Spin Imbalance Research Articles

Spin-selective transport in a correlated double quantum dot-Majorana wire system

In this work we investigate the spin-dependent transport through a double quantum dot embedded in a ferromagnetic tunnel junction and side attached to a topological superconducting nanowire hosting Majorana zero-energy modes. We focus on the transport regime when the Majorana mode leaks into the double quantum dot competing with the two-stage Kondo effect and the ferromagnetic-contact-induced exchange field. In particular, we determine the system’s spectral properties and analyze the temperature dependence of the spin-resolved linear conductance by means of the numerical renormalization group method. Our study reveals unique signatures of the interplay between the spin-resolved tunneling, the Kondo effect and the Majorana modes, which are visible in the transport characteristics. In particular, we uncover a competing character of the coupling to topological superconductor and that to ferromagnetic leads, which can be observed already for very low spin polarization of the electrodes. This is signaled by an almost complete quenching of the conductance in one of the spin channels which is revealed through perfect conductance spin polarization. Moreover, we show that the conductance spin polarization can change sign depending on the magnitude of spin imbalance in the leads and strength of interaction with topological wire. Thus, our work demonstrates that even minuscule spin polarization of tunneling processes can have large impact on the transport properties of the system.

Quantum degeneracy and spin entanglement in ideal quantum gases

Quantum degeneracy is the central many-body feature of ideal quantum gases stemming from quantum mechanics. In this work we address its relationship to the most fundamental form of non-classicality in many-body system, i.e. many-body entanglement. We aim at establishing a quantitative link between quantum degeneracy and entanglement in spinful ideal gases, using entanglement witness criteria based on the variance of the collective spin of the spin ensemble. We show that spin-1/2 ideal Bose gases do not possess entanglement which can be revealed from such entanglement criteria. On the contrary, ideal spin-1/2 Fermi gases exhibit spin entanglement revealed by the collective-spin variances upon entering quantum degeneracy, due to the formation of highly non-local spin singlets. We map out the regime of detectable spin entanglement for Fermi gases in free space as well as in a parabolic trap, and probe the robustness of spin entanglement to thermal effects and spin imbalance. Spin entanglement in degenerate Fermi gases is amenable to experimental observation using state-of-the-art spin detection techniques in ultracold atoms.

Generation and decay of Higgs mode in a strongly interacting Fermi gas

We investigate the life cycle of the large amplitude Higgs mode in strongly interacting superfluid Fermi gas. Through numerical simulations with time-dependent density functional theory and the technique of the interaction quench, we verify the previous theoretical predictions on the mode’s frequency. Next, we demonstrate that the mode is dynamically unstable against external perturbation and qualitatively examine the emerging state after the mode decays. The post-decay state is characterized by spatial fluctuations of the order parameter and density at scales comparable to the superfluid coherence length scale. We identify similarities with FFLO states, which become more prominent at higher dimensionalities and nonzero spin imbalances.

Spin-singlet topological superconductivity in the attractive Rashba-Hubbard model

Fully gapped, spin-singlet superconductors with antisymmetric spin-orbit coupling in a Zeeman magnetic field provide a promising route to realize superconducting states with non-Abelian topological order and therefore fault-tolerant quantum computation. Here we use a quantum Monte Carlo dynamical cluster approximation to study the superconducting properties of a doped two-dimensional attractive Hubbard model with Rashba spin-orbit coupling in a Zeeman magnetic field. We generally find that the Rashba coupling has a beneficial effect towards $s$-wave superconductivity. In the presence of a finite Zeeman field, when superconductivity is suppressed by Pauli pair breaking, the Rashba coupling counteracts the spin imbalance created by the Zeeman field by mixing the spins, and thus restores superconductivity at finite temperatures. We show that this favorable effect of the spin-orbit coupling is traced to a spin-flip driven enhancement of the amplitude for the propagation of a pair of electrons in time-reversed states. Moreover, by inspecting the Fermi surface of the interacting model, we show that for sufficiently large Rashba coupling and Zeeman field, the superconducting state is expected to be topologically nontrivial.

Inducing Shifted Edge Modes by the Spin‐Dependent Time‐Periodical Driving and the Corresponding Topological Phase Transitions

AbstractTime periodic driving can serve as synthetic gauge fields and plays a key role in simulating dynamical topological materials. The periodically driven systems, where different spins (or sublattices) are engaged in the different dynamical driving processes are investigated. It is demonstrated that spin‐dependent time‐periodical periodic driving can result in shifted topological edge modes with non‐zero (nor ) quasi‐energies. Such shifted topological edge modes are not only related to the spin imbalance at each instantaneous time, but also the details of the dynamical driving. Here, it is also illustrated that the spin‐dependent time‐periodical driving can be conceived as the time‐spin coupling, and similar to the static spatial spin‐orbit coupling, tuning time‐spin coupling parameters can lead to topological phase transitions. Experimental simulations on the spin‐dependent time‐periodic driving are proposed by shaking optical super‐lattices and Raman assisted tunneling.

Light-Controlled Magnetoelastic Effects in Ni/BaTiO3 Heterostructures

Magnetoelastic and magnetoelectric coupling in the artificial multiferroic heterostructures facilitate valuable features for device applications such as magnetic field sensors and electric-write magnetic-read memory devices. In ferromagnetic/ferroelectric heterostructures, the intertwined physical properties can be manipulated by an external perturbation, such as an electric field, temperature, or a magnetic field. Here, we demonstrate the remote-controlled tunability of these effects under visible, coherent, and polarized light. The combined surface and bulk magnetic study of domain-correlated Ni/BaTiO3 heterostructures reveals that the system shows strong sensitivity to the light illumination via the combined effect of piezoelectricity, ferroelectric polarization, spin imbalance, magnetostriction, and magnetoelectric coupling. A well-defined ferroelastic domain structure is fully transferred from a ferroelectric substrate to the magnetostrictive layer via interface strain transfer. The visible light illumination is used to manipulate the original ferromagnetic microstructure by the light-induced domain wall motion in ferroelectric substrates and consequently the domain wall motion in the ferromagnetic layer. Our findings mimic the attractive remote-controlled ferroelectric random-access memory write and magnetic random-access memory read application scenarios, hence facilitating a perspective for room temperature spintronic device applications.

Unlocking perpendicular magnetic anisotropy with Gd substitution in SmN

A unique bulk-origin perpendicular magnetic anisotropy (PMA) is reported in thin films of the cubic ferromagnetic semiconductor GdxSm1−xN. PMA behavior is clearly signaled by in-vs-out-of-plane magnetic hysteresis showing out-of-plane remanence of ∼80% of the saturation magnetization in films with up to 30% Gd. Anomalous Hall effect data show further that the conduction-band spin imbalance shows complete 100% remanence to retain half-metallic conduction even as the out-of-plane applied field is reduced to zero, in stark contrast to the 80% magnetization remanence. The unusual occurrence of PMA in this cubic material is discussed as stabilized by a mix of two in-plane crystal orientations in the epitaxial (111)-oriented films.

A spintronic analog of the Landauer residual resistivity dipole on the surface of a topological insulator containing a line defect *

The Landauer ‘residual resistivity dipole’ is a well-known concept in electron transport through a disordered medium. It is formed when a defect/scatterer reflects an impinging electron causing negative charges to build up on one side of the scatterer and positive charges on the other. This charge imbalance results in the formation of a microscopic electric dipole that affects the electrical resistivity of the medium. Here, we show that an equivalent entity forms in spin polarized electron transport on the surface of a real topological insulator (TI) such as Bi2Te3 containing a line defect. When electrons reflect from such a scatterer, a local spin imbalance forms owing to spin accumulation on one side and depletion on the other side of the scatterer, resulting in a spin current that flows either in the same or in the opposite direction as the injected spin current, and hence, either decreases or increases the spin resistivity. Spatially varying local magnetic fields appear in the vicinity of the scatter, which will cause transiting spins to precess and emit electromagnetic waves. If the current injected into the TI is an alternating current, then the magnetic field’s polarity will oscillate in time with the frequency of the current and if the spins can follow quasi-statically, then they will radiate electromagnetic waves of the same frequency, thereby making the scatterer act as a miniature antenna.

Observing a topological phase transition with deep neural networks from experimental images of ultracold atoms.

Although classifying topological quantum phases have attracted great interests, the absence of local order parameter generically makes it challenging to detect a topological phase transition from experimental data. Recent advances in machine learning algorithms enable physicists to analyze experimental data with unprecedented high sensitivities, and identify quantum phases even in the presence of unavoidable noises. Here, we report a successful identification of topological phase transitions using a deep convolutional neural network trained with low signal-to-noise-ratio (SNR) experimental data obtained in a symmetry-protected topological system of spin-orbit-coupled fermions. We apply the trained network to unseen data to map out a whole phase diagram, which predicts the positions of the two topological phase transitions that are consistent with the results obtained by using the conventional method on higher SNR data. By visualizing the filters and post-convolutional results of the convolutional layer, we further find that the CNN uses the same information to make the classification in the system as the conventional analysis, namely spin imbalance, but with an advantage concerning SNR. Our work highlights the potential of machine learning techniques to be used in various quantum systems.

Stability of (N+1) -body fermion clusters in a multiband Hubbard model

We start with a variational approach and derive a set of coupled integral equations for the bound states of $N$ identical spin-$\ensuremath{\uparrow}$ fermions and a single spin-$\ensuremath{\downarrow}$ fermion in a generic multiband Hubbard Hamiltonian with an attractive on-site interaction. As an illustration, we apply our integral equations to the one-dimensional sawtooth lattice up to $N\ensuremath{\le}3$, i.e., to the $(3+1)$-body problem, and we reveal not only the presence of tetramer states in this two-band model but also their quasiflat dispersion when formed in a flat band. Furthermore, for $N={4,5,\ensuremath{\cdots},10}$, our density-matrix renormalization-group simulations and exact diagonalization suggest the presence of larger and larger multimers with lower and lower binding energies, conceivably without an upper bound on $N$. These peculiar $(N+1)$-body clusters are in sharp contrast with the exact results on the single-band linear-chain model where none of the $N\ensuremath{\ge}2$ multimers appear. Hence their presence must be taken into account for a proper description of the many-body phenomena in flat-band systems, e.g., they may suppress superconductivity especially when there exists a large spin imbalance.

Non-Hermitian many-body localization with open boundaries

The explorations of non-Hermiticity have been devoted to investigate the disorder-induced many-body localization (MBL). However, the sensitivity of the spatial boundary conditions and the interplay of the non-Hermitian skin effect with many-body phenomena are not yet clear. For a MBL system in the presence of nonreciprocal tunnelings and random disorder potential, we identify two different complex-real spectral transitions, one is present for both open and periodic boundaries while the other is present only for open boundaries of coupled non-Hermitian chains. The later is driven due to the interchain coupling at weak disorder where the level statistics of the real eigenenergy phase follows a Gaussian orthogonal ensemble. We further characterize wave functions through the (biorthogonal) inverse participation ratio and fractal dimension, which reveal the suppression of skin effect in the non-Hermitian MBL phase. Finally, we demonstrate that the quench dynamics of the local particle density, spin imbalance, and entanglement entropy also signify the hallmark of the boundary effects and nonergodic character of many-body localization.

Magnetic Interactions in Substitutional Core-Doped Graphene Nanoribbons.

The design of a spin imbalance within the crystallographic unit cell of bottom-up engineered 1D graphene nanoribbons (GNRs) gives rise to nonzero magnetic moments within each cell. Here, we demonstrate the bottom-up assembly and spectroscopic characterization of a one-dimensional Kondo spin chain formed by a chevron-type GNR (cGNR) physisorbed on Au(111). Substitutional nitrogen core doping introduces a pair of low-lying occupied states per monomer within the semiconducting gap of cGNRs. Charging resulting from the interaction with the gold substrate quenches one electronic state for each monomer, leaving behind a 1D chain of radical cations commensurate with the unit cell of the ribbon. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal the signature of a Kondo resonance emerging from the interaction of S = 1/2 spin centers in each monomer core with itinerant electrons in the Au substrate. STM tip lift-off experiments locally reduce the effective screening of the unpaired radical cation being lifted, revealing a robust exchange coupling between neighboring spin centers. First-principles DFT-LSDA calculations support the presence of magnetic moments in the core of this GNR when it is placed on Au.

Many-body localization and delocalization dynamics in the thermodynamic limit

Disordered quantum systems undergoing a many-body localization (MBL) transition fail to reach thermal equilibrium under their own dynamics. Distinguishing between asymptotically localized or delocalized dynamics based on numerical results is however nontrivial due to finite-size effects. Numerical linked cluster expansions (NLCE) provide a means to tackle quantum systems directly in the thermodynamic limit, but are challenging for models without translational invariance. Here, we demonstrate that NLCE provide a powerful tool to explore MBL by simulating quench dynamics in disordered spin-$1/2$ two-leg ladders and Fermi-Hubbard chains. Combining NLCE with an efficient real-time evolution of pure states, we obtain converged results for the decay of the imbalance on long time scales and show that, especially for intermediate disorder below the putative MBL transition, NLCE outperform direct simulations of finite systems with open or periodic boundaries. Furthermore, while spin is delocalized even in strongly disordered Hubbard chains with frozen charge, we unveil that an additional tilted potential leads to a drastic slowdown of the spin imbalance and nonergodic behavior on accessible times. Our work sheds light on MBL in systems beyond the well-studied disordered Heisenberg chain and emphasizes the usefulness of NLCE for this purpose.

Exotic Superfluid Phases in Spin-Polarized Fermi Gases in Optical Lattices.

Leveraging cutting-edge numerical methodologies, we study the ground state of the two-dimensional spin-polarized Fermi gas in an optical lattice. We focus on systems at high density and small spin polarization, corresponding to the parameter regime believed to be most favorable to the formation of the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid phase. Our systematic study of large lattice sizes, hosting nearly 500 atoms, provides strong evidence of the stability of the FFLO state in this regime, as well as a high-accuracy characterization of its properties. Our results for the density correlation function reveal the existence of density order in the system, suggesting the possibility of an intricate coexistence of long-range orders in the ground state. The ground-state properties are seen to differ significantly from the standard mean-field description, providing a compelling avenue for future theoretical and experimental explorations of the interplay between spin imbalance, strong interactions, and superfluidity in an exotic phase of matter.

Ab initio расчет зонной структуры и свойств модификаций соединения Ti-=SUB=-3-=/SUB=-Sb, допированного литием

Using the density functional theory (DFT) in the local electron spin density approximation (LSDA), 2×2×2 supercells based on the Ti3Sb compound have been studied. The supercells contained their own vacancies and doped lithium atoms replacing Ti and/or Sb. The DFT-LSDA method was used to calculate the structural, electronic, and magnetic properties, the enthalpy of formation, and the cohesion energy of supercells of two modifications of the Ti3Sb compound. We studied supercells with cubic system (A15 type structure; a = 5.217 Å) and tetragonal system (D8_m type structure; а = 10.457 Å, с = 5.258 Å). It has been established that the distribution of the density of states of the Ti3Sb cubic system has a more metallic character than for the D8_mTi3Sb modification. The enhancement of "metallicity" in the cubic modification A15 Ti3Sb is associated with an increase in the Ti–Sb interatomic distance in the crystal. Due to this, the degree of metallic bonding increases during the electronic interaction between Ti and Sb atoms near the Fermi level in Ti3Sb. In DFT calculations, the spin-polarized density of states was taken into account. It has been established that in both modifications (A15 and D8_m) of Ti3Sb near the Fermi level for s, p, d-states with spin "up" and spin "down" there is a spin imbalance in the population of energy levels. Defect-containing supercells based on Ti3Sb were studied by DFT-LSDA calculations. It is shown that Ti and/or Sb vacancies in the lattices of Ti3Sb crystals of both modifications (A15 and D8_m) increase the magnetic moment (M) as compared to the value of M (M_tot = 0.08 μ_B) of a “pure” Ti3Sb crystal. Considering that Al5 Ti3Sb is also interesting as a material for Li-ion batteries, we studied Ti3Sb–Li supercells doped with lithium. Li-doping and the creation of own Ti and/or Sb vacancies changes the distance between atoms in Ti3Sb. Correspondingly, the resulting local magnetic moments near the Ti or Sb vacancies also change. The enthalpy of formation and magnetic moment of Ti3Sb and Ti3Sb–Li based supercells are calculated. DFT calculations of the structural stability of binary compounds (phases) have been carried out, and the stability of conodes between phases in the Ti–Sb–Li system has been established. The isothermal section of the Ti–Sb–Li system was plotted at 298 K. It is shown that the introduction of various lithium concentrations (<6.25 at. % Li) into the Ti3Sb (Space group Pm3 ̅n; № 223; a = 5.217 Å) crystal lattice reduces the partial magnetic moment of Ti in the Ti3Sb–Li supercell.

Ab initio calculation of the band structure and properties of modifications of the Ti-=SUB=-3-=/SUB=-Sb compound doped with lithium

Using the density functional theory (DFT) in the local electron spin density approximation (LSDA), 2x2x2 supercells based on the Ti3Sb compound have been studied. The supercells contained their own vacancies and doped lithium atoms replacing Ti and/or Sb. The DFT-LSDA method was used to calculate the structural, electronic, and magnetic properties, the enthalpy of formation, and the cohesion energy of supercells of two modifications of the Ti3Sb compound. We studied supercells with cubic system (A15 type structure; a=5.217 Angstrem) and tetragonal system (D8m type structure; a=10.457 Angstrem, c=5.258 Angstrem). It has been established that the distribution of the density of states of the Ti3Sb cubic system has a more metallic character than for the D8m Ti3Sb modification. The enhancement of "metallicity" in the cubic modification A15 Ti3Sb is associated with an increase in the Ti-Sb interatomic distance in the crystal. Due to this, the degree of metallic bonding increases during the electronic interaction between Ti and Sb atoms near the Fermi level in Ti3Sb. In DFT calculations, the spin-polarized density of states was taken into account. It has been established that in both modifications (A15 and D8m) of Ti3Sb near the Fermi level for s-, p-, d-states with spin "up" and spin "down" there is a spin imbalance in the population of energy levels. Defect-containing supercells based on Ti3Sb were studied by DFT-LSDA calculations. It is shown that Ti and/or Sb vacancies in the lattices of Ti3Sb crystals of both modifications (A15 and D8m) increase the magnetic moment (M) as compared to the value of M (M=0.08 μB) of a "pure" Ti3Sb crystal. Considering that Al5 Ti3Sb is also interesting as a material for Li-ion batteries, we studied Ti3Sb-Li supercells doped with lithium. Li-doping and the creation of own Ti and/or Sb vacancies changes the distance between atoms in Ti3Sb. Correspondingly, the resulting local magnetic moments near the Ti or Sb vacancies also change. The enthalpy of formation and magnetic moment of Ti3Sb and Ti3Sb-Li based supercells are calculated. DFT calculations of the structural stability of binary compounds (phases) have been carried out, and the stability of conodes between phases in the Ti-Sb-Li system has been established. The isothermal section of the Ti-Sb-Li system was plotted at 298 K. It is shown that the introduction of various lithium concentrations (≤6.25 at.% Li) into the Ti3Sb (Space group Pm3 n; N 223; a=5.217 Angstrem) crystal lattice reduces the partial magnetic moment of Ti in the Ti3Sb-Li supercell. Keywords: Ti3Sb intermetallic compounds, A15 cubic modification, D8m tetragonal modification, doping with lithium, Ti3Sb-Li, DFT-LSDA calculations, supercell, electronic and magnetic properties, Ti-Sb-Li phase stability.

Phenomenology of Spectral Functions in Disordered Spin Chains at Infinite Temperature.

Studies of disordered spin chains have recently experienced a renewed interest, inspired by the question to which extent the exact numerical calculations comply with the existence of a many-body localization phase transition. For the paradigmatic random field Heisenberg spin chains, many intriguing features were observed when the disorder is considerable compared to the spin interaction strength. Here, we introduce a phenomenological theory that may explain some of those features. The theory is based on the proximity to the noninteracting limit, in which the system is an Anderson insulator. Taking the spin imbalance as an exemplary observable, we demonstrate that the proximity to the local integrals of motion of the Anderson insulator determines the dynamics of the observable at infinite temperature. In finite interacting systems our theory quantitatively describes its integrated spectral function for a wide range of disorders.

Many-body localization in the interpolating Aubry-André-Fibonacci model

We investigate the localization properties of a spin chain with an antiferromagnetic nearest-neighbour coupling, subject to an external quasiperiodic on-site magnetic field. The quasiperiodic modulation interpolates between two paradigmatic models, namely the Aubry-Andr\'e and the Fibonacci models. We find that stronger many-body interactions extend the ergodic phase in the former, whereas they shrink it in the latter. Furthermore, the many-body localization transition points at the two limits of the interpolation appear to be continuously connected along the deformation. As a result, the position of the many-body localization transition depends on the interaction strength for an intermediate degree of deformation of the quasiperiodic modulation. Moreover, in the region of parameter space where the single-particle spectrum contains both localized and extended states, many-body interactions induce an anomalous effect: weak interactions localize the system, whereas stronger interactions enhance ergodicity. We map the model's localization phase diagram using the decay of the quenched spin imbalance in relatively long chains. This is accomplished employing a time-dependent variational approach applied to a matrix product state decomposition of the many-body state. Our model serves as a rich playground for testing many-body localization under tunable potentials.

Dynamics of spin-polarized impurity in ultracold Fermi gas

We show that the motion of spin-polarized impurity (ferron) in ultracold atomic gas is characterized by a certain critical velocity which can be traced back to the amount of spin imbalance inside the impurity. We have calculated the effective mass of ferron in two dimensions. We show that the effective mass scales with the surface of the ferron. We discuss the impact of these findings; in particular, we demonstrate that ferrons become unstable in the vicinity of a vortex.

Energy-efficient spin injector into semiconductors driven by elastic waves

Generation of spin imbalance in nonmagnetic semiconductors is crucial for the functioning of many spintronic devices. An attractive design of spin injectors into semiconductors is based on a spin pumping from a precessing ferromagnet, typically excited by a microwave magnetic field leading to a high power consumption of the device. Here we describe theoretically a spin injector with greatly reduced energy losses, in which the magnetic dynamics is excited by an elastic wave injected into a ferromagnet-semiconductor heterostructure. To demonstrate the efficient functioning of such an injector, we perform micromagnetoelastic simulations of the coupled elastic and magnetic dynamics in Ni films and Ni/GaAs bilayers. For thick Ni films, it is shown that a monochromatic acoustic wave generates a spin wave with the same frequency and wavelength, which propagates over distances of several micrometers at the excitation frequencies close to the frequency of ferromagnetic resonance. The simulations of Ni/GaAs bilayers with Ni thicknesses comparable to the wavelength of the injected acoustic wave demonstrate the development of a steady-state magnetization precession at the Ni/GaAs interface. The amplitude of such a precession has a maximum at Ni thickness amounting to three quarters of the wavelength of the elastic wave, which is explained by an analytical model. Using simulation data obtained for the magnetization precession at the Ni/GaAs interface, we evaluate the spin current pumped into GaAs and calculate the spin accumulation in it by solving the spin diffusion equation. Then the electrical signals resulting from the spin flow and the inverse spin Hall effect are determined via the numerical solution of the Laplace's equation. It is shown that amplitudes of these ac signals are large enough for experimental measurement, which indicates an efficient acoustically driven spin pumping into GaAs.