Controlling electronic spin relaxation of cold molecules with electric fields

We present a theoretical study of atom-molecule collisions in superimposed electric and magnetic fields and show that dynamics of electronic spin relaxation in molecules at temperatures below 0.5 K can be manipulated by varying the strength and the relative orientation of the applied fields. The mechanism of electric field control of Zeeman transitions is based on an intricate interplay between intramolecular spin-rotation couplings and molecule-field interactions. We suggest that electric fields may affect chemical reactions through inducing nonadiabatic spin transitions and facilitate evaporative cooling of molecules in a magnetic trap.     

Orientational transitions in ferromagnetic liquid crystals with bistable coupling between colloidal particles and the matrix

We study the orientational response of a ferromagnetic liquid crystal that is induced by magnetic and electric fields. A modified form of the energy of the orientational interaction between magnetic impurity particles and the liquid crystal matrix that leads to bistable coupling is considered. It is shown that apart from magnetic impurity segregation, first-order orientational transitions can be due to the bistability of the potential of the orientational coupling between the director and the magnetization. The ranges of material parameters that lead to optical bistability are determined. The possibility of first-order orientational transitions is analyzed for the optical phase difference between the ordinary and extraordinary light rays transmitted through a ferronematic cell. It is shown that an electric field applied in the given geometry considerably enhances the magneto-orientational response of the ferronematic.     

Phase diagram of diluted magnetic semiconductor quantum wells

The phase diagram of diluted magnetic semiconductor quantum wells is investigated. The interaction between the carriers in the hole gas can lead to first-order ferromagnetic transitions, which remain abrupt in applied fields. These transitions can be induced by magnetic fields or, in double-layer systems, by electric fields. We make a number of precise experimental predictions for observing these first-order phase transitions.     

Cyclotron resonance in asymmetric double quantum wells

Far-infrared magnetotransmission measurements in magnetic fields are carried out on asymmetric coupled double wells. We observe a splitting in the cyclotron resonance (CR) line for a wide range of intermediate magnetic fields and only one line at high magnetic fields. Two peaks observed in the CR spectra correspond to transitions between Landau levels in individual wells. We propose that phase transition between weak and strong coupling regimes may be responsible for the features. The characteristics of the transition are studied via an analysis of CR masses, CR splitting and line widths as a function of the magnetic field.     

Spin chirality and electric polarization in multiferroic compounds (, Er)

Polarized neutron diffraction experiments have been performed on multiferroic materials RMn₂O₅ (R=Ho, Er) under electric fields in the ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM) phases, where the former has the highest electric polarization and the latter has reduced polarization. It is found that, after cooling in electric fields down to the CM phase, the magnetic chirality is proportional to the electric polarization. Also we confirmed that the magnetic chirality can be switched by the polarity of the electric polarization in both the CM and LT-ICM phases. These facts suggest an intimate coupling between the magnetic chirality and the electric polarization. However, upon the transition from the CM to LT-ICM phase, the reduction of the electric polarization is not accompanied by any reduction of the magnetic chirality, implying that the CM and LT-ICM phases contain different mechanisms of the magnetoelectric coupling.     

Precursor phenomena at the magnetic ordering of the cubic helimagnet FeGe

We report on detailed magnetic measurements on the cubic helimagnet FeGe in external magnetic fields and temperatures near the onset of long-range magnetic order at T(C)=278.2(3) K. Precursor phenomena display a complex succession of temperature-driven crossovers and phase transitions in the vicinity of T(C). The A-phase region, present below T(C) and fields H<0.5 kOe, is split in several pockets. The complexity of the magnetic phase diagram is theoretically explained by the confinement of solitonic kinklike or Skyrmionic units that develop an attractive and oscillatory intersoliton coupling owing to the longitudinal inhomogeneity of the magnetization.     

On the determination of small electric dipole moments of excited electronic states in diatomic molecules

A new method is proposed for the determination of small electric-dipole moments in diatomic molecules by measuring the induced transitions due to crossed electric and magnetic fields. A theoretical treatment is given for states belonging to Hund's cases (a) and (b), and for states in the intermediate coupling case between Hund's cases (a) and (b). The method is restricted to non-Σ states.     

Unusual thickness-dependent thermal behavior and anticlinic coupling in chiral smectic free-standing liquid-crystal films

We observe, in free-standing films of a chiral smectic liquid crystal, a series of discrete transitions in the relative orientation of the tilt of the interior and surface layers. These transitions include a remarkable reentrant synclinic-anticlinic-synclinic ordering sequence of the film surfaces in the presence of an electric field upon cooling. The profiles of the associated heat-capacity anomalies are found to be strongly thickness dependent and exhibit a novel crossover behavior in reduced dimensions. We measure the anticlinic coupling between tilted surface layers in the smectic- A phase.     

微波驱动精细结构能级跃迁引起的电磁诱导负折射效应

本文通过建立Λ形四能级原子系统, 研究了微波驱动精细结构能级跃迁引起的电磁诱导负折射效应. 微波场作用于基态精细结构能级之间, 与不同精细结构能级之间的电偶极矩或磁偶极矩发生耦合, 使系统在某些频率处呈现负折射特性.同时, 两个耦合场各自激励一对基态和激发态之间的光学跃迁. 通过改变两个耦合场的频率失谐量控制负折射区域的频带宽度.结果表明, 耦合场失谐时出现负折射特性的频率范围比耦合场共振时迅速缩小, 而且耦合场负失谐和正失谐时的变化规律不同.     

Induction of ferroelectric transitions by the noncollinear exchange-modulated magnetic structure and magnetic fields in terbium manganate

The magnetic transition from the exchange-modulated collinear to the noncollinear state in terbium manganate, accompanied by the appearance of electric polarization, is explained within the Landau theory of phase transitions. The experimentally observed reorientation or disappearance of electric polarization in magnetic fields along the Y and Z axes are explained by the spin-flop transitions of manganese spins from the incommensurate noncollinear to the commensurate magnetic phase.     

Radiation of the Point Source from an Anisotropic Plasma Cylinder

We obtain and analyze an analytical solution to the problem of electromagnetic-wave radiation of the point electric dipole from an anisotropic plasma cylinder to free space. Two cases of the dipole orientation are considered, where the electric dipole is directed along and across a horizontal magnetic field whose direction does not coincide with the axes of a cylindrical coordinate system. We analyze how the conditions and characteristics of the resonance influence of the anisotropic plasma cylinder depend on the strength of the magnetic field and its direction with respect to the dipole moment of the source. Comparative analysis of the resonance influence of the plasma cylinder with horizontal and axial external magnetic fields is performed.     

Interaction of Wannier–Stark levels in a wide well superlattice

We have investigated the energy spectrum of a superlattice with wide quantum wells under the bias of an electric field perpendicular to the superlattice layers. By using photocurrent spectroscopy, transitions of Wannier–Stark levels for the various electron and hole states are observed, and at low fields, further structures corresponding to miniband edge transitions are found. Various anticrossings could be observed at higher and lower electric fields. The anticrossings at high electric fields are due to energy alignment of different electronic sublevels in adjacent wells. The anticrossing structures at low fields could be interpreted as resonances between intrawell and interwell excitonic Wannier–Stark states with equal sublevel states, where the anticrossing is caused by differences in exciton binding energy. Fitting of transitions and anticrossings was done by using a semi-empirical model and we have extracted relevant fitting parameters like the quantum-confined Stark coefficient, binding energies for the excitonic Wannier–Stark levels and the resonant coupling strength for states involved in the various anticrossing transitions. Finally, insight into the excitonic influences on the coupling of the WS states could be obtained by comparing the fitted parameters for the various transitions.     

Response of prestretched nematic elastomers to external fields

We investigate the response of prestretched nematic side-chain liquid single-crystal elastomers to superimposed external shear, electric, and magnetic fields of small amplitude. The prestretching direction is oriented perpendicular to the initial nematic director orientation, which enforces director reorientation. Furthermore, the shear plane contains the direction of prestretch. In this case, we obtain a strongly decreased effective shear modulus in the vicinity of the onset and the completion of the enforced director rotation. For the same regions, we find that it becomes comparatively easy to reorient the director by external electric and magnetic fields. These results were derived using conventional elasticity theory and its coupling to relative director-network rotations.     

Quantum interference of electrons in a ring: tuning of the geometrical phase

We calculate the oscillations of the dc conductance across a mesoscopic ring, simultaneously tuned by applied magnetic and electric fields orthogonal to the ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit coupling. They result from mixing of the dynamical phase, including the Zeeman spin splitting, and of geometric phases. By changing the applied fields, the geometric phase contribution to the conductance oscillations can be tuned from the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a realistic device, we also include nonzero backscattering at the connection between ring and contacts, and a random phase for electron wave function, accounting for dephasing effects.     

Role of the spin anisotropy of the interchain interaction in weakly coupled antiferromagnetic Heisenberg chains

In quasi-one-dimensional(q1D) quantum antiferromagnets, the complicated interplay of intrachain and interchain exchange couplings may give rise to rich phenomena. Motivated by recent progress on field-induced phase transitions in the q1D antiferromagnetic(AFM) compound YbAlO_3, we study the phase diagram of spin-1/2 Heisenberg chains with Ising anisotropic interchain couplings under a longitudinal magnetic field via large-scale quantum Monte Carlo simulations,and investigate the role of the spin anisotropy of the interchain coupling on the ground state of the system. We find that the Ising anisotropy of the interchain coupling can significantly enhance the longitudinal spin correlations and drive the system to an incommensurate AFM phase at intermediate magnetic fields, which is understood as a longitudinal spin density wave(LSDW). With increasing field, the ground state changes to a canted AFM order with transverse spin correlations. We further provide a global phase diagram showing how the competition between the LSDW and the canted AFM states is tuned by the Ising anisotropy of the interchain coupling.     

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells. It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the crystallographic orientation of quantum wells and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.     

Photon-induced vanishing of magnetoconductance in 2D electrons on liquid helium

We report on a novel transport phenomenon realized by optical pumping in surface state electrons on helium subjected to perpendicular magnetic fields. The electron dynamics is governed by the photon-induced excitation and scattering-mediated transitions between electric subbands. In a range of magnetic fields, we observe vanishing longitudinal conductivity σ(xx)→0. Our result suggests the existence of radiation-induced zero-resistance states in the nondegenerate 2D electron system.     

Multiwell potentials of endohedral complexes of icosahedral symmetry

We examine multiwell potentials that correspond to the displacements of off-center ions in complexes with icosahedral symmetry (dodecahedrons, icosahedrons, fullerenes, etc.) along symmetry directions (toward vertices, midpoints of edges, and centers of faces). An expression is derived for an effective Hamiltonian, which describes the behavior of endohedral complexes with off-center ions placed in external electric fields of arbitrary strength and orientation. We find the eigenvalues of this Hamiltonian and calculate the intensities of the lines of all possible transitions between tunneling levels. We also predict and analyze the spectra of paraelectric resonances in the absence of an external static electric field (zero-field resonances) and in the presence of such a field. Finally, we provide recommendations for detecting these effects and discuss the specific features of the effects and the possibility of studying them. Zh. éksp. Teor. Fiz. 114, 222–238 (July 1998)     

Coupled magnetic-ferroelectric metal-insulator transition in epitaxially strained SrCoO3 from first principles

First-principles calculations are presented for the epitaxial-strain dependence of the ground-state phase stability of perovskite SrCoO(3). Through the combination of the large spin-phonon coupling with polarization-strain coupling and the coupling of the band gap to the polar distortion, both tensile and compressive epitaxial strain are seen to drive the bulk ferromagnetic-metallic (FM-M) phase to antiferromagnetic-insulating-ferroelectric (AFM-I-FE) phases, the latter having unusually low elastic energy. For compressive strain, there is a single coupled magnetic-ferroelectric metal-insulator transition. At this phase boundary, cross responses to applied electric and magnetic fields and stresses are expected. In particular, a magnetic field or compressive uniaxial stress applied to the AFM-FE(z) phase could induce an insulator-metal transition, and an electric field applied to the FM-M phase could induce a metal-insulator transition.