Anomalous compressibility and magnetostriction of beryllium under the conditions of diamagnetic domain formation

Magnetostriction oscillations are measured for a single-crystal beryllium sample shaped like a plate perpendicular to the direction of the magnetic field. In the range of 2–5 T at a temperature of 1.5 K, i.e., in the region of diamagnetic domain formation (Condon domains), the striction signal has the saw-tooth shape corresponding to the alternation of homogeneous and nonhomogeneous (domain) states. The formation of the domain structure is accompanied by an anomalous increase in compressibility; the oscillations in this coefficient are more than one hundred times greater than the value given by the standard theory. An analysis of the results indicates that the domain wall width should increase with increasing plate thickness.     

Coherent spin precession in normal Fermi liquids

In the collisionless limit the Fermi liquid interaction gives rise to nondissipative spin currents. Due to these currents, a coherently precessing spin state may be formed in NMR-experiments with normal Fermi liquids. This state consists of two domains separated by a coherently precessing domain wall. In one of the domains the magnetization is oriented along, and in the other opposite, to the direction of the magnetic field. Such a state has been studied in NMR-experiments in liquid³He and³He-⁴He solutions. The conditions necessary for the formation of the state are considered. Possible other substances in which similar states may be observed are discussed.     

Quantum oscillations of the resistivity and hall coefficient and the quantum limit in Bi<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript> alloys in a magnetic field along the trigonal axis

The dependences of the electrical resistivity ρ and the Hall coefficient R on the magnetic field have been measured for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconductor alloys with electron concentrations in the range 1 × 10¹⁶ cm⁻³ < n < 2 × 10¹⁸ cm⁻³. It has been found that the measured dependences exhibit Shubnikov-de Haas quantum oscillations. The magnetic fields corresponding to the maxima of the quantum oscillations of the electrical resistivity are in good agreement with the calculated values of the magnetic fields in which the Landau quantum level with the number N intersects the Fermi level. The quantum oscillations of the Hall coefficient with small numbers are characterized by a significant spin splitting. In a magnetic field directed along the trigonal axis, the quantum oscillations of the resistivity ρ and the Hall coefficient R are associated with electrons of the three-valley semiconductor and are in phase with the magnetic field. In the case of a magnetic field directed parallel to the binary axis, the quantum oscillations associated both with electrons of the secondary ellipsoids in weaker magnetic fields and with electrons of the main ellipsoid in strong magnetic fields (after the overflow of electrons from the secondary ellipsoids to the main ellipsoid) are also in phase. In magnetic fields of the quantum limit ħω _c /2 ≥ E _F, the electrical conductivity increases with an increase in the magnetic field: σ₂₂(H) ∼ H ^k . A theoretical evaluation of the exponent in this expression for a nonparabolic semiconductor leads to values of k close to the experimental values in the range 4 ≤ k ≤ 4.6, which were obtained for samples of the semiconductor alloys with different electron concentrations. A further increase in the magnetic field results in a decrease of the exponent k and in the transition to the inequality σ₂₂(H) ≤ σ₂₁(H).     

The distribution of equilibrium magnetization currents in systems with dimensional quantization in a finite magnetic field

The distribution of equilibrium magnetization currents in two-dimensional bounded systems placed in an external magnetic field is studied. A half-plane, a quantum disk, and a wide quantum ring are considered. The passage from classical to quantizing magnetic fields is investigated. The edge currents near the boundary of the half-plane are shown to experience damped (far from the boundaries) spatial oscillations related to the Fermi electron wavelength. The region occupied by currents was found to narrow with increasing field. Apart from these oscillations, the current contains a component that smoothly changes with distance but oscillationally depends on the position of the Fermi level relative to the Landau levels. The suppression of the oscillations by temperature is studied. The spatial distribution of the current in a circular disk and a ring is shown to significantly depend on the position of the Fermi level.     

Thermopower of a semiconductor film with parabolic potential in a strong magnetic field

Relationships for the transverse magnetothermopower of a semiconductor film with parabolic potential are derived for the cases of nondegenerate statistics and strong degeneracy. It is demonstrated that, in the case of strong degeneracy, quantum levels lying below the Fermi level intersect the Fermi level with an increase in the magnetic field, which leads to jumpwise oscillations of the thermopower magnitude.     

Doppler-shifted cyclotron resonance in tungsten plates with atomic pure surface

The surface resistance of thin monocrystalline W plates as a function of the constant magnetic field H directed along the normal to the sample surface is studied in the r.f. spectrum region. The sample surface was cleaned in high vaccum (10^-11 torr) or coated with the monomolecular impurity film. The oscillating with the magnetic field part R_osc due to the Doppler-shifted cyclotron resonance is studied. The doppleron oscillation amplitude is found to depend on the surface state and increases with the crystal cleaning. The observed changes are caused by the increase of the specular reflection coefficient for resonance electrons. With the deviation of the magnetic field from the normal to the plate surface, the doppleron wave undergoes a collisionless magnetic Landau damping and the signal amplitude decreases down to values comparable with that of Gantmakher-Kaner oscillations. Cleaning of the surface (and related increase of specularity) gives rise to a further decrease of the doppleron amplitude and appearance of additional interference maxima induced by the Gantmakher-Kaner effect.     

Density-of-state oscillation of quasiparticle excitation in the spin density wave phase of (TMTSF)2ClO4

Systematic measurements of the magnetocaloric effect, heat capacity, and magnetic torque under a high magnetic field up to 35 T are performed in the spin density wave (SDW) phase of a quasi-one-dimensional organic conductor (TMTSF)2ClO4. In the SDW phase above 26 T, where the quantum Hall effect is broken, rapid oscillations (ROs) in these thermodynamic quantities are observed, which provides clear evidence of the density-of-state (DOS) oscillation near the Fermi level. The resistance is semiconducting and the heat capacity divided by temperature is extrapolated to zero at 0 K in the SDW phase, showing that all the energy bands are gapped, and there is no DOS at the Fermi level. The results show that the ROs are ascribed to the DOS oscillation of the quasiparticle excitation.     

Magnetic measurements of Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B in the vicinity of the Curie temperature

The study of the transition between ferromagnetic and paramagnetic states has been investigated on selected metallic glass systems based on Fe-Ni-Nb-B and Fe-Co-Mo-Cu-B with T_C close to room temperature. Samples in the form of ribbons were prepared by planar flow casting and magnetostriction in parallel and perpendicular directions and saturation magnetostrictions have been determined on these samples in as-cast states together with hysteresis loops. In addition, a magneto-optic device for dynamic domain observation has been used for observation of domain structure. Magnetostriction measurements using direct method of measurement show the decrease of saturation magnetostriction towards zero upon approaching T_C. In paramagnetic state the field dependencies of magnetostriction in parallel and perpendicular configurations exhibit a linear dependence on the external magnetic field. In the transition region of temperatures the dependencies are a combination of ferromagnetic and paramagnetic field dependencies. The coercivity H_C in the materials investigated exhibits values below 20 A/m. The observed magnetic domains are typical for this class of amorphous alloys. The polarization in paramagnetic state increases gradually with increase in magnetizing field, reflecting the increasing amount of polarized regions.     

Corrugated graphene: effects of in-plane and tilted out-of-plane magnetic fields

The electronic energy level structure of the corrugated graphene electron subjected to a magnetic field tilted with respect to the graphene plane and an in-plane homogeneous magnetic field is investigated theoretically within the perturbation framework. It is shown that the anisotropy induced by the tilted magnetic field strongly modifies the Fermi velocity of the corrugated graphene electron, and the corrugated structure yields intrinsic Weiss oscillations in both Fermi velocity and the graphene Landau levels.     

Giant magnetoresistance oscillations induced by microwave radiation and a zero-resistance state in a 2D electron system with a moderate mobility

The effect of a microwave field in the frequency range from 54 to 140 GHz on the magnetotransport in a GaAs quantum well with AlAs/GaAs superlattice barriers and with an electron mobility no higher than 10⁶ cm²/V s is investigated. In the given two-dimensional system under the effect of microwave radiation, giant resistance oscillations are observed with their positions in the magnetic field being determined by the ratio of the radiation frequency to the cyclotron frequency. Earlier, such oscillations had only been observed in GaAs/AlGaAs heterostructures with much higher mobilities. When the samples under study are irradiated with a 140-GHz microwave field, the resistance corresponding to the main oscillation minimum, which occurs near the cyclotron resonance, appears to be close to zero. The results of the study suggest that a mobility value lower than 10⁶ cm²/V s does not prevent the formation of zero-resistance states in a magnetic field in a two-dimensional system under the effect of microwave radiation.     

Magnetostatic energy and stripe domain structure in a ferromagnetic plate of finite width with in-plane anisotropy

The magnetostatic energy and domain structure (DS) in a long ferromagnetic plate of a finite width with in-plane anisotropy are calculated for the case of the domain magnetization vectors lying in the plane of the plate. The situation where the DS period is much shorter than the width but is considerably larger than the thickness of the plate is analyzed in detail. The equilibrium DS period and the width ratio of two adjacent domains are determined as functions of an external magnetic field parallel to the plane of the plate by minimizing the energy. The DS period is found to be proportional to the plate width and the domain wall energy and inversely proportional to the squared saturation magnetization. While the width of the favorable domains (with the magnetization parallel to the field) grows with increasing field, the unfavorable domains, rather than disappearing completely, form relatively narrow transition regions between the favorable domains, i.e., 360° domain walls.     

Electronic topological transition in an <Emphasis Type="Italic">n</Emphasis>-BiSb semiconductor alloy in the quantum limit range of magnetic fields for H‖<Emphasis Type="Italic">C</Emphasis><Subscript>2</Subscript>

The galvanomagnetic properties of single-crystal samples of the Bi_0.93Sb_0.07 semiconductor alloy with the electron density n = 1.6 × 10¹⁷ cm⁻³ in magnetic fields up to 14 T at T = 1.6 K have been investigated. The resistivity ρ and Hall coefficient R have been measured as functions of the magnetic field directed along the binary axis of a crystal for a current flowing through a sample along the bisector axis; i.e., the components ρ₂₂ and R _{32, 1} have been measured. The strong anisotropy of the electron spectrum of the samples makes it possible to separately observe quantum oscillations of the magnetoresistance ρ₂₂(H) for H ‖ C ₂ in low magnetic fields for two equivalent ellipsoids with small extremal cross sections (secondary ellipsoids) and in high magnetic fields for electrons of the ellipsoid with a large extremal cross section (main ellipsoid). An increase in the energy of the electrons of secondary ellipsoids in the quantum limit magnetic fields is accompanied by the flow of electrons to the main ellipsoid; i.e., an electronic topological transition occurs from the three-valley electron spectrum to the single-valley one. After the flow stops, the Fermi energy E _F increases from 18 meV to 27.8 meV. With an increase in the quantizing magnetic field, the Fermi energy of the electrons decreases both in the region of quantum oscillations of the resistance that are attributed to the electrons of the secondary ellipsoids and in the region of oscillations associated with the electrons of the main ellipsoid. The Hall coefficient R _{32, 1} decreases in high magnetic fields; this behavior indicates the absence of the electron magnetic freezing effect.     

Local flattening of the Fermi surface and quantum oscillations in the magnetoacoustic response of a metal

In the present work, we theoretically analyze the effect of the Fermi surface local geometry on quantum oscillations in the velocity of an acoustic wave travelling in metal across a strong magnetic field. We show that local flattenings of the Fermi surface could cause significant amplification of quantum oscillations. This occurs due to enhancement of commensurability oscillations modulating the quantum oscillations in the electron density of states on the Fermi surface. The amplification in the quantum oscillations could be revealed at fitting directions of the magnetic field.     

180° domain structure in a cubic U3P4 crystal

In a thick plate cut from a U₃P₄ single crystal perpendicularly to the 111 direction, changes in the domain structure under the influence of a strong magnetic field perpendicular to the plate surface were studied. Experiments were carried out at T = 28 K, applying the Kerr effect technique to make the domain structure visible. In the cubic crystal studied, the applied field was found to force the appearance of a 180° branched maze domain structure, typical of thick plates of highly anisotropic uniaxial materials. The Hubert-Bodenberger model of this structure was improved and extended by considering the change in the shape of domains occurring with increase in the distance from the sample surfaces, by describing the sample in a non-zero magnetic field and by analyzing the sample in the state of remanence. The model was applied to the case of U₃P₄ and the energy density of 180° domain walls was estimated (γ = 12 erg/cm²).     

Types of Fermi surface segments of cuprate stripe phases

The mean field method was used to study zones and Fermi surfaces of the stripe phases of the Hubbard model, which describes the electronic properties of cuprates. It was shown for structures with stripes parallel to bonds that the Fermi surface consisted of open surface segments and closed surfaces around hole pockets. Segments of the first type originate from quasi-one-dimensional zones of states localized on domain walls. The properties of zones in comparison with the zones observed in angle-allowed photoemission spectra and the possible influence of the corresponding two types of carriers on the dependence of the Hall constant on temperature and doping are discussed. In view of the observed magnetic quantum oscillations, the search for electron pockets was undertaken. Their formation was shown to be possible in two-layer (but not one-layer) models.     

The effect of temperature on the nonlinear dynamics of charge in a semiconductor superlattice in the presence of a magnetic field

The space-time dynamics of electron domains in a semiconductor superlattice is studied in a tilted magnetic field with regard to the effect of temperature. It is shown that an increase in temperature substantially changes the space-time dynamics of the system. This leads to a decrease in the frequency and amplitude of oscillations of a current flowing through the semiconductor superlattice. The quenching of oscillations is observed, which is attributed to the change in the drift velocity as a function of electric-field strength under the variation of temperature.     

n-Hg1-xCdxTe的Shubnikov-de Haas振荡

在T=2.3—24K的温度范围里,研究了不同组分的n-Hg_1-xCd_xTe样品的SdH效应。测量了纵向和横向的磁阻振荡峰,能清晰地分辨n>3的自旋分裂峰。由此计算了n-Hg_1-xCd_xTe的能带参数,讨论了导带的非抛物线性和费密能级随磁场的变化对实验结果的影响。 关键词：     

Surface magnetostatic oscillations in elliptical bubble domains

A theory of surface magnetostatic oscillations in magnetic bubble domains with an elliptical cross section is presented. The dependences of the eigenfrequencies of resonant modes on the applied magnetic field are analyzed for a barium hexaferrite sample with allowance made for the change in the domain size due to a variation in the bias magnetic field. The range of frequency tuning in response to a magnetic field ranging from the elliptical instability field to the collapse field is estimated. It is demonstrated that elliptical bubble domains can be used as microminiature resonators operating in the millimeter range.     

Quenching of the quantum Hall effect in multilayered epitaxial graphene: the role of undoped planes

We propose a mechanism for the quenching of the Shubnikov-de Haas oscillations and the quantum Hall effect observed in epitaxial graphene. Experimental data show that the scattering time of the conduction electron is magnetic field dependent and of the order of the cyclotron orbit period, i.e., it can be much smaller than the zero field scattering time. Our scenario involves the extraordinary graphene n=0 Landau level of the uncharged layers which is pinned at the Fermi level. We find that the coupling between this n=0 Landau level and the conducting states of the doped plane leads to a scattering mechanism having the right magnitude to explain the experimental data.     

Magnetostriction studies of magnetic phase transitions in the copper metaborate CuB2O4

The field dependences of the longitudinal and transversal magnetostriction of the copper metaborate CuB₂O₄ were measured at various temperatures below the Néel point in magnetic fields directed along the tetragonal axis or in the basal plane. Magnetostriction was found to exhibit jumps at magnetic-field-induced phase transitions to a commensurate weak ferromagnetic state, as well as to grow smoothly in fields above and below the critical level. The magnetostriction observed in a magnetic field directed along the tetragonal axis is shown to be primarily caused by volume dilatation of the crystal. The experimental data obtained were used to construct the magnetic phase diagram of copper metaborate magnetized along the tetragonal axis.