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We report a direct observation of fast Type II twin boundary motion under a pulsed magnetic field excitation in Ni–Mn–Ga 5 M martensite, utilizing high-speed camera technique. Our results demonstrate saturation velocities up to ~ 39 m/s measured along [100] direction for individual boundaries moving at RT, showing an order of magnitude larger values as compared to the earlier studies. Actuation response times of ~ 1 μs were observed using laser vibrometry technique. The reported results show a minor role of magneto-static forces and emphasize the role of inertia and internal friction in twin boundary dynamics at the measured speeds.

Low-temperature conductance measurements of strongly coupled ballistic one-dimensional (1D) wires show the formation of bonding and antibonding 1D states. Two parallel 1D wires are defined by a split-gate structure deposited on a GaAs/AlGaAs double quantum well with a 25 A AlGaAs barrier. A midline gate positioned at the centre of the split-gate allows both the widths and electron densities of the two wires to be tuned. At resonance when the wires are matched, symmetric and antisymmetric 1D energy levels are formed, and are separated by an energy gap Δ SAS . The overlap integral that describes the mixing of the 1D wave functions of different index in the two wires can be altered with a small in-plane magnetic field B . We also present preliminary measurements of the mixing of edge states in the quantum Hall regime.

We have developed a technique to measure the thermopower of a mesoscopic device. Absolute calibration is possible, and we find that a constriction in a 2D electron gas obeys theoretical predictions sufficiently well that its thermopower can be used for electron thermometry. This has been applied to measure heat loss rates from the 2DEG at zero field, leading to the electron–phonon scattering rate. The same technique can determine the effect of a quantising magnetic field on the coupling, and the electron thermometry also allows a calibrated measurement of the thermal conductance of a mesoscopic device. We discuss how such measurements might be realised.

The physics of low-dimensional electron systems continues to be an active field of research; using two examples we show how double-layer electron gases can extend these investigations. First, after reviewing the properties of 2D–2D tunnelling, we present equilibrium tunnelling measurements between a two-dimensional electron gas (2DEG) and an array of independent and identical 1D wires. Using a parallel magnetic field full information about the wires can be obtained, their 1D energies and wave functions, and hence the functional form of the confining potential. In the second case, the Coulomb drag (where a voltage is induced in one layer when a current is passed through the other) is measured between two separately contacted and electrically isolated 2DEGs. At temperatures comparable to the Fermi temperature, the measured transresistivity shows a plasmon enhancement. A comparison between theory and experiment shows the importance of many-body corrections to the coupled plasmon modes.

We have investigated the transport properties of one-dimensional (1D) constrictions defined by split-gates in high quality GaAs/AlGaAs heterostructures. In addition to the usual quantized conductance plateaus, the equilibrium conductance shows a structure close to $0.7(2e^2/h)$, and in consolidating our previous work [K.~J. Thomas et al., Phys. Rev. Lett. 77, 135 (1996)] this 0.7 structure has been investigated in a wide range of samples as a function of temperature, carrier density, in-plane magnetic field $B_{\parallel}$ and source-drain voltage $V_{sd}$. We show that the 0.7 structure is not due to transmission or resonance effects, nor does it arise from the asymmetry of the heterojunction in the growth direction. All the 1D subbands show Zeeman splitting at high $B_{\parallel}$, and in the wide channel limit the $g$-factor is $\mid g \mid \approx 0.4$, close to that of bulk GaAs. As the channel is progressively narrowed we measure an exchange-enhanced $g$-factor. The measurements establish that the 0.7 structure is related to spin, and that electron-electron interactions become important for the last few conducting 1D subbands., Comment: 8 pages, 7 figures (accepted in Phys. Rev. B)

Focusing on the last spin-degenerate one-dimensional subband, conductance measurements of split-gate devices were carried out in the linear and nonlinear regimes. In the linear regime, we observed a structure at 0.7(2e 2/h) in addition to the quantized plateaux. In a strong in-plane magnetic field B this 0.7 structure evolves into the spin-split plateau at e 2/h, suggesting a possible spin polarization for the last subband at B = 0. The 0.7 structure and the e 2/h plateau show similar source—ain bias dependence, suggesting that the 0.7 structure has origin in the lower spin subband.

We present measurements in a perpendicular magnetic field of the frictional drag between two closely spaced, but electrically isolated, two-dimensional electron gases. At high temperatures, when the Landau level structure is smeared out, the transresistivity shows a magnetic field dependence and an approximately linear temperature dependence. As the temperature is lowered below 20 K, the transresistivity shows structure reflecting the formation of Landau levels and, in general, the drag is more sensitive to the spin splitting of the Landau levels than the Shubnikov - de Haas oscillations. When the Fermi energy lies at the centre of a Landau level, the drag can be enhanced by two orders of magnitude over the zero field signal. In contrast, when lies between Landau levels in the quantum Hall regime, the drag signal tends to zero. We have also measured the transverse voltage in the drag layer and find no evidence for a Hall transresistance.

Recent theoretical and experimental work demonstrates that at filling factor v = 1/2, a two-dimensional electron gas can be described in terms of composite fermions (CFs), for which the effective magnetic field vanishes. We present low-temperature measurements on a wide split-gate (SG) device investigating CFs in the quantum point contact (QPC) defined by such a structure. Negative magnetoresistance due to suppression of backscattering in the QPC was observed both around zero magnetic field and v = 1/2. We have also measured the resistance of a CF QPC at zero effective magnetic field as a function of gate voltage, with a compensating field to maintain v = 1/2 in the QPC. Using a simple model, we have determined the channel widths for CFs, which are narrower than those for electrons.

We have measured the tunneling between two two-dimensional electron gases at high magnetic fields $B$, when the carrier densities of the two electron layers are matched. For filling factors $��, 9 pages, cond-mat/yymmnnn

We present low-temperature transport measurements on the two-dimensional electron gas in delta -doped GaAs which undergoes an insulator-quantum Hall-insulator transition as the magnetic field is increased. Both low- and high-held transitions are marked by peaks in sigma xx and the temperature-independent critical value of sigma xy of 0.5e2/h per spin. We map out the phase diagram versus disorder and magnetic field and study the temperature dependence of sigma xx throughout. In the quantum Hall region we observe Mott variable range hopping and, around the high-field transitions, scaling via a single parameter: z=(B-B*)T-0.45. The functional dependence on z above this transition is fitted by recent network percolation calculations.

Low temperature measurements of ballistic one-dimensional (1D) wires show an additional structure at 0.7(2e(2)/h) in the conductance characteristics (Phys. Rev. Lett, 77 (1996) 135: Phys. Rev. B 62 (2000) 10950). This 0.7 structure develops into a plateau at e(2)/h, if. either the in-plane magnetic field is increased (Phys. Rev. Lett. 77 ( 1996) 135) or the carrier density is decreased (Phys. Rev. B 61 (2000) 13 365). A suggested explanation for this effect is based on spin polarisation induced by many-body interactions. As temperature increases, the e(2)/h plateau in a magnetic field shifts to 0.6(2e(2)/h). whereas other plateaus remain unchanged. We study the effect of temperature on the evolution of the 0.7 structure with increasing in-plane magnetic field. (C) 2002 Elsevier Science B.V. All rights reserved.

The authors present calculations of the differential conductance, G, of a constriction, defined by a saddle-point potential in a two-dimensional electron gas, in the non-linear regime of transport as a function of Fermi energy, source-drain voltage and magnetic field. The manner in which the potential is dropped along the device is considered phenomenologically. The dependence of G on the parameters that define the potential drop is investigated, extending the model proposed by Glazman and Khaetskii. A method for measuring the sub-band energies and spin-splitting energies in a bottle-neck of the constriction is also proposed. Finally, a comparison between experimental data and theoretical calculations is presented.

We have measured the non‐equilibrium current noise in a ballistic one‐dimensional wire which exhibits an additional conductance plateau at 0.7 × 2e2/h. The Fano factor shows a clear reduction on the 0.7 structure, and eventually vanishes upon applying a strong parallel magnetic field. These results provide the first experimental evidence that the 0.7 structure is associated with two conduction channels which have different transmission probabilities.

We have measured the temperature dependence of the tunnelling conductance G = d I d V sd between two parallel two-dimensional electron gases in perpendicular magnetic fields, supplementing earlier measurements as a function of applied d.c. bias. For filling factors below v = 10 there is a suppression of the conductance near Vsd = 0, which develops into a strong gap when v d G d T > 0 , sandwiched between two regions where d G d T .

We have measured the nonequilibrium current noise in a ballistic one-dimensional wire which exhibits an additional conductance plateau at 0.7x2e(2)/h. The Fano factor shows a clear reduction on the 0.7 structure, and eventually vanishes upon applying a strong parallel magnetic field. These results provide experimental evidence that the 0.7 structure is associated with two conduction channels that have different transmission probabilities.

We show that small structures can be defined in high mobility two‐dimensional electron gases formed at a depth of 2770 A below the surface in GaAs/Al0.33Ga0.67As heterostructures. The differential conductance of one‐dimensional constrictions defined by split gates in such deep electron gases showed more than 20 quantised plateaus. The absence of resonant structures on the plateaus demonstrates the absence of potential fluctuations in the constrictions. By applying a dc source‐drain bias we have measured the energy spacings of the first 18 subbands, and the effect of a small perpendicular magnetic field on the energy spacings has been investigated.

We have measured the tunnelling between 2 two-dimensional electron gases at magnetic fields B up to 25 T. When the carrier densities of the two electron layers are matched and for filling factors ν < 1, we observe a clear gap in the differential conductance dI/dV as a function of the voltage V sd applied between the layers. We have measured tunnelling peak positions and fitted gap parameters Δ that are proportional to B and do not depend on the carrier concentration of the two layers. These results suggest that we are observing a gap that is different in origin to that proposed by current theories

We investigate the transport properties of two-dimensional electron gases (2DEGs) formed in a GaAs/Al(0.33)Ga(0.67)As quantum well, where InAs has been inserted into the center of the GaAs well. Depending on the growth conditions, the InAs forms either self-assembled quantum dots or dashes, and due to the resulting strain fields repulsive short-range scattering is experienced by the conduction electrons in the 2DEG. In a perpendicular magnetic field there are transitions between quantum Hall liquids at filling factors υ=1 and υ=2 and the insulating phase. Depending on the amount of InAs coverage, the degree of disorder in the 2DEG can be varied, changing the relative size of the υ=1 and υ=2 regions in the disorder-magnetic-field phase diagrams. Thermal studies show a spin gap at filling factor υ=1, which collapses as the magnetic field is decreased. Microscopic information about the dots was obtained from structural characterization, as well as from conductance measurements through individual dots isolated using a submicron split gate.

We present data for the differential conductance of a quantum point contact under both strong electric and magnetic fields that shows quantisation in units of e 2 /2h, proving that the spin-splitting and the bias-induced-splitting of the plateaux are two independent effects. Calculations of the non-linear differential conductance, using a saddle-point potential to model (1) the one-dimensional channel and assuming that half of the applied source-drain voltage is dropped between each of the contacts and the centre of the channel (2) , show excellent agreement with the experimental data.

We investigate the transport properties of two strongly coupled ballistic one-dimensional (1D) wires, defined by a split-gate structure deposited on a GaAs/AlGaAs double quantum well. Matching the widths and electron densities of the two wires brings them into resonance, forming symmetric and antisymmetric 1D subbands separated by energy gaps that are measured to be larger than their two-dimensional counterpart. Applying a magnetic field parallel to the wire axes enhances wavefunction mixing at low fields, whereas at high fields the wires become completely decoupled., Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. B

We report ac magnetic susceptibility (χ=χ’−iχ‘) measurements of stage‐1 CoCl2‐graphite intercalation compounds (GICs) carried out under hydrostatic pressures of 0–15 kbar. For stage‐1 CoCl2 ‐GICs, it is found that the three‐dimensional ordering temperature Tc2=TN=9.7 K, where the ferromagnetic planes of Co2+ spins become ordered into an antiferromagnetic arrangement along the c axis, shifts to a higher temperature with increasing pressure. Additionally, the transition field Ht, measured when a magnetic field is applied in the x‐y plane, shifts to a higher field value with increasing pressure. From the increase of the transition field, we can estimate the pressure dependence of the antiferromagnetic interlayer coupling J’.

We present results of a detailed study of equilibrium magnetotunneling between an array of independent identical one-dimensional wires and a two-dimensional electron gas. From the tunneling differential conductance, measured as a function of the in-plane magnetic field, we find directly the number of occupied one-dimensional subbands, the subband energies, and the wave functions as a function of the strength of confinement in these wires. As many as 14 one-dimensional subbands were probed. We show that an analysis of our results based on the Bardeen tunneling Hamiltonian formalism allows the determination of the functional form of one-dimensional confining potentials. We give two examples, one for a narrow wire and one for a wide wire.

The tunneling between two parallel two-dimensional electron gases has been investigated as a function of temperature $T$, carrier density $n$, and the applied perpendicular magnetic field $B$. In zero magnetic field the equilibrium resonant lineshape is Lorentzian, reflecting the Lorentzian form of the spectral functions within each layer. From the width of the tunneling resonance the lifetime of the electrons within a 2DEG has been measured as a function of $n$ and $T$, giving information about the density dependence of the electron-impurity scattering and the temperature dependence of the electron-electron scattering. In a magnetic field there is a general suppression of equilibrium tunneling for fields above $B=0.6$ T. A gap in the tunneling density of states has been measured over a wide range of magnetic fields and filling factors, and various theoretical predictions have been examined. In a strong magnetic field, when there is only one partially filled Landau level in each layer, the temperature dependence of the conductance characteristics has been modeled with a double-Gaussian spectral density., Comment: LaTeX requires REVTeX macros. Eighteen pages. Fourteen postscript figures are included. (All figures have been bitmapped to save space. The original can be requested by email from jtn11@cam.ac.uk). Accepted for publication in Phys. Rev. B

Variable-range hopping (VRH) conductance fluctuations in the gate-voltage characteristics of mesoscopic GaAs and Si transistors are analyzed by means of their full distribution functions (DFs). The forms of the DF predicted by the theory of Raikh and Ruzin have been verified under controlled conditions for both the long, narrow wire and the short, wide channel geometries. The variation of the mean square fluctuation size with temperature in wires fabricated from both materials is found to be described quantitatively by Lee's model of VRH along a 1D chain. Armed with this quantitative validation of the VRH model, the DF method is applied to the problem of magnetoconductance in the insulating regime. Here a non-monotonic variation of the magnetoconductance is observed in Si MOSFETS whose sign at low magnetic fields is dependent on the channel geometry. The origin of this defect is discussed within the framework of the interference model of VRH magnetoconductance in terms of narrowing of the DF in a magnetic field., Comment: Revtex, 24 pages, 9 figures, figures are added

In zero magnetic field, conductance measurements of clean one-dimensional (1D) constrictions defined in GaAs/AlGaAs heterostructures show twenty-six quantized ballistic plateaux, as well as a structure close to $0.7(2e^2/h)$. In an in-plane magnetic field all the 1D subbands show Zeeman splitting and in the wide channel limit the $g$-factor is $\mid g \mid = 0.4$, close to that of bulk GaAs. For the last subband spin-splitting originates from the structure at $0.7(2e^2/h)$, indicating spin polarization at $B=0$. The measured enhancement of the $g$-factor as the subbands are depopulated suggests that the ``0.7 structure'' is induced by electron-electron interactions., Ten pages with four ps figures. Accepted for publication in Phys. Rev. Lett. (17. June 1996)

A system of two parallel constrictions in a two-dimensional hole gas has been studied. At zero magnetic-field conductance quantization is observed through the individual constrictions. At high magnetic fields Aharonov-Bohm oscillations result from resonant tunneling through states encircling the repulsive potential between the constrictions. The oscillations demonstrate directly phase-coherent transport of holes.

We have investigated the magnetic properties of a ballistic quasi-one-dimensional channel, in a GaAs-${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunction, when the magnetic field is applied in the plane of the two-dimensional electron gas. At high magnetic fields, and high applied source-drain voltage, the devices show differential conductance plateaus quantized in units of ${\mathit{e}}^{2}$/2h. This is a consequence of the combined effect of the electric field, which causes the number of conducting subbands to be different in the two current directions, and the Zeeman splitting of the one-dimensional subbands. When the magnetic field is parallel to the current direction, the g factor is 1.1\ifmmode\pm\else\textpm\fi{}0.1; and when the field is perpendicular to the current, the g factor is reduced.

We report low-temperature, high-field (T≤4.2 K, H=0-18 teslas) magnetization measurements of CoCl2- and NiCl2-graphite intercalation compounds (GIC’s), both parallel (M?) and perpendicular (M⊥) to the c axis. From the saturation magnetization values of the stage-1 and stage-2 CoCl2-GIC’s, we have been able to measure the anisotropy of the g values, and thus calculate the amount of XY spin anisotropy in these compounds. Magnetization measurements show that upon intercalation, the Co2+ ions retain their anisotropic magnetic behavior. In addition, the state-1 CoCl2-GIC’s exhibit unusual hysteresis phenomena when the magnetization is measured along the c axis. Upon doping of the stage-1 CoCl2-GIC’s with nonmagnetic Mg2+ ions, the magnitude of the hysteresis in M?(H) decreases and the magnetization saturates. In contrast to the behavior of the stage-1 CoCl2-GIC’s, state-1 NiCl2-GIC’s exhibit a very small XY spin anisotropy. The magnetization M?(H) is featureless for magnetic fields H>1 tesla and exhibits no high-field hysteresis. The high-field c-axis magnetization of unintercalated highly oriented pyrolytic graphite was also examined; the susceptibility was χ==M/H=-28.5×10-6 emu/g, and low-field de Haas–van Alphen oscillations were observed with a frequency consistent with previously identified majority hole carriers.

0I, =9.36+0.05 A, and the intercalate has a rhombohedral interplanar stacking sequence similar to pristine NiCl, . 3ust as with stage-1 CoC12-GIC's, the magnetic behavior of stage-1 NiC12-GIC's appears always to be three dimensional. We interpret the magnetic phase transition at T =22.0+0.5 K as the Neel temperature T&. At lower temperatures, the ac susceptibility shows two magnetic field-induced transitions when a magnetic field is applied in the x-y plane. At T = 11.5 K, a temperature much less than T&, the transition field values are H„=380 Oe and H, 2=780 Oe. Compared with similar measurements in stage-2 NiC12-GIC s and pristine NiC12, we estimate that the ratios of the interplanar antiferromagnetic couplings J~„;„;„, :J, 't g — ] J f g — p are 720:24:1. These new results now provide us with two families, stage-1 and stage-2 CoC12-GIC's and NiC1~-GIC's, from which some general trends about quasi-two-dimensional systems can be inferred.

We present the results of a detailed study of equilibrium magnetotunneling between an array of independent identical quasi-one-dimensional electron systems and a two-dimensional electron system. From the tunneling differential conductance, measured as a function of in-plane magnetic field, we map out the principal features of the spectral functions of the quasi-one-dimensional systems for sub-band occupancy between three and fourteen. We find that these spectral functions are mono-modal just as they are for higher-dimensional systems. By performing an analysis of them, based on the tunneling Hamiltonian formalism, we are able to find the different characteristic forms for confining potentials in narrow (parabolic) and wide (square) systems.

Recent theoretical and experimental work demonstrates that a two-dimensional electron gas at Landau-level filling factor $\ensuremath{\nu}=\frac{1}{2}$ can be described in terms of composite fermions (CF's) for which the effective magnetic field ${B}_{\mathrm{eff}}$ vanishes. We have measured the transport properties of CF's in a quantum point contact (QPC) defined by a wide split-gate device. Negative magnetoresistance due to suppression of backscattering in the QPC was observed both around zero magnetic field $B=0$ and ${B}_{\mathrm{eff}}=0$. We have also measured the resistance of a composite fermion QPC at ${B}_{\mathrm{eff}}=0$ as a function of gate voltage, with an applied magnetic field to maintain $\ensuremath{\nu}=\frac{1}{2}$ in the QPC. Using a simple model to compare the results for $B=0$ and $\ensuremath{\nu}=\frac{1}{2}$, we have determined that the channel widths for CF's are narrower than those for electrons.