High In-plane Magnetic Fields Research Articles

Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy

We study the effects of electron-electron interactions on the energy levels of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy spectrum at zero magnetic field, and at crossings of opposite-spin-levels in high in-plane magnetic field $B$. Our results constitute direct evidence that spin-up (higher energy) levels pin to the chemical potential $\ensuremath{\mu}$ as they populate. We also show that spin-up and spin-down levels abruptly rearrange at the crossing in a manner resembling the magnetic phase transitions predicted to occur at crossings of Landau levels. This rearranging and pinning of subbands provides a phenomenological explanation for the 0.7 structure, a one-dimensional (1D) nanomagnetic state, and its high-$B$ variants.

Soft magnetic properties of (Ni 50Fe 50) SiO 2 granular thin films for high frequency application

A series of (Ni 50Fe 50) (1-x) films with different volume fraction x was fabricated by magnetron co-sputtering technique. The microstructure, magnetic and electrical properties were investigated systematically by using X-ray diffraction, transmission electronic microscope, vibrating sample magnetometer and the traditional four point measurement method of resistivity. The results show that the samples consist of nano-scaled Ni 50Fe 50 metallic particles with fee structure uniformly embedded in amorphous insulating SiO 2 matrix, and the particle size decreases with the decrease of x. The rapid change of coercivity with x is observed, and a minimum value 160 A·m −1 of H c was obtained for the sample of x = 0.83 with film thickness of 180 nm, which can be contributed to the exchange coupling between nano-scaled Ni 50Fe 50 particles. At the frequency lower than 1 GHz, the real part μ of complex permeability keeps about 110 and the image part μ” is less than 15. Besides, this film exhibits high resistivity τ = 263 μΩ·cm, high saturation magnetization 4π M c = 1.25 T, high in-plane magnetic anisotropy field H k = 6.37 kA·m −1, and the ferromagnetic resonance (FMR) frequency is estimated to be 2.8 GHz. Therefore, this film can be used in high frequency devices operating over 2 GHz.

Luminescence of indirect excitons in high in-plane magnetic fields

We report on low-temperature magneto-luminescence measurements of biased Ga 1 - x Al x As / GaAs double quantum well structures. The luminescence of spatially indirect excitons (IX) is studied up to the magnetic field of 22 T. In contradiction to theoretical predictions and their several experimental confirmations, the IX line survives in PL spectra of our double quantum well structure up to high in-plane magnetic fields. We explain this difference by the IX localization which enables the relaxation of the IX in-plane momentum, whose conservation is responsible for the damping of the IX luminescence intensity observed by other groups. This localization is supported by a relatively weak non-radiative recombination in comparison to the radiative one observed in our double quantum well structure. The IX luminescence was studied under various excitation intensities to obtain deeper insight into the IX localization mechanism. This paper confirms and broadens our previously published results obtained from a double quantum well structure of other width.

In-plane magnetic-field-induced Wigner crystallization in a two-electron quantum dot

The orbital effects of the in-plane magnetic field on a two-electron harmonic quantum dot are studied using a variational method. For flat quantum dots the singlet-triplet transitions appearing in a perpendicular magnetic field are absent in a magnetic field oriented parallel to the plane of confinement. Instead, a degeneracy of orbital energies for symmetric and antisymmetric states at high in-plane magnetic field is observed. This degeneracy is due to the formation of Wigner molecules in the laboratory frame of reference with charge islands elongated along the direction of the magnetic field and localized within the plane perpendicular to it.

Luminescence of coupled quantum wells: Effects of indirect excitons in high in-plane magnetic fields

Luminescence measurements of a ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ double quantum well in in-plane magnetic fields up to $22\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ are reported. The properties of spatially direct and indirect excitons are studied. We show that the strong indirect exciton luminescence survives in samples with low nonradiative recombination up to high in-plane magnetic fields. This contrasts with previously published results, where its strong suppression, observed for magnetic fields as low as of $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, was explained by the exciton center-of-mass momentum conservation. We attribute the discrepancy to a relatively low nonradiative recombination in the studied sample in comparison with the radiative recombination of localized indirect excitons.

London theory of the crossing vortex lattice in highly anisotropic layered superconductors

A novel description of Josephson vortices (JVs) crossed by the pancake vortices (PVs) is proposed on the basis of the anisotropic London theory. The field distribution of a JV and its energy have been calculated for both dense ($a<\lambda_J$) and dilute ($a>\lambda_J$) PV lattices with distance $a$ between PVs, and the nonlinear JV core size $\lambda_J$. It is shown that the ``shifted'' PV lattice (PVs displaced mainly along JVs in the crossing vortex lattice structure), formed in high out-of-plane magnetic fields transforms into the PV lattice ``trapped'' by the JV sublattice at a certain field, lower than $\Phi_0/\gamma^2s^2$, where $\Phi_0$ is the flux quantum, $\gamma$ is the anisotropy parameter and $s$ is the distance between CuO$_2$ planes. With further decreasing $B_z$, the free energy of the crossing vortex lattice structure (PV and JV sublattices coexist separately) can exceed the free energy of the tilted lattice (common PV-JV vortex structure) in the case of $\gamma s<\lambda_{ab}$ with the in-plane penetration depth $\lambda_{ab}$ if the low ($B_x<\gamma\Phi_0/\lambda_{ab}^2$) or high ($B_x\gtrsim \Phi_0/\gamma s^2$) in-plane magnetic field is applied. It means that the crossing vortex structure is realized in the intermediate field orientations, while the tilted vortex lattice can exist if the magnetic field is aligned near the $c$-axis and the $ab$-plane as well. In the intermediate in-plane fields $\gamma\Phi_0/\lambda_{ab}^2\lesssim B_x \lesssim \Phi_0/\gamma s^2$, the crossing vortex structure with the ``trapped'' PV sublattice seems to settle in until the lock-in transition occurs since this structure has the lower energy with respect to the tilted vortex structure in the magnetic field ${\vec H}$ oriented near the $ab$-plane.

Spin splitting of one-dimensional subbands in high quality quantum wires at zero magnetic field

We have studied the transport propel-ties of a high quality one-dimensional constriction formed in an undoped GaAs/AlxGa1-xAs heterostructure and therefore largely free of the random potential of ionized donors. We induce an electron gas electrostatically and are able to vary the sheet carrier density (n(2D)) by a factor of at least seven. The constriction shows resonance-free integer conductance plateaus and the additional structure, a plateaulike feature, the conductance of which decreases from about 0.80 towards 0.5 x 2e(2)/h at low and high n(2D) This low value is unaffected by a high in-plane magnetic field, supporting previous evidence and theories that the breaking of the spin degeneracy at high fields persists in some form, even at zero field. The height of the feature generally seen at a conductance of about 0.85 x 2 e(2)/h at high de bias also varies, and we show that this is in reasonable agreement with a simple relation linking the conductances of the two features. We use a source-drain bias to study the spin splitting of the lowest one-dimensional subbands, and find a spin gap that is independent of n(2D) for the first subband. We discuss possible reasons for the splitting, and show how various models for the 0.7 structure can be applied in the finite bias regime.

Spin-dependent transport in a quasiballistic quantum wire

We describe the transport properties of a 5 $\mu$m long one-dimensional (1D) quantum wire. Reduction of conductance plateaux due to the introduction of weakly disorder scattering are observed. In an in-plane magnetic field, we observe spin-splitting of the reduced conductance steps. Our experimental results provide evidence that deviation from conductance quantisation is very small for electrons with spin parallel and is about 1/3 for electrons with spin anti-parallel. Moreover, in a high in-plane magnetic field, a spin-polarised 1D channel shows a plateau-like structure close to $0.3 \times e^2/h$ which strengthens with {\em increasing} temperatures. It is suggested that these results arise from the combination of disorder and the electron-electron interactions in the 1D electron gas.

Magnetic trap for excitons

High in-plane magnetic field gradients are used to control the lateral motion of excitons in an InGaAs quantum well. The gradients, as large as 6.10 4 T/m, are produced by positioning a thin magnetized stripe of dysprosium on top of the quantum well. By measuring the exciton photoluminescence energy and intensity, spatially resolved with μm resolution, it is shown that the diamagnetic excitons are forced to regions of low magnetic field, because of the interaction of their magnetic moment with the inhomogeneous field. These results show the possibility to magnetically confine excitons to a limited region of space, which opens the way for the realization of the first magnetic trap in solids.

Tunnelling and transfer between 1D and 2D electrons in adjusted quantum wells with thin barrier

By the technique of in situ focused ion-beam implantation in an underlying doping layer followed by molecular beam epitaxy overgrowth of a double quantum-well we have fabricated adjusted one-dimensional and two-dimensional electrons with very thin barrier between them. Each of the electron systems could be contacted separately. The tunnelling resistance between electron layers increases at high in-plane magnetic fields. With magnetic fields perpendicular to the wire we can investigate the coupling of lower one-dimensional subband while in the parallel direction higher subband states hybridise. In the first case, we observed clear resonance in the tunnelling and electron transfer.

Real-space approach to the multicomponent-envelope-function problem in semiconductor heterostructures.

We describe a real-space numerical method for the solution of the multicomponent-envelope-function problem in semiconductor heterostructures. The method, based on a shooting technique, provides, with a very modest computational effort, an exact solution for arbitrarily shaped one-dimensional confining potentials, including high in-plane magnetic fields. Boundary conditions at interfaces are automatically taken into account. We apply our method to the 4\ifmmode\times\else\texttimes\fi{}4 Luttinger Hamiltonian and indicate how larger k\ensuremath{\cdot}p Hamiltonians can be implemented. To demonstrate the flexibility of the method, we show the calculated hole subbands and envelope functions in a GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum well with a magnetic field parallel to the interfaces and with an applied bias along the growth direction.

Theoretical analysis of resonant magnetotunneling spectroscopy of holes

We calculate the hole energy band structure in GaAs-AlAs Quantum Wells (QWs) in high in-plane magnetic fields and simulate Resonant Magnetotunneling Spectroscopy (RMS) experiments, recently proposed as a probe of the hole energy dispersion. We find that the high magnetic fields used in RMS result in small modifications of the QW hole subbands in thin QWs. However, for wide QWs or very large magnetic fields, the field strongly affects the hole dispersion by changing the heavy hole-light hole mixing. We give a simple expression to evaluate to which extent the mixing is modified as a function of the QW width and field strength.

Hole states in quantum wells under high in-plane magnetic fields: Implications for resonant magnetotunneling spectroscopy.

We calculate the hole energy-level structure in quantum wells (QW's) in high in-plane magnetic fields. On the basis of our results, we analyze the conditions of validity of resonant magnetotunneling spectrosocpy (RMS) as a probe of the unperturbed hole subband dispersion. The high magnetic field used in RMS does not change appreciably, apart from a modest ``spin'' splitting, the hole subbands for thin wells, but have a non-negligible effect on the dispersion for thicker wells. We use a simplified analytical model to evaluate the field-related heavy-hole\char21{}light-hole mixing terms and we give their explicit dependence on the field strength and the width of the QW. This allows us to estimate the range of validity of RMS as a probe of hole subbands.

Vertical transport in a GaAs/(Ga,Al)As superlattice containing an enlarged quantum well studied by photoluminescence in high in-plane magnetic fields

Vertical transport in a GaAs/(Ga,Al)As superlattice has been studied using photoluminescence and magnetophotoluminescence measurements with in-plane fields of up to 25 T at temperatures in the range 4-80 K. Vertical transport at 4 K (which is determined by exciton transport) has been found to be less efficient than free-carrier-like transport at high temperatures (47 and 80 K). Application of a high, in-plane magnetic field tends to reduce the hole transport considerably. In addition, a drastic enhancement of the total photoluminescence yield with in-plane field has been observed.