultracold polar molecules with both electron spin and electric dipole momentum. However, such a rich mixture of inelastic processes, and the experimental advantage of immunity from intraspecies inelastic collisions for metastable fermionic allow for precise investigations on the interspecies scattering properties for systems with excited-state atoms included.

Orthogonal Cherenkov sound in spin-orbit coupled systems.

Conventionally the Cherenkov sound is governed by orbital degrees of freedom and is excited by supersonic particles. Additionally, it usually has a forward nature with a conic geometry known as the Cherenkov cone whose axis is oriented along the supersonic particle motion. Here we predict Cherenkov sound of a unique nature entirely resulting from the electronic spin degree of freedom and demonstrate a fundamentally distinct Cherenkov effect originating from essentially subsonic electrons in two-dimensional gases with both Bychkov-Rashba and Dresselhaus spin-orbit interactions. Specifically, we show that the axis of the conventional forward Cherenkov cone gets a nontrivial quarter-turn and at the same time the sound distribution strongly localizes around this rotated axis being now orthogonal to the subsonic particle motion. Apart from its fundamentally appealing nature, the orthogonal Cherenkov sound could have applications in planar semiconductor technology combining spin and acoustic phenomena to develop, e.g., acoustic amplifiers or sound sources with a flexible spin dependent orientation of the sound propagation.

Ab initio non-relativistic spin dynamics.

Many magnetic materials do not conform to the (anti-)ferromagnetic paradigm where all electronic spins are aligned to a global magnetization axis. Unfortunately, most electronic structure methods cannot describe such materials with noncollinear electron spin on account of formally requiring spin alignment. To overcome this limitation, it is necessary to generalize electronic structure methods and allow each electron spin to rotate freely. Here, we report the development of an ab initio time-dependent non-relativistic two-component spinor (TDN2C), which is a generalization of the time-dependent Hartree-Fock equations. Propagating the TDN2C equations in the time domain allows for the first-principles description of spin dynamics. A numerical tool based on the Hirshfeld partitioning scheme is developed to analyze the time-dependent spin magnetization. In this work, we also introduce the coupling between electron spin and a homogenous magnetic field into the TDN2C framework to simulate the response of the electronic spin degrees of freedom to an external magnetic field. This is illustrated for several model systems, including the spin-frustrated Li3 molecule. Exact agreement is found between numerical and analytic results for Larmor precession of hydrogen and lithium atoms. The TDN2C method paves the way for the ab initio description of molecular spin transport and spintronics in the time domain.

Ultracold Heteronuclear Mixture of Ground and Excited State Atoms

We report on the realization of an ultracold mixture of lithium atoms in the ground state and ytterbium atoms in an excited metastable (3P2) state. Such a mixture can support broad magnetic Feshbach resonances which may be utilized for the production of ultracold molecules with an electronic spin degree of freedom, as well as novel Efimov trimers. We investigate the interaction properties of the mixture in the presence of an external magnetic field and find an upper limit for the background interspecies two-body inelastic decay coefficient of K2'<3×10(-12) cm3/s for the 3P2 mJ=-1 substate. We calculate the dynamic polarizabilities of the Yb(3P2) magnetic substates for a range of wavelengths, and find good agreement with our measurements at 1064 nm. Our calculations also allow the identification of magic frequencies where Yb ground and metastable states are identically trapped and the determination of the interspecies van der Waals coefficients.

A New Spin on ARPES

The electronic spin degree of freedom is of fundamental importance to all matter, with consequences ranging from the structure of the periodic table to the refrigerator magnet. Understanding the behavior and role of electronic spin in magnetic, correlated, and novel quantum materials is a problem of growing prominence in materials science, both in terms of fundamental science and our ever-growing desire for stricter comprehension and control of electronic spin and magnetic phenomena for technology and energy applications. Spin-resolved ARPES, with its unique simultaneous sensitivity to electronic spin, momentum, and energy, is a powerful method for directly probing the spin dependence of materials and electronic structures of current research interest.

Intervalley coupling for interface-bound electrons in silicon: An effective mass study

Orbital degeneracy of the electronic conduction band edge in silicon is a potential roadblock to the storage and manipulation of quantum information involving the electronic spin degree of freedom in this host material. This difficulty may be mitigated near an interface between Si and a barrier material, where intervalley scattering may couple states in the conduction ground state, leading to nondegenerate orbital ground and first excited states. The level splitting is experimentally found to have a strong sample dependence, varying by orders of magnitude for different interfaces and samples. The basic physical mechanisms leading to such coupling in different systems are addressed. We expand our recent study based on an effective mass approach, incorporating the full plane-wave expansions of the Bloch functions at the conduction band minima. Physical insights emerge naturally from a simple Si/barrier model. In particular, we present a clear comparison between ours and different approximations and formalisms adopted in the literature and establish the applicability of these approximations in different physical scenarios.

Effect of spin-orbit interaction on the spin transport of quantum dot systems

Spintronics is an emerging cross-disciplinary, and its central theme is that of solid electronic systems for effective electronic spin degree of freedom to operate and control. Spin effect in quantum dot system has attracted considerable attention theoretically and experimentally. This paper introduces the effect of spin-orbit interaction on the transport properties of quantum dot system, and to explore how to use spin-orbit coupling to achieve effective filtering of spin and pure spin current generation. Based on the mechanism of the exchange interaction between electrons in a double quantum dot system coupled with four ferromagnetic metals, we propose a way to enhance the injection efficiency of spin-polarized current from ferromagnetic metal into semiconductor quantum dot.

Noncollinear magnetism in density functional calculations

We generalize the treatment of the electronic spin degrees of freedom in density functional calculations to the case where the spin vector variables employed in the definition of the energy functional can vary in any direction in space. The expression for the generalized exchange-correlation potential matrix elements is derived for general functionals which among their ingredients include the electron density, its gradient and Laplacian, the kinetic energy density, and non-local Hartree-Fock type exchange. We present calculations on planar Cr clusters that exhibit ground states with noncollinear spin densities due to geometrically frustrated antiferromagnetic interactions.

Half-metallic graphene nanoribbons

Electrical current can be completely spin polarized in a class of materials known as half-metals, as a result of the coexistence of metallic nature for electrons with one spin orientation and insulating nature for electrons with the other. Such asymmetric electronic states for the different spins have been predicted for some ferromagnetic metals--for example, the Heusler compounds--and were first observed in a manganese perovskite. In view of the potential for use of this property in realizing spin-based electronics, substantial efforts have been made to search for half-metallic materials. However, organic materials have hardly been investigated in this context even though carbon-based nanostructures hold significant promise for future electronic devices. Here we predict half-metallicity in nanometre-scale graphene ribbons by using first-principles calculations. We show that this phenomenon is realizable if in-plane homogeneous electric fields are applied across the zigzag-shaped edges of the graphene nanoribbons, and that their magnetic properties can be controlled by the external electric fields. The results are not only of scientific interest in the interplay between electric fields and electronic spin degree of freedom in solids but may also open a new path to explore spintronics at the nanometre scale, based on graphene.

Selective pulse implementation of two-qubit gates for spin-32-based fullerene quantum-information processing

We propose two potentially practical schemes to carry out two-qubit quantum gates on endohedral fullerenes $\mathrm{N}@{\mathrm{C}}_{60}$ or $\mathrm{P}@{\mathrm{C}}_{60}$. The qubits are stored in electronic spin degrees of freedom of the doped atom N or P. By means of the magnetic dipolar coupling between two neighboring fullerenes, the two-qubit controlled-NOT gate and the two-qubit conditional phase gate are performed by selective microwave pulses assisted by the refocusing technique. We will discuss the necessary additional steps for the universality of our proposal. We will also show that our proposal is useful for both quantum gating and the readout of quantum information from the spin-based qubit state.

Implementation of an all‐optical spin‐based quantum computer

AbstractAn all‐optical implementation scheme of a spin‐based quantum computer is presented. Our quantum memory consists of the spin of electrons confined to quantum dots. Utilizing the Pauli blocking effect we are able to have ultra‐fast control and read out of the electronic spin degrees of freedom by conditionally coupling them with charged excitations of the quantum dot. Imperfections effecting gate operations are discussed and we show that final readout can still be performed by a quantum‐jump technique even in the presence of hole mixing, when the Pauli‐blocking selection rule is violated.

Fine-structure effects in vibrational relaxation at ultralow temperatures

We have calculated cross sections for vibrational quenching in collisions of O217(X 3∑g−) with He3 at cold and ultracold temperatures, explicitly accounting for the electronic spin degree of freedom. The observed population of the final spin-rotational levels can be interpreted on the basis of useful “propensity rules” governing the intensity of the collision-induced transitions. In addition, it is found that the orientation of the molecular spin is reasonably robust against collisional relaxation, even in vibrationally excited states. This finding implies that magnetic trapping of vibrationally excited, but rotationally and translationally cold, molecules may be possible.