Spin-orbital Entanglement Research Articles

Spin Polarization and Transport of Surface States in the Topological InsulatorsBi2Se3andBi2Te3from First Principles

We investigate the band dispersion and the spin texture of topologically protected surface states in the bulk topological insulators Bi2Se3 and Bi2Te3 by first-principles methods. Strong spin-orbit entanglement in these materials reduces the spin polarization of the surface states to ∼50% in both cases; this reduction is absent in simple models but of important implications to essentially any spintronic application. We propose a way of controlling the magnitude of spin polarization associated with a charge current in thin films of topological insulators by means of an external electric field. The proposed dual-gate device configuration provides new possibilities for electrical control of spin.

Quantum criticality of vanadium chains with strong relativistic spin-orbit interaction

We study quantum phase transitions induced by the on-site spin-orbit interaction lambda(L.S) in a toy model of vanadium chains. In the lambda->0 limit, the decoupled spin and orbital sectors are described by a Haldane and an Ising chain, respectively. The gapped ground state is composed of a ferro-orbital order and a spin liquid with finite correlation lengths. In the opposite limit, strong spin-orbital entanglement results in a simultaneous spin and orbital-moment ordering, which can be viewed as an orbital liquid. Using a combination of analytical arguments and density-matrix renormalization group calculation, we show that an intermediate phase, where the ferro-orbital state is accompanied by a spin Neel order, is bounded on both sides by Ising transition lines. Implications for vanadium compounds CaV2O4 and ZnV2O4 are also discussed.

Single-photon spin-orbit entanglement violating a Bell-like inequality

Single photons emerging from q-plates (or Pancharatnam-Berry phase optical element) exhibit entanglement in the degrees of freedom of spin and orbital angular momentum. We put forward an experimental scheme for probing the spin-orbit correlations of single photons. It is found that the Clauser-Horne-Shimony-Holt (CHSH) parameter S for the single-photon spin-orbit entangled state could be up to 2.828, evidently violating the Bell-like inequality and thus invalidating the noncontextual hidden variable (NCHV) theories.

Large-Japproach to strongly coupled spin-orbital systems

We present an approach to study the ground-state and elementary excitations in compounds where spins and orbitals are entangled by on-site relativistic spin-orbit interaction. The appropriate degrees of freedom are localized states with an effective angular momentum $J$. We generalize $J$ to arbitrary large values while maintaining the delicate spin-orbital entanglement. After projecting the intersite exchange interaction to the manifold of effective spins, a systematic $1/J$ expansion of the effective Hamiltonian is realized using the Holstein-Primakoff transformation. Applications to representative compounds ${\text{Sr}}_{2}{\text{IrO}}_{4}$ and particularly vanadium spinels $A{\text{V}}_{2}{\text{O}}_{4}$ are discussed.

Encoding orbital angular momentum onto multiple spin states based on a Huffman tree

Based on single-photon spin–orbit entanglement, we propose in this paper an experimental scheme to sort orbital angular momentum (OAM) by cascading conventional polarizing beam splitters. An economical algorithm, taking advantage of the pair's complementary notation of OAM numbers, is designed to separate a mixture of definite OAM. Our scheme provides an alternative technique to encode OAM onto multiple spin states based on a Huffman tree and its potential in optical communication is demonstrated.

Spin-Orbital Physics in Transition Metal Oxides

We present the main features of the spin-orbital superexchange which describes the magnetic and optical properties of Mott insulators with orbital degrees of freedom. In contrast to the SU(2) symmetry of spin superexchange, the orbital part of the superexchange obeys the lower cubic symmetry of the lattice and is intrinsically frustrated. This intrinsic frustration and spin-orbital entanglement induce enhanced quantum fluctuations, and we point out a few situations where this leads to disordered states. Strong coupling between the spin and orbital degrees of freedom is discussed on the example of the $R$VO$_3$ perovskites, with $R$ standing for rare-earth ion, La,...,Lu. We explain the observed evolution of the orbital $T_{\rm OO}$ and N\'eel $T_{N1}$ transition temperature in the $R$VO$_3$ series with decreasing ionic radius $r_R$. A few open problems and the current directions of research in the field of spin-orbital physics are pointed out.

Spin–orbital entanglement near quantum phase transitions

AbstractSpin–orbital entanglement in the ground state of a one‐dimensional SU(2) ⊗ SU(2) spin–orbital model is analyzed using exact diagonalization of finite chains. For S = 1/2 spins and T = 1/2 pseudospins one finds that the quantum entanglement is similar at the SU(4) symmetry point and in the spin–orbital valence bond state. We also show that quantum transitions in spin–orbital models turn out to be continuous under certain circumstances, in constrast to the discontinuous transitions in spin models with SU(2) symmetry. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-orbital entanglement and quantum phase transitions in a spin-orbital chain withSU(2)×SU(2)symmetry

Spin-orbital entanglement in quantum spin-orbital systems is quantified by a reduced von Neumann entropy, and is calculated for the ground state of a coupled spin-orbital chain with $SU(2)\times SU(2)$ symmetry. By analyzing the discontinuity and local extreme of the reduced entropy as functions of the model parameters, we deduce a rich phase diagram to describe the quantum phase transitions in the model. Our approach provides an efficient and powerful method to identify phase boundaries in a system with complex correlation between multiply degrees of freedom.

Electronic Spin Precession and Interferometry from Spin-Orbital Entanglement in a Double Quantum Dot

A double quantum dot inserted in parallel between two metallic leads can entangle the electron spin with the orbital (dot index) degree of freedom. An Aharonov-Bohm orbital phase can be transferred to the spinor wave function, providing a geometrical control of the spin precession around a fixed magnetic field. A fully coherent behavior occurs in a mixed orbital-spin Kondo regime. Evidence for the spin precession can be obtained, either using spin-polarized metallic leads or by placing the double dot in one branch of a metallic loop.