Spin-dependent Shift Research Articles

Theory of superconducting and magnetic proximity effect in S/F structures with inhomogeneous magnetization textures and spin-active interfaces

We present a study of the proximity effect and the inverse proximity effect in a $\text{superconductor}\ensuremath{\mid}\text{ferromagnet}$ bilayer, taking into account several important factors which mostly have been ignored in the literature so far. These include spin-dependent interfacial phase shifts (spin-DIPS) and inhomogeneous textures of the magnetization in the ferromagnetic layer, both of which are expected to be present in real experimental samples. Our approach is numerical, allowing us to access the full proximity effect regime. In Sec. II of this work, we study the superconducting proximity effect and the resulting local density of states in an inhomogeneous ferromagnet with a nontrivial magnetic texture. Our two main results in Sec. II are a study of how Bloch and N\'eel domain walls affect the proximity-induced superconducting correlations and a study of the superconducting proximity effect in a conical ferromagnet. The latter topic should be relevant for the ferromagnet Ho, which was recently used in an experiment to demonstrate the possibility to generate and sustain long-range triplet superconducting correlations. In Sec. III of this work, we investigate the inverse proximity effect with emphasis on the induced magnetization in the superconducting region as a result of the ``leakage'' from the ferromagnetic region. It is shown that the presence of spin-DIPS modifies conclusions obtained previously in the literature with regard to the induced magnetization in the superconducting region. In particular, we find that the spin-DIPS can trigger an antiscreening effect of the magnetization, leading to an induced magnetization in the superconducting region with the same sign as in the proximity ferromagnet.

Long storage time of collective coherence in an optically trapped Bose-Einstein condensate

Our recent work has demonstrated the superradiant writing and reading of collective coherence in a BoseEinstein condensate Phys. Rev. Lett. 99, 220407 2007 . This time we report the drastic improvement of the storage time realized by loading the atoms into an optical dipole trap, wherein an unfavorable spin-dependent phase shift and spatial diffusion of atoms can be suppressed. The measured storage time was 0.57 2 ms, which is limited by the temporal variation of atomic momentum due to harmonic oscillations in the trap.

Mott-Schwinger scattering of polarized low energy neutrons up to thermal energies

The availability of new, high-intensity, cold and thermal neutron sources has opened the possibility of performing high-precision fundamental neutron physics experiments, including measurements that study the hadronic weak interaction and standard model test measurements, using neutron $\ensuremath{\beta}$ decay. The observables in these experiments are usually correlated with the direction of neutron polarization and are often very small $({10}^{\ensuremath{-}8}\ensuremath{\rightarrow}{10}^{\ensuremath{-}6})$. Mott-Schwinger scattering of polarized neutrons can produce spin-dependent shifts in beam centroids, which has the potential to produce significant systematic effects for these types of experiments. An accurate calculation of this process for neutral atoms and basic molecules has not been carried out for low neutron energies. In this work, we derive a general expression for the electromagnetic (Mott-Schwinger) contributions to the analyzing power for low-energy neutron scattering. We obtain numerical results for 11 nuclei in the range of $A=1$ to $A=208$ and provide a series of graphs for easy reference and interpolation between $A$ values. We also estimate the contribution of spin-dependent nucleon-nucleon forces and apply our results to determine the analyzing power of parahydrogen. Numerical calculations are performed to determine the analyzing power for the parahydrogen molecule and are compared to results obtained using analytical expressions.

Coulomb corrections in quasi-elastic scattering: Tests of the effective-momentum approximation

Coulomb corrections for quasielastic scattering of electrons by nuclei are calculated using eikonal distorted waves. Corrections to the lowest-order eikonal approximation are included to obtain accurate results. Spin-dependent eikonal phase shifts are evaluated and they yield very small corrections to the longitudinal and transverse cross sections at electron energies of 500 MeV or higher. Because of this the Rosenbluth procedure is accurate for separation of the longitudinal and transverse response functions. The effective-momentum approximation is also found to be accurate with regard to removal of the remaining Coulomb effects from the distorted waves. Calculations are presented for electron scattering from $^{208}\mathrm{Pb}$ and $^{56}\mathrm{Fe}$ nuclei at energies of 500 and 800 MeV and momentum transfers $q=550$ and 900 MeV/$c$.

Describing ferromagnetism in the Hubbard model

The survival of magnetic phases beyond the mean field level is a challenging issue in different models of strongly correlated electron systems. By applying the equation of motion method we develop an alternative version of the modified alloy analogy to account for the ferromagnetic phase of the Hubbard model. The method incorporates the quasiparticle damping and a spin-dependent band shift which strongly affects the distribution of spectral weight in the densities of states. The selfconsistent spontaneous magnetization and the nearest-neighbor correlation functions 〈 n i σ n k σ ′ 〉 are calculated as a function of temperature for finite U and for different band fillings.

Spin-filtering effect and proximity effect in normal metal/ferromagnetic insulator/normal metal/superconductor junctions

Taking into account the thickness of the ferromagnetic insulator (FI), the spin-filtering effect and proximity effect in normal metal/ferromagnetic insulator/normal metal/superconductor (NM/FI/NM/SC) junctions are studied based on an extended Blonder–Tinkham–Klapwijk (BTK) theory. It is shown that a spin-dependent energy shift during the tunneling process induces splitting of the sub-energy gap conductance peaks and the spin polarization in the ferromagnetic insulator causes an imbalance of the peak heights. Different from the ferromagnet the spin-filtering effect of the FI cannot cause the reversion of the normalized conductance in NM/FI/NM/SC junctions.

Spin-filter effect in normal metal/ferromagnetic insulator/normal metal/superconductor structures

Taking into account the thickness of the ferromagnetic insulator, the spin-filter effect in normal metal/ferromagnetic insulator/normal metal/superconductor (NM/FI/NM/SC) junctions is studied based on the Blonder–Tinkham–Klapwijk (BTK) theory. It is shown that a spin-dependent energy shift during the tunneling process induces splitting of the subgap resonance peaks. The spin polarization due to the spin-filter effect of the FI causes an imbalance of the peaks heights and can enhance the Zeeman splitting of the gap peaks caused by an applied magnetic field. The spin-filter effect has no contribution to the proximity-effect-induced superconductivity in NM interlayer.

Josephsonπstate in superconductor–Luttinger liquid hybrid systems

Josephson current through a Luttinger liquid (LL) under a magnetic field is theoretically studied. We derive an analytical expression of Josephson current for clean interfaces, by using quasiclassical Green's function and functional bosonization procedure. We show that critical currents can be renormalized by electron-electron interactions at perfect transparency when LL is adiabatically connected with superconductors. We also find that a generation of $\pi$-state, due to spin-dependent energy shift in Andreev bound states (ABS), is prohibited even at zero temperature when the strength of repulsive interactions reaches some critical value. The suppression of $\pi$-state is caused by the low energy fluctuations propagating in LL, and making the Zeeman splitting in ABS blurred.

Spin-dependent localization effects in GaAs:Mn/MnAs granular paramagnetic–ferromagnetic hybrids at low temperatures

We compare the magneto-transport in paramagnetic–ferromagnetic GaAs:Mn/MnAs granular hybrids and paramagnetic GaAs:Mn reference samples. The differences in the hole transport between the two systems at low temperatures arise due to carrier localization effects at the cluster–matrix interface in the hybrids. The localization is caused by a Schottky barrier formation at the interface as well as spin-dependent shifts of the hole bands caused by the stray field of the ferromagnetic clusters. The application of an external magnetic field leads to a delocalization of the carriers and thus a negative magneto-resistance effect. These effects can be simulated using a network model approach.

Many-Body Interactions in the CdTe-Based Quantum Well under Strong Optical Excitation

We performed pump-probe time-resolved absorption measurements on a modulation p-doped (Cd,Mn)Te quantum well. The density of the 2D hole gas was controlled, in the 10 cm−2 range, by additional cw illumination. Interactions between photocreated neutral (X) and charged (X) excitons and the 2D hole gas were analyzed. We found that the strongest effect is the influence of the carriers on both X and X optical resonances. Neutral and charged excitons are screened by holes, which results in the decrease in their intensities. We conclude from polarization resolved experiments that this screening is spin-dependent: it is more efficient between holes with opposite spins. Binding holes into the charged excitons reduces the screening of the neutral excitons and leads to an enhancement of the neutral exciton intensity. We also analyzed weaker effects, due to exciton–exciton interaction, at a constant hole density. We found that the reduction of the neutral exciton intensity due to two different mechanisms (phase-space filling and biexciton formation) is almost equal. We observed a spin-dependent blue shift of the neutral exciton line in the presence of a population of neutral excitons created by the pump pulse. Due to the attractive interaction between excitons with opposite spins, the corresponding shift of the neutral exciton is smaller than that observed due to the interaction between excitons with the same spins.

Optical band edge of diluted magnetic semiconductors

Applying the dynamical coherent potential approximation to a simple model, we have theoretically studied the behavior of the optical band edge in diluted magnetic semiconductors (DMS). For ${A}_{1\ensuremath{-}x}^{II}{\mathrm{Mn}}_{x}{B}^{VI}$-type DMS, the present study reveals that the linear relationship between spin splitting due to the exchange interaction $\ensuremath{\Delta}{E}_{\mathit{ex}}$ and averaged magnetization $\ensuremath{\mid}x⟨Sz⟩\ensuremath{\mid}$ widely holds for different values of $x$. The ratio, $\ensuremath{\Delta}{E}_{\mathit{ex}}∕x⟨{S}_{z}⟩$, however, does not agree with the exchange integral, but depends on both the exchange strength and the band offset. Furthermore, the present study reveals that in the low dilution of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$, the optical band edge exists not at the band edge of the impurity band but near the bottom of the host band. The optical band edge behaves as if the exchange interaction is ferromagnetic although the antiferromagnetic exchange interaction actually operates at the Mn site. We conclude that the spin-dependent shift of the carrier states between the impurity band and the host band accompanied by the change of magnetization causes the anomalous behavior of the optical band edge (the reversal of the apparent sign of the exchange integral) that was reported in the magnetoreflection measurement of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$.

Loss-free polarization of pulsed neutron beams

We study the feasibility and efficiency of the so-called “dynamical polarization” of neutrons under realistic conditions. This loss-free technique exploits the spin-dependent energy shift of neutrons traversing a NMR-like magnetic field arrangement in combination with Larmor precession of their magnetic moments to end up in a situation where all particles finally are in the same spin state. We present possible field configurations for the application of this dynamical polarization method to pulsed polychromatic beams and investigate their stability under realistic experimental conditions.

Stabilization ofd-band ferromagnetism by hybridization with uncorrelated bands

We investigate the influence of s-d or p-d hybridization to d-band ferromagnetism to estimate the importance of hybridization for the magnetic properties of transition metals. To focus our attention to the interplay between hybridization and correlation we investigate a simple model system consisting of two non-degenerated hybridized bands, one strongly correlated, the other one quasi-free. To solve this extended Hubbard model, we apply simple approximations, namely SDA and MAA, that, concerning ferromagnetism in the single-band model, are known to give qualitatively satisfactory results. This approach allows us to discuss the underlying mechanism, by which d-band ferromagnetism is influenced by the hybridization on the basis of analytical expressions. The latter clearly display the order and the functional dependencies of the important effects. It is found, that spin-dependent inter-band particle fluctuations cause a spin-dependent band shift and a spin-dependent band broadening of the Hubbard bands. The shift stabilizes, the broadening tends to destabilize ferromagnetism. Stabilization requires relatively high band distances and small hybridization matrix elements. Super-exchange and RKKY coupling are of minor importance.

Observation of spin polarized transport across a ferromagnet–two-dimensional electron gas interface (invited)

Spin injection at a ferromagnet–semiconductor interface has been demonstrated by convolving the spin-split density of states of carriers in a high mobility InAs quantum well with the spin polarization of conduction electrons in a thin ferromagnetic film. Transport measurements on a diode structure detected a spin-dependent interfacial resistance, with ΔRi/R̄i≈1%. In a different configuration, open circuit voltage experiments measure the spin-dependent shifts in the chemical potential of the carriers in the quantum well. Results of the two techniques are compared over the temperature range 50<T<296 K, demonstrating both spin injection and detection at the ferromagnet–semiconductor interface.

High precision measurement of the topological Aharonov–Casher effect with neutrons

The phase shift predicted by Aharonov and Casher (AC) for a magnetic dipole diffracting around a line charge was first observed by Cimmino et al. using a neutron interferometer. A number of subsequent atom interferometry experiments have been performed to observe this effect. These experiments measured the v× E phase shift due to the magnetic field induced in the rest frame of the atom, with no indication of the topological nature of the AC interaction. We intend to perform a high precision AC experiment with neutrons to improve the accuracy of our previous results and to highlight the topological nature of the effect. Finally, we present a novel geometry to achieve a spin-dependent magnetic phase shift.

Magnetism in the single-band Hubbard model

A self-consistent spectral density approach (SDA) is applied to the Hubbard model to investigate the possibility of spontaneous ferro- and antiferromagnetism. The starting point is a two-pole ansatz for the single-electron spectral density, the free parameter of which can be interpreted as energies and spectral weights of respective quasiparticle excitations. They are determined by fitting exactly calculated spectral moments. The resulting self-energy consists of a local and a non-local part. The higher correlation functions entering the spin-dependent local part can be expressed as functionals of the single-electron spectral density. Under certain conditions for the decisive model parameters (Coulomb interaction U, Bloch bandwidth W, band occupation n, temperature T) the local part of the self-energy gives rise to a spin-dependent band shift, thus allowing for spontaneous band magnetism. As a function of temperature, second-order phase transitions are found away from half-filling, but close to half-filling, the system exhibits a tendency towards first-order transitions. The non-local self-energy part is determined by use of proper two-particle spectral densities. Its main influence concerns a (possibly spin-dependent) narrowing of the quasiparticle bands with the tendency to stabilize magnetic solutions. The non-local self-energy part disappears in the limit of infinite dimensions. We present a full evaluation of the Hubbard model in terms of quasiparticle densities of states, quasiparticle dispersions, magnetic phase diagram, critical temperatures ( T c, T N) as well as spin and particle correlation functions. Special attention is focused on the non-locality of the electronic self-energy, for which some rigorous limiting cases are worked out.

Ferromagnetism in the strongly correlated Hubbard model.

We use a self-consistent ``spectral-density approach'' for the strongly correlated Hubbard model in order to find out under what circumstances spontaneous band ferromagnetism may appear. The magnetic polarization m=${n}_{\ensuremath{\uparrow}}$-${n}_{\ensuremath{\downarrow}}$ is examined for a bcc lattice in terms of the temperature T and the band occupation n=${n}_{\ensuremath{\uparrow}}$+${n}_{\ensuremath{\downarrow}}$ (0\ensuremath{\le}n\ensuremath{\le}2). For T=0 and less than half-filled bands (n<1), ferromagnetism becomes possible as soon as n exceeds the critical value ${n}_{\mathrm{I}=0.54}$ and becomes saturated (m=n) above ${n}_{s}^{\mathrm{*}}$=0.74. A further, less polarized ferromagnetic solution appears for n\ensuremath{\ge}${n}_{\mathrm{II}=0.79}$. It turns out that a spin-dependent band shift, which consists of ``higher'' correlation functions, decisively determines the possibility of spontaneous moment ordering. This is demonstrated by a set of self-consistently calculated quasiparticle densities of states. The Curie temperature ${T}_{c}$ appears as strongly n dependent. Starting at ${0}^{+}$ for n=0.54, ${T}_{c}$ increases with n, reaching a maximum of about 710 K near n=0.75, and decreases again for n>0.8 down to ${0}^{+}$ for n=1. According to the free energy F, in cases of more than one solution, that solution with the highest polarization is always stable.

Effects of Exchange Interaction in Diluted Magnetic Semiconductor Quantum Wells

AbstractThe presence of transition metal ions (typically Mn2+) in diluted magnetic semiconductors (DMS) results in a strong spin-spin coupling between localized magnetic moments and band electrons. This leads to considerable modifications of the semiconductor band structure in the presence of strong magnetic fields, e.g., to large spin-dependent shifts of the electronic states at the band edge. This feature is of particular interest in the context of quantum wells involving DMS. Starting with the original idea of a “spin-superlattice”, we concentrate on various opportunities which arise due to the tunability of the depth of the quantum wells by the magnetic field and/or temperature associated with the aforementioned spindependent effects. Thus, we discuss boil-off and freeze-out of electrons to and from quantum wells, selective spin tunneling across the barriers, tunable infrared emitters, enhancement of electronic g-factors in shallow non-magnetic wells surrounded by DMS barriers, the possibility of transition from a type-1 to a type-il superlattice induced by the magnetic field, and quantum oscillations anomalies in DMS quantum wells.

Глуболы, образованные тремя глуонами, в модеи «мешка»

The spin-dependent energy shifts of glueballs composed of three gluons are studied in the MIT bag model systematically. The mass spectra of the three (TE)3 states and the seven (TE)2(TM) states are given, assuming that ι(1440) and θ(1640) are the(TE) (TM) and (TE)2 glueballs, respectively. We propose the possibility that ξ(2200) state is the 0++(TE)3 glueball. An exotic narrow 1−+ state is also expected to be seen in the radiative decay of J/ψ.

Antiferromagnetism in not exactly half‐filled narrow energy bands (hubbard model)

AbstractIt is investigated if the ground state of a system of interacting electrons in a narrow energy band can be of antiferromagnetic order within the limits of the Hubbard model. By means of the “method of spectral moments” an attempt is made to find antiferromagnetic solutions for all values of the band occupation, which then are checked for stability toward para‐ and ferromagnetism. The numerical evaluation for the b.c.c. lattice yields that antiferromagnetism is stable toward paramagnetism in a small region around the half‐filled band, but is unstable against ferromagnetism. Special attention is paid to the determination of a spin dependent band shift, which turns out to be a decisive quantity for the possibility of antiferromagnetic ordering.