Magneto-tunneling spectroscopy of GaAs---AlGaAs double barrier tunneling devices in pulsed high magnetic fields up to 40T

The magneto-tunneling effect was investigated in GaAs---AlGaAs double barrier resonant tunneling devices in pulsed high magnetic fiels up to 40T applied parallel(B) and perpendicular (B) to the barrier layers. In a sample with , oscillatory structures due to the 2D electrons in the emitter and the LO phonon assisted resonant tunneling were observed when the magnetic field (B) was swept at constant bias voltages. A large drop of the current was found in the quantum limit at applied voltages below the negative differential conductivity region. A striking hysteresis was observed in the voltage-current (V - I) curves. In a wide well sample with , rich structures were observed in the V - I curve for B, corresponding to the tunneling to different cyclotron orbits from the emitter.     

Magnetotunneling spectroscopy of ring-shaped (InGa)As quantum dots: Evidence of excited states with 2pz character

We study the effect of a high magnetic field (B) on the current–voltage characteristics, I(V), of a GaAs/(AlGa)As resonant tunneling diode incorporating a layer of ring-shaped quantum dots (QDs) in the quantum well (QW). The dots give rise to a series of four unusual resonances in I(V) which show a high degree of reproducibility across the epitaxial wafer. By combining data for B parallel and perpendicular to the growth axis z, we identify that the unusual resonances arise from resonant tunneling into QD excited states with 2p_z-like symmetry. The two series of magneto-oscillations in I for Bz allow us to determine the resonant charging and discharging of the QW with varying bias.     

N-type ferrimagnetism of SmVO3

DC and AC magnetizations of the perovskite ortho-vanadate compound SmVO₃ with GdFeO₃ structure were investigated by a SQUID magnetometer. The low field cooled magnetic susceptibility became negative between 68 and 122 K, and the negative remanent and spontaneous magnetizations were confirmed at 90 K with cooling fields below 10 kOe. These results are discussed on the basis of the molecular field theory for the N-type ferrimagnetism of V³⁺ magnetic moments. By the present analysis, the ferrimagnetic transition temperature of T_c=130 K, and the inter- and intra-sublattice exchange integrals of J_AB/k_B=−62.5 K, J_AA/k_B=−16.2 K and J_BB/k_B=−13.8 K were obtained.     

Mapping of the hole wave functions of self-assembled InAs-quantum dots by magneto-capacitance–voltage spectroscopy

We have performed magneto-capacitance–voltage spectroscopy for the valence band states of InAs quantum dots embedded in a p-type Schottky diode. By choosing the right measurement frequency and applying an in-plane magnetic field, we were able to map the k-space wave functions corresponding to the individual charging peaks. The wave functions belonging to the first two charging peaks show no nodes as expected for an s-like ground state. In contrast, nodes are observed for the next four charging peaks supporting the identification as excited states with finite orbital angular momentum. Peaks 3 and 4 show different wave functions compared to peaks 5 and 6, which points to different angular momenta for this two pairs of charging peaks.     

Condition for adiabatic passage in the earth’s-field NMR technique

The equation of motion dM/dt=γ M×B(t) is solved for the case B(t)=jB_p(t)+kB_e. The field B_e is a small static field, typically the earth’s field. The field B_p(t) decays exponentially toward zero with time constant T. This decay is produced by an overdamped switching transient that occurs near the end of the rapid cutoff of the coil current used to polarize the sample. It is assumed that B_p is initially large compared to B_e, and that magnetization M is initially along the resultant field B. Exact solutions are obtained numerically for several decay time constants of B_p, and the motion of M is depicted graphically. It is found that for adiabatic passage, the final cone angle β of the precession in field B_e is related to the decay time constant of B_p by β=2e^{−(π/2)ω_eT}. This is confirmed by measurements of the amplitudes of the ensuing free-precession signals for various decay rates of B_p. Near-perfect adiabatic passage (magnetization aligned within 2° of the earth’s field) can be achieved for time constants T2.6/ω_e. For the case of sudden passage, an approximate analytic solution is developed by linearizing the equation of motion in the laboratory frame of reference. For the adiabatic case, an approximate analytic solution is obtained by linearizing the equation of motion in a rotating frame of reference that follows the resultant field B=B_p+B_e.     

Magnetic order in CaMn2Sb2 studied via powder neutron diffraction

This paper reports a neutron powder diffraction study of CaMn₂Sb₂ in the temperature range of 20–300 K. Collinear long-range antiferromagnetic order of manganese ions occurs below 85 K, where a transition is observed in the dc magnetic susceptibility measured with a single crystal. Short-range magnetic order, characterized by a broad diffraction peak corresponding to a d-spacing of approximately 4 Å (2θ≈22°), is also observed above 20 K. The long-range antiferromagnetic order is indexed by the chemical unit cell, indicating a propagation vector k=(0 0 0), with a refined magnetic moment of 3.38 μ_B at 20 K. Two possible magnetic models have been identified, which differ in spin orientation for the two manganese ions with respect to the ab plane. The model with spins oriented at a 25±2° angle relative to the ab plane gives an improved fit compared to the other model in which the spins are constrained to the ab plane. Representational analysis can account for a model involving a c-axis component only by the mixing of two irreducible representations.     

超导膜的能隙和磁场的依赖关系

本文计算出超导膜在磁场中的能隙。所得到的表达式适用于所有的温度,但磁场要满足((ehp₀/(mc) A/(πk_BT_c))²《1条件,其中A是矢势,hp₀是Fermi动量。理论结果和实验进行了比较,符合得还好。     

Dynamics of magnetic insulation failure and self-organization of an electron flow in a magnetron diode

The dynamics of back cathode bombardment (BCB) instability in a magnetron diode (a coaxial diode in a magnetic field, B ≡ B _0z ≡ B ₀) is numerically simulated. The quasi-stationary regime of electron leakage across the high magnetic field (B ₀/B _cr > 1.1, where B _cr is the insulation critical field) is realized. An electron beam in the electrode gap is split into a series of bunches in the azimuthal direction and generates the electric field component E _θ(r, θ, t), which accelerates some of the electrons. Having gained an extra energy, these electrons bombard the cathode, causing secondary electron emission. The rest of the electrons lose kinetic energy and move toward the anode. Instability is sustained if the primary emission from the cathode is low and the secondary emission coefficient k _se=I _se/I _{e, BCB} is greater than unity. The results of numerical simulation are shown to agree well with experimental data. A physical model of back-bombardment instability is suggested. Collective oscillations of charged flows take place in the gap with crossed electric and magnetic fields (E × B field) when the electrons and E × B field exchange momentum and energy. The self-generation and self-organization of flows are due to secondary electron emission from the cathode.     

Alternating magneto-birefringence of ionic ferrofluids in crossed fields

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated by another smaller field, normal and alternating. The optical magneto-birefringence under these crossed magnetic fields is recorded as a function of the frequency for different field intensities and different sizes of the magnetic nanoparticles. A general reduced behavior is found for the in-phase and the out-of-phase optical response which is well-described by a simple mechanical model. Depending on the value H _ani of the anisotropy field of the nanoparticles, we can distinguish two different high magnetic field regimes: - a rigid dipole regime (large anisotropy energy with respect to k _B T) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity H _C(H _C < H _ani), - a soft dipole regime (anisotropy energy of the order of k _B T) for maghemite nanoparticles with a relaxation time independent of the field intensity H _C(H _C > H _ani). Received 5 June 2000 and Received in final form 8 January 2001     

Neutron diffraction study of the U(Pd1−xFex)2Ge2 magnetic structure

The neutron diffraction and magnetic susceptibility studies have shown that the magnetic structure of UPd₂Ge₂ changes dramatically even under very low iron doping. Though the general magnetic structure of pure UPd₂Ge₂ and of 1%Fe-doped samples is the same, the temperature intervals of existence of different magnetic phases are different. The values of transition temperatures, where (i) the ‘square’ modulated longitudinal spin-density wave (LSDW) structure with the propagation vector k=(0; 0; ) starts to transform into the sinusoidal modulated LSDW structure and (ii) the commensurate phase transforms into incommensurate one, shift under the 1%Fe doping to the higher temperatures (from 50 to 65 K and from 80 to 90 K, respectively). In the pure and 1%Fe-doped UPd₂Ge₂, the magnetic transition from the commensurate to incommensurate phase is accompanied by the drastic decrease of the propagation vector k_z. In the 2%Fe-doped sample, besides the Néel point of T_N=135 K, we have found two additional characteristic temperatures of 65 and 93 K. Below 65 K, the material has a simple antiferromagnetic (AF) structure with the propagation vector k=(0; 0; 1) and, at 65 K<T<T_N, the magnetic structure is LSDW with sinusoidal modulation. Over almost the total region 65 K<T<T_N, the LSDW magnetic structure is incommensurate. Only at about 93 K, the propagation vector passes the commensurate value of , whereas at 65<T<93 K and at 93 K<T<T_N. We have found that the magnetic susceptibility and the uranium magnetic moment are sensitive to the transition. With increasing iron concentration to x0.15, the simple AF structure with k=(0; 0; 1) develops over all temperature region up to the Néel point. Below T_N, the uranium magnetic moments are always parallel to the tetragonal c-axis.     

The sub-micrometer thickness n-InSb/i-GaAs epilayers for magnetoresistor applications at room temperatures of operation

Magnetotransport at fields up to 500 mT and LF-noise characteristics are reported for miniature magnetoresistors with ferrite concentrators based on Sn-doped n-InSb/i-GaAs heterostructures grown by MBE. The thickness of the InSb epilayers lie in the range 0.55–1.5 μm giving room temperature mobilities of 2.5–5.5 m² V⁻¹ s⁻¹ with carrier densities of (0.5–1.5)×10¹⁷ cm⁻³. The room temperature magnetoresistance (MR) for our two terminal devices could be as high as 115% at 50 mT which is comparable to the extraordinary MR (ExMR) recently reported in microscopic composite van der Pauw disks four terminal devices [Science 289 (2000) 1530]. In addition, a high signal-to-noise ratio and a good temperature stability of R(B)/R₀=0.5–0.83% K⁻¹ was observed for B<60 mT (below the saturation field B_sat for ferrite). Device resistance stability R₀(T) was equal to 0.27–0.66% K⁻¹ in zero field with a nominal device resistance R₀=197–224 Ω for DC currents in the range I=0.01–1.0 mA. The minimum detectable magnetic field is estimated from the reduced differential MR (∂R/∂B)/R=2000% T⁻¹ at B=31 mT and normalised 1/f current noise power spectral density measured at the same field. The resolution limit B_min=2.6 nT at 10² Hz and B_min=0.82 nT at 10³ Hz. These resolution limits are seven times better than those recently reported for the same material n-InSb/i-GaAs and ferrite fabricated Hall sensors [Magnetotransport and Raman characterization of n-InSb/i-GaAs epilayers, for Hall sensors applications over extremely wide ranges of temperature and magnetic field, Proceedings NGS 10, IPAP Conference Series 2, IPAP, Tokyo, 2001, pp. 151–154].     

The study of superexchange interaction of ordered Li 0.5 Fe 1.0 Rh 1.5 O 4

Li_0.5Fe_1.0Rh_1.5O₄ has been studied by X-ray diffraction, Mössbauer spectroscopy. The crystal structure is characterized by the additional reflection (200) that is described by 1:1 ordered structure of Li, Fe at tetrahedral (A) site and can be assigned to the space group F $\bar{4}Li_0.5Fe_1.0Rh_1.5O₄ has been studied by X-ray diffraction, M?ssbauer spectroscopy. The crystal structure is characterized by the additional reflection (200) that is described by 1:1 ordered structure of Li, Fe at tetrahedral (A) site and can be assigned to the space group F3m. The lattice constant (a ₀) is 8.4348 ?. The temperature dependence of the magnetic hyperfine field is analyzed by the Néel theory of ferrimagnetism. The inter-sublattice superexchange interaction is found to be antiferromagnetic with a strength of J _A–B = –3.78 k _B while the intra-sublattice superexchange interactions are ferromagnetic with strengths of J _A–A = 5.40 k _B and J _B−B = 7.39 k _B . The Debye temperatures of the tetrahedral and octahedral sites are determined to be 388 and 464 ± 3 K, respectively, and the Néel temperature has been found to be 260 ± 3 K.     

Two-step flux penetration in classic antiferromagnetic superconductor

The influence of antiferromagnetic order on the mixed state of a superconductor may result in creation of spin-flop domains along vortices. This may happen when an external magnetic field is strong enough to flip over magnetic moments in the vortex core from their ground state configuration. The formation of domain structure causes modification of the surface energy barrier, and creation of the new state in which magnetic flux density is independent of the applied field. The modified surface energy barrier has been calculated for parameters of the antiferromagnetic superconductor DyMo₆S₈. The prediction of two-step flux penetration process has been verified by precise magnetization measurements performed on the single crystal of DyMo₆S₈ at milikelvin temperatures. A characteristic plateau on the virgin curve B(H ₀) has been found and attributed to the modified surface energy barrier. The end of the plateau determines the critical field, which we call the second critical field for flux penetration. Received 16 August 2002 / Received in final form 22 October 2002 Published online 29 November 2002     

Wave shape of de Haas-van Alphen oscillations and effective mass in the two-dimensional organic conductor α-(BEDT-TTF)2KHg(SCN)4

De Haas-van Alphen (dHvA) oscillations in the two dimensional organic conductor α-(BEDT-TTF)₂KHg(SCN)₄ have been measured by a canti-lever technique at high magnetic fields up to 30 T. The wave shape of the dHvA oscillations undergoes a drastic change around the transition field B_K (24 T). The effective mass shifts 1.5 m₀ in the density wave phase (B<B_K) to 1.65 m₀ in the normal metallic phase (B>B_K). Based on the simulation, we have determined that the primary cause of the characteristic field dependence of the wave shape is the change in the effective mass.     

Resonant Γ–X–Γ tunneling in GaAs/AlAs/GaAs single barrier heterostructures at zero and elevated magnetic field

We report the observation of resonant tunneling and magneto-tunneling between states of different effective mass derived from zone centre (Γ) and zone edge (X) points of the Brillouin zone in single AlAs barrier diodes. The nature of the X states involved (longitudinal X_zor transverseX_xy ) is deduced from the observed resonances in the conductance versus bias characteristics at zero magnetic field (B). At finite B, the σ–V curves exhibit resonant magneto-tunneling with X_zLandau levels (LL), whilst no evidence of resonances withX_xy LLs is found. Clear observation of both LL index (in-plane momentum) conserving and non-conserving tunneling to X_zallows the transverse effective mass in AlAs to be determined. As a consequence of the different effective masses, momentum-conserving tunneling is inhibited at B = 0, but is restored when high B is applied.     

Magnetotransport and magnetotunneling in single-layer, two-layer, and three-layer Bi2Sr2Can−1CunOz (n=1,2,3) single crystals

In continuous magnetic fields H up to 28 T, we have studied the out-of-plane transport properties and tunneling characteristics of high-quality nondoped single crystals of the Bi-cuprate family: Bi₂Sr₂CuO_6+δ (Bi2201), Bi₂Sr₂CaCu₂O_8+δ (Bi2212) and Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi2223) grown by an identical method. For all compounds the out-of-plane magnetotransport ρ_c(H) is negative in the temperature region where ρ_c(T) shows in the normal state a semiconducting-like temperature dependence. The negative magnetoresistance of ρ_c corresponds to the suppression of the semiconducting temperature dependence of ρ_c(T) which is found to be isotropic. For the Bi2201 compound, where the normal state can be reached in the available magnetic fields (28 T), a nearly complete suppression of the low-temperature upturn in ρ_c(T) is observed in the highest magnetic fields with a tendency towards a metallic behavior down to the lowest temperatures (0.4 K). Using the break-junction technique, especially for the Bi2212 and Bi2232 compounds, a clear superconducting gap structure can be observed. Both for temperatures above the critical temperature and for magnetic fields above the upper critical field, a pseudogap structure remains present in the tunneling spectra. The applied magnetic fields yield a stronger suppression of the superconducting state compared to that of the normal-state gap structures as manifested in ρ_c(T) transport and tunneling.     

Effects of linking chain length and magnetic field on the kinetics of photoprocesses in covalently bonded porphyrin—viologen dyads

The formation and decay kinetics of chain linked triplet radical pairs derived from photo-induced electron transfer reactions in a series of 21 zinc porphyrin-flexible spacer-viologen (ZnP-Sp _n -Vi²⁺) dyads containing 2–138 atoms (n) in the spacer, have been examined by nanosecond laser flash photolysis techniques in an external magnetic field. In non-viscous polar solvents (acetone and CHCl₃ plus CH₃OH = 1:1 v/v), the effect of the spacer length on the rate constant of forward electron transfer can be described by the equation: k _et = k ⁰ _et(n + 6)^?1.5, with k ⁰ _et = 3 × 10¹⁰ s^?1 and 1.2 × 10¹⁰ s^?1 for electron transfer from the singlet and triplet states of ZnP, respectively. In zero magnetic field, the value of the triplet radical pair recombination rate constant, k _r(0) = 0.7 × 10⁶-8 × 10⁶ s^?1, is significantly smaller than k _et. The dependence of k _r(0) on n has an extremum with the maximum near n = 20. In a strong magnetic field (B = 0.21 T), significant retardation of triplet radical pair recombination is observed. In strong magnetic fields the effect of the chain length on triplet radical pair recombination rates is rather small and k _r(B) may vary in the range 0.3 × 10⁶-1 × 10⁷ s^?1. The phenomena observed are discussed in terms of the interplay of molecular and spin dynamics in the limits of slow and fast encounters, taking into account the exchange-interaction.     

Observation of magnetic breakdown in double quantum wells

We report on our studies of magnetic breakdown (MB) in coupled GaAs/Al_0.3Ga_0.7As double quantum wells (DQWs) subject to crossed magnetic fields. MB is a failure of semiclassical theory that occurs when a magnetic field causes electrons to tunnel across a gap ink-space from one Fermi surface (FS) branch to another. We study MB in a two-branch FS created by subjecting a DQW to an in-plane magnetic field (B_∥). The principal effect ofB_∥is a distortion in the dispersion curve of the system, yielding a FS consisting of two components, a lens-shaped inner orbit and an hour-glass-shaped outer orbit. The perpendicular field (B_⊥) causes Landau level formation and Shubnikov–de Haas (SdH) oscillations for each branch of the FS. At higher perpendicular fields MB occurs and electrons tunnel throughk-space from one FS orbit to the other. MB is observed by noting which peaks are present in the Fourier power spectrum of the magnetoresistance versus 1/B_⊥at constantB_∥. We observe MB in two DQW samples over a range ofB_∥.     

The small momentum transfer k L o C→K S 0 C regeneration at high energies

Results of the first elastic K _S ^o regeneration experiment on carbon, using magnetic spark chamber spectrometer, are presented in the beam momentum interval 10p50 GeV/c. The d ifferentia cross section d/dt is reconstructed in the range 0·0025–t0·02 (GeV/c)² and its slopeB is found to be momentum independent with an average valueB=(65±11) (GeV/c)^–2. The results are in agreement with the calculations using the coherent production model.     

Dynamics of a quantum particle in a static modulated magnetic field B=(0,0,B/cosh ((x−x 0)/δ))

Quantum motion of a particle with its massm and its chargee>0 in thex–y plane under the influence of the static unidirectionally modulated magnetic fieldB=(0,0,B/cosh²((x–x ₀)/)) is studied in this paper. We solved the single-particle problem exactly. Expressions for eigenenergies and eigenfunctions are found. Several physical phenomena are described: the energy spectrum separates into two parts which we call a discrete part and a continuous part—the discrete part of the spectrum corresponds with those states states which describe localized behavior in the direction of the field modulation; these states are extended in the perpendicular direction; the effective mass of this quasi-one-dimensional motion is found to be negative and dependent on the discrete quantum numbern—the states corresponding to the continuous part of the energy spectrum are found to be of two types: the reflection states, which are extended only in that part of thex–y plane where the effective potential barrier is small, and the states describing overbarrier motion in the whole plane; there exists a minimum of the energy for the states from this part of the spectrum which corresponds with a nonzero momentum of the particle motion in they-direction and almost zero momentum for thex-direction motion; there exists twofold degeneracy for those states from the continuous part of the spectrum for which their energy is lower than a certain value.