High-field State Research Articles

Neutron experiments on solid3He in high field at very low temperature

The magnetic transition from the paramagnetic phase to the magnetically ordered high-field phase of solid3He is observed by polarized neutron transmission experiments. The transition shows the feasibility of neutron diffraction experiments on the high-field ground state of solid3He. The behavior of the boundary resistance between solid3He and silver powder in a magnetic field is discussed.

Interaction electromagnetic field and anisotropic superconductors: Application to high Tc

The specialities of interaction between high frequency electromagnetic field and anisotropic superconducting state are considered. It is shown that an interpretation of the high frequency surface resistance data for oxide superconductors could be based on the nonzero orbital momentum (spin) model.

Theory of reentrance of spin-density-wave transitions in bis-tetramethyltetraselenafulvalenium perchlorate

The stability of spin-density-wave formation in the organic superconductor bis-tetramethyltetraselenafulvalenium perchlorate (TMTSF${)}_{2}$${\mathrm{ClO}}_{4}$ is studied. The anion gap in the electron spectrum causes reentrance of the metallic state in high magnetic fields. It also leads to fast oscillations of physical properties, in particular, the phase-transition temperature versus magnetic field. Both these phenomena have been observed experimentally.

Quantum Hall effect in a bulk crystal.

We have observed remarkably flat Hall plateaus (step ratios . . .5:4:3:2:1) with correspondingly sharp peaks in the magnetoresistance in the Bechgaard salt (TMTSF${)}_{2}$${\mathrm{PF}}_{6}$. This highly anisotropic conductor exhibits magnetic-field-induced spin-density-wave (FISDW) states at low temperatures and pressures above about 6 kbar. Unlike previous measurements on similar materials we find almost complete confirmation of the theoretical model for the FISDW transitions, their quantum Hall effects, and the predicted high-field semiconducting state.

NMR studies of chalcogenide glasses: The system phosphorus-selenium

The feasibility of modern pulsed high-field high resolution solid state NMR techniques in non-oxide chalcogenide glasses is demonstrated for the system phosphorus-selenium. Isotropic and anisotropic chemical shieldings as well as 31P-31P dipolar couplings from 31P MAS and spin-echo NMR are reported on the glasses (10 to 50 mole% P), and the crystalline model compounds P4Se3 and P4Se4. Measurement of spin-echo decays show a monotonic increase of 31P-31P homonuclear dipole-dipole couplings with increasing phosphorus contents. The latter result suggests that the structure is more random than implied by the cluster model previously suggested.

On the vortex lattice transition in heavy-fermionic UPt3

The author argues that the observed phase transition in the UPt3 superconductor in applied magnetic field indicates the unconventional superconductivity in this heavy-fermion compound. It is shown that if the superconducting state corresponds to the two-dimensional representation and exhibits spontaneous anisotropy in the basal plane then the applied magnetic field splits the superconducting state into two phases. Each phase has a different structure of the Abrikosov vortex lattice: the high-field state has the vortex lattice with the hexagonal symmetry, while in the low-field state this symmetry is spontaneously broken. The transition between these two vortex states may be identified with the observed transition.

Cyclotron radiation in strong magnetic fields

Radiative transitions of electrons between low-lying Landau levels and the ground state in high magnetic fields are investigated. The relevant transition rates are calculated in the framework of spinor electrodynamics. Analytical approximations for various cases are compared to numerical evaluations. Special attention is given to the X-ray emission and absorption lines in magnetic fields of the order of 400 MT. 9 references.

Electronic structure of an isolated GaAs-GaAlAs quantum well in a strong electric field.

An exact numerical calculation for an isolated quantum well of a GaAs-GaAlAs system subjected to an external electric field is presented. Resonance positions and widths, and density-of-states profiles are reported for a 30-\AA{} well to demonstrate that the rigorous description of the high-field states as resonances leads to qualitatively new phenomena. In particular, in fields of 4\ifmmode\times\else\texttimes\fi{}${10}^{5}$ V ${\mathrm{cm}}^{\ensuremath{-}1}$ the effective hole energy shift is reduced to zero and a new antiresonance state emerges from the continuum into the well.

NMR imaging in solids by multiple-quantum resonance

NMR imaging is now a well-established technique for studying biological systems (1). In its most general form, an imaging method uses a magnetic field gradient to encode the positions of the nuclear spins with a spatially varying Larmor frequency. Once the variations in resonant frequency have been decoded appropriately, an image of the nuclear spin density or, more generally, of any mix of NMR parameters can be created. In a linear magnetic field gradient, g, the spread of frequencies across a thickness AZ is ghz. If features on the order of AZ are to be resolved, then the externally imposed field, gAz, must itself be resolved relative to any background or internal field. For solids, the dominant background field is usually the local dipolar field, BL. In biological systems familiar from ‘H imaging, rapid isotropic molecular motion often averages these internal dipolar fields to zero. However, in a strongly protonated solid, where molecular motion is restricted, a typical value for BL might be 5 G so that a gradient greater than 50 G/cm (0.5 T/m) would be needed to achieve a resolution of 1 mm. One approach (2-4) to this problem is to reduce the effective local field by a multiple-pulse line-narrowing sequence (5, 6). The alternative approach is to leave the local field untouched, but to impose a gradient large enough to meet the condition g % &/AZ. In this communication, we demonstrate a prototype imaging experiment for solids based in spirit on this “brute force” method of increasing the gradient, but which relies instead on the properties of multiplequantum NMR transitions (7, 8) to increase the effective gradient strength by an order of magnitude. Specifically, we intensify the effect of the gradient upon the evolution of the spin system by creating high-order multiple-quantum coherences and following their development in the static field gradient. A multiple-quantum coherence of order n = Mi Mj, where Mi and Mj are the magnetic-quantum numbers for high-field states Ii) and lj), evolves n times more rapidly in an inhomogeneous field than the usual single-quantum coherence. That is, if a singlequantum transition in the presence of a field gradient appears with resonance offset AU, then an n-quantum transition appears at nAo. The effective local dipolar fields are, however, roughly comparable for highand low-order coherences in very large

Electric quadrupole refocusing of CaBr: Differential transmission of hyperfine levels and assignment of low-J lines in optical spectra

Abstract The refocusing of a molecular beam of CaBr X 2 Σ + by an electric quadrupole is investigated. The enhancement by electric deflection of the intensities of lines originating from levels of low J in the fluorescence excitation spectrum of CaBr B 2 Σ + −X 2 Σ + is shown to be a useful technique of line assignment in a dense electronic band system. It is also found that the hyperfine levels are not transmitted equally through the quadrupole; for f levels, the hyperfine populations are actually reversed. A qualitative explanation, based on the correlation of zero-field hyperfine levels with high-field states, is given.

Field ionization of highly excited states of sodium

We have observed the selective ionization of the degenerate $|{m}_{j}|$ levels of $s$, $p$, and $d$ states of sodium in the range of $n$ from 15 to 20 and have shown that atoms produced in these low-field (\ensuremath{\sim} 10 V/cm) states pass adiabatically from low-field $|{m}_{j}|$ to intermediate-field $|{m}_{l}|$ states. Experiments on the Stark manifold states in intermediate fields (\ensuremath{\sim} 1 kV/cm) and high fields (\ensuremath{\sim} 4 kV/cm) show that the $|{m}_{l}|=0 \mathrm{and} 1$ states pass adiabatically from intermediate- to high-field states and that the passage of the $|{m}_{l}|=2$ states is very nearly adiabatic. These findings suggest a general approach for predicting the threshold fields for ionization of atomic Rydberg states.

Proton Resonance Study of the Antiferromagnetic State of Two-Dimensional Cu(HCOO)2·4D2O

Proton resonance experiments up to 60 MHz(14 k0e) were made at 4.2 K. The observed strong field-dependence of nmr spectra is attributed to a field-induced structure change from a 4-sublattice state to a 2-sublattice state. The origin of the transition is ascribed to the two-dimensional character of the interaction path. Rotation of the antiferromagnetic axis was observed in the 2-sublattice state by changing the field direction. The origin of this movement is attributed to the weak ferromagnetism mostly arising from the inequivalence of g -tensors of two copper ions in the unit cell. The large spin-reduction reported by Dupas and Renard was not found in the high field state.

Shallow impurity states in InSb in magnetic fields: High-field donor states and acceptor states

The high-field donor and acceptor states associated with Landau ladders are determined using the effective-mass approximation. Nonparabolic effects as well as band-band interaction are included in this calculation. Based on the calculated results, the coexisting feature of the Landau series and Rydberg series is reexamined. A quantitative comparison with the magneto-optical measurements is made in order to obtain information about the central-cell correction and nonparabolic effects. Selection rules for optical transitions and Raman transitions are also derived.

Exciton levels in a magnetic field

A variational calculation for the first six levels of m=0 even parity states of hydrogenic system in a magnetic field is given. Our calculation, which bridges the gap between low field states and high field states, strongly supports the non-crossing rule and stands completely against either of the proposed conservation criteria for the connection of high field states to the low field states. Plots showing the evolution of the nodal surfaces of excited state wavefunctions with changing magnetic field are presented. The relevance of our results to the interpretation of experimental excition spectra in TlCl and TlBr and donor spectra in GaAs is pointed out.

Pulse discrimination by Gunn-effect switching

Starting with simple equations for the domain dynamics the switching behaviour of a Gunn device with an ohmic load resistance is investigated, in a way similar to tunnel diode switching. An appropriate load line shall define three bias states on the current voltage characteristic, i.e. a homogeneous low-field state I ( E I < E peak), a homogeneous high-field state III ( E III > E valley) and a steady-state domain state IV. Then triggering the device from the only d.c. stable state I by (rectangular and triangular) input pulses of various heights and lengths leads to three types of switching processes namely direct return to I, return via state III or via state IV, these transitions yielding three typical output pulses. Discrimination curves for the input-pulse parameters are presented. Simple limiting cases for the three switching processes are treated in some detail yielding the switching times typical for each transition. Finally, the long-time stability of the bias states I, III and IV is discussed.

Effect of Orbital Quantization on the Critical Field of Type-II Superconductors

We discuss the effect of the discreteness of the Landau levels on the upper critical field of a type-II superconductor. At $T=0\ifmmode^\circ\else\textdegree\fi{}$K an ideal superconductor will remain superconducting in an arbitrarily large magnetic field. However, the high-field state is destroyed by a minute amount of impurity scattering or by a small misalignment of up- and down-spin Landau levels. The residual effects are (a) a small shift in the low-temperature critical field [$\frac{\ensuremath{\Delta}{H}_{c2}}{{H}_{c2}}\ensuremath{\approx}{({k}_{F}{\ensuremath{\xi}}_{0})}^{\ensuremath{-}1}$] and (b) oscillatory structure in the transition temperature ${T}_{c2}(H)$ for temperatures $T\ensuremath{\lesssim}\frac{\ensuremath{\hbar}{\ensuremath{\omega}}_{c}}{2{\ensuremath{\pi}}^{2}k}$.

Proposed Gunn-effect switch

The possibility of using a Gunn device as a switch similar to a tunnel diode is investigated theoretically. Three switching types are obtained, one of which leads to the homogeneous high-field state with predicted switching times of a few |RN|C (corresponding to the homogeneous sample).

Paramagnetic Superconductivity in Extreme Type-II Superconductors

Magnetization measurements in applied magnetic fields $0&lt;H\ensuremath{\le}55$ kG and at temperatures $1.2\ensuremath{\le}T\ensuremath{\le}4.2$ \ifmmode^\circ\else\textdegree\fi{}K have been made on some extremely "dirty" (short electron mean free path) type-II superconducting transition-metal alloys with Gor'kov-Goodman-calculated Ginzburg-Landau ${\ensuremath{\kappa}}_{G}$ values in the range 30-100: Ti(16 at.% Mo), V(30 at.% Ti) (10 at.% Cr), Ti(22.5 at.% V), and Ti(25 at.% V). Down to the lowest temperatures of measurement, the data show that the high-field superconducting mixed state of such materials is characterized by (a) reversible paramagnetic magnetization, (b) second-order transitions at upper critical fields ${H}_{u}(T)$ where the paramagnetic superconducting magnetization ${M}_{s}(H)$ becomes equal to the paramagnetic normal-state magnetization ${M}_{n}(H)$, and (c) parameters ${\ensuremath{\kappa}}_{1}(T)\ensuremath{\equiv}\frac{{H}_{u}(T)}{[\sqrt{2}{H}_{c}(T)]}$ (where ${H}_{c}\ensuremath{\equiv}\mathrm{the}\mathrm{thermodynamic}\mathrm{critical}\mathrm{field}$) and ${\ensuremath{\kappa}}_{2}(T)\ensuremath{\propto}{{[\frac{d({M}_{s}\ensuremath{-}{M}_{n})}{\mathrm{dH}}]}_{{H}_{u}}}^{\ensuremath{-}\frac{1}{2}}$ which decrease with decrease of $T$. The second-order nature of the upper-critical-field transition implies a mixed-state Pauli-paramagnetic conduction-electron spin alignment near ${H}_{u}(T)$ which is comparable to that in the high-field normal state. Comparison of the ${H}_{u}(T)$, ${\ensuremath{\kappa}}_{1}(T)$, and ${\ensuremath{\kappa}}_{2}(T)$ data with recent extreme type-II theories of Maki and of Werthamer, Helfand, and Hohenberg suggests that electronic spin-flip scattering induced by spin-orbit coupling effectively acts to decouple superconductive spin pairing and thus enhance mixed-state Pauli paramagnetism.