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The cross-sectional dimensions of nanowires set the quantization conditions for the electronic subbands they host. These can be used as a platform to realize one-dimesional topological superconductivity. Here we develop a protocol that forces such nanowires to kink and change their growth direction. Consequently, a thin rectangular nanoplate is formed, which gradually converges into a very thin square tip. We characterize the resulting tapered nanowires structurally and spectroscopically by scanning and transmission electron microscopy and scanning tunneling microscopy and spectroscopy and model their growth. A unique structure composed of ordered rows of atoms on the (110) facet of the nanoflag is further revealed by atomically resolved topography and modeled by simulations. We discuss possible advantages tapered InAs nanowires offer for Majorana zero-mode realization and manipulation.

In the pursuit of magneto-electronic systems nonstoichiometric magnetic elements commonly introduce disorder and enhance magnetic scattering. We demonstrate the growth of (EuIn)As shells, with a unique crystal structure comprised of a dense net of Eu inversion planes, over InAs and InAs

Correlated charge inhomogeneity breaks the electron-hole symmetry in two-dimensional (2D) bilayer heterostructures which is responsible for non-zero drag appearing at the charge neutrality point. Here we report Coulomb drag in novel drag systems consisting of a two-dimensional graphene and a one dimensional (1D) InAs nanowire (NW) heterostructure exhibiting distinct results from 2D-2D heterostructures. For monolayer graphene (MLG)-NW heterostructures, we observe an unconventional drag resistance peak near the Dirac point due to the correlated inter-layer charge puddles. The drag signal decreases monotonically with temperature ($\sim T^{-2}$) and with the carrier density of NW ($\sim n_{N}^{-4}$), but increases rapidly with magnetic field ($\sim B^{2}$). These anomalous responses, together with the mismatched thermal conductivities of graphene and NWs, establish the energy drag as the responsible mechanism of Coulomb drag in MLG-NW devices. In contrast, for bilayer graphene (BLG)-NW devices the drag resistance reverses sign across the Dirac point and the magnitude of the drag signal decreases with the carrier density of the NW ($\sim n_{N}^{-1.5}$), consistent with the momentum drag but remains almost constant with magnetic field and temperature. This deviation from the expected $T^2$ arises due to the shift of the drag maximum on graphene carrier density. We also show that the Onsager reciprocity relation is observed for the BLG-NW devices but not for the MLG-NW devices. These Coulomb drag measurements in dimensionally mismatched (2D-1D) systems, hitherto not reported, will pave the future realization of correlated condensate states in novel systems., Accepted for publication in Physical Review Letters

Following significant progress in the visualization and characterization of Majorana end modes in hybrid systems of semiconducting nanowires and superconducting islands, much attention is devoted to the investigation of the electronic structure at the buried interface between the semiconductor and the superconductor. The properties of that interface and the structure of the electronic wavefunctions that occupy it determine the functionality and the topological nature of the superconducting state induced therein. Here we study this buried interface by performing spectroscopic mappings of superconducting aluminum islands epitaxially grown in-situ on indium arsenide nanowires. We find unexpected robustness of the hybrid system as the direct contact with the aluminum islands does not lead to any change in the chemical potential of the nanowires, nor does it induce a significant band bending in their vicinity. We attribute this to the presence of surface states bound to the facets of the nanowire. Such surface states, that are present also in bare nanowires prior to aluminum deposition, pin the Fermi-level thus rendering the nanowires resilient to surface perturbations. The aluminum islands further display Coulomb blockade gaps and peaks that signify the formation of a resistive tunneling barrier at the InAs-Al interface. At low energies we identify a potential energy barrier that further suppresses the transmittance through the interface. A corresponding barrier exists in bare semiconductors between surface states and the accumulation layer, induced to maintain charge neutrality. Our observations elucidate the delicate interplay between the resistive nature of the InAs-Al interface and the ability to proximitize superconductivity and tune the chemical potential in semiconductor-superconductor hybrid nanowires., Comment: 32 pages, 10 figures

Thermoelectric measurements have the potential to uncover the density of states of low-dimensional materials. Here, we present the anomalous thermoelectric behaviour of mono-layer graphene-nanowire (NW) heterostructures, showing large oscillations as a function of doping concentration. Our devices consist of InAs NW and graphene vertical heterostructures, which are electrically isolated by thin ($\sim$ 10nm) hexagonal boron nitride (hBN) layers. In contrast to conventional thermoelectric measurements, where a heater is placed on one side of a sample, we use the InAs NW (diameter $\sim 50$ nm) as a local heater placed in the middle of the graphene channel. We measure the thermoelectric voltage induced in graphene due to Joule heating in the NW as a function of temperature (1.5K - 50K) and carrier concentration. The thermoelectric voltage in bilayer graphene (BLG)- NW heterostructures shows sign change around the Dirac point, as predicted by Mott's formula. In contrast, the thermoelectric voltage measured across monolayer graphene (MLG)-NW heterostructures shows anomalous large-amplitude oscillations around the Dirac point, not seen in the Mott response derived from the electrical conductivity measured on the same device. The anomalous oscillations are a signature of the modified density of states in MLG by the electrostatic potential of the NW, which is much weaker in the NW-BLG devices. Thermal calculations of the heterostructure stack show that the temperature gradient is dominant in the graphene region underneath the NW, and thus sensitive to the modified density of states resulting in anomalous oscillations in the thermoelectric voltage. Furthermore, with the application of a magnetic field, we detect modifications in the density of states due to the formation of Landau levels in both MLG and BLG., Comment: 27 pages, 16 figures

The prediction that Majorana Fermions obey nonabelian exchange statistics can only be tested by interchanging such carriers in "Y'- or 'X'- (or 'K'-) shaped nanowire networks. Here we report the molecular beam epitaxy (MBE) growth of 'K'-shaped InAs nanowires consisting of two interconnected wurtzite wires with an additional zinc-blende wire in between. Moreover, occasionally, the growth results in formation of a purely wurtzite two-dimensional plate between the zinc-blende nanowire and one (sometimes both) intersecting wurtzite arm. By modeling the crystal structure we explain the transformation from wurtzite to zinc-blende and the coexistence of both crystallographic phases in such nanowire structures. To the best of our knowledge neither the MBE growth of an InAs nano-object showing combination of wurtzite and zinc-blende crystal structures nor the growth of pure wurtzite InAs nanoplates in this geometry has been reported before.

We study the role of gold droplets in the initial stage of nanowire growth via the vapor liquid solid method. Apart from serving as a collections center for growth species, the gold droplets carry an additional crucial role that necessarily precedes the nanowire emergence, that is, they assist the nucleation of nanocraters with strongly faceted 111B side walls. Only once these facets become sufficiently large and regular, the gold droplets start nucleating and guiding the growth of nanowires. We show that this dual role of the gold droplets can be detected and monitored by high energy electron diffraction during growth. Moreover, gold induced formation of craters and the onset of nanowires growth on the 111B facets inside the craters are confirmed by the results of Monte Carlo simulations. The detailed insight into the growth mechanism of inclined nanowires will help to engineer new and complex nanowire based device architectures., 17 pages with 5 figures (main). 7 pages with 7 figures (Supplementary Information). This manuscript was published in Nano Letters on 2018

It was recently shown that in situ epitaxial aluminum coating of indium arsenide nanowires is possible and yields superior properties relative to ex-situ evaporation of aluminum ( Nat. Mater. 2015 , 14 , 400 - 406 ). We demonstrate a robust and adaptive epitaxial growth protocol satisfying the need for producing an intimate contact between the aluminum superconductor and the indium arsenide nanowire. We show that the (001) indium arsenide substrate allows successful aluminum side-coating of reclined indium arsenide nanowires that emerge from (111)B microfacets. A robust, induced hard superconducting gap in the obtained indium arsenide/aluminum core/partial shell nanowires is clearly demonstrated. We compare epitaxial side-coating of round and hexagonal cross-section nanowires and find the surface roughness of the round nanowires to induce a more uniform aluminum profile. Consequently, the extended aluminum grains result in increased strain at the interface with the indium arsenide nanowire, which is found to induce dislocations penetrating into round nanowires only. A unique feature of proposed growth protocol is that it supports in situ epitaxial deposition of aluminum on all three arms of indium arsenide nanowire intersections in a single growth step. Such aluminum coated intersections play a key role in engineering topologically superconducting networks required for Majorana based quantum computation schemes.

The higher the energy of a particle is above equilibrium, the faster it relaxes because of the growing phase space of available electronic states it can interact with. In the relaxation process, phase coherence is lost, thus limiting high-energy quantum control and manipulation. In one-dimensional systems, high relaxation rates are expected to destabilize electronic quasiparticles. Here, we show that the decoherence induced by relaxation of hot electrons in one-dimensional semiconducting nanowires evolves nonmonotonically with energy such that above a certain threshold hot electrons regain stability with increasing energy. We directly observe this phenomenon by visualizing, for the first time, the interference patterns of the quasi-one-dimensional electrons using scanning tunneling microscopy. We visualize the phase coherence length of the one-dimensional electrons, as well as their phase coherence time, captured by crystallographic Fabry-Pèrot resonators. A remarkable agreement with a theoretical model reveals that the nonmonotonic behavior is driven by the unique manner in which one-dimensional hot electrons interact with the cold electrons occupying the Fermi sea. This newly discovered relaxation profile suggests a high-energy regime for operating quantum applications that necessitate extended coherence or long thermalization times, and may stabilize electronic quasiparticles in one dimension.

Semiconducting nanowires are expected to have applications in various areas as transistors, sensors, resonators, solar cells, and thermoelectric systems. Understanding the surface properties is crucial for the fabrication of high-performance devices. Due to the large surface-to-volume ratio of nanowires, their surface electronic properties, like surface states, can a have a large effect on the performance of both electronic and optoelectronic devices. At present, determination of the surface state density depends on a combination of experimental measurements of the capacitance and/or drain current, in a nanowire field-effect transistor, and a fitting to simulation. This technique follows certain assumptions, which can severely harm the accuracy of the extracted density of states. In this report, we demonstrate a direct measurement of the surface state density of individual InAs and silicon nanowires. The method is based on measuring the surface potential of a nanowire field-effect transistor, with respect to a changing gate bias. The extracted density of states at the surface helps to explain various electronic phenomena in such devices.

Double quantum dots are proving themselves to be an excellent test bed for many-body physics. These artificial atoms now demonstrate a phenomenon in which the capacitive coupling between them causes the spin and charge degrees of freedom of the electrons in the system to become entangled—the so-called SU(4) Kondo effect.

Molecular beam epitaxy growth of merging InAs nanowire intersections, that is, a first step toward the realization of a network of such nanowires, is reported. While InAs nanowires play already a leading role in the search for Majorana fermions, a network of these nanowires is expected to promote their exchange and allow for further development of this field. The structural properties of merged InAs nanowire intersections have been investigated using scanning and transmission electron microscope imaging. At the heart of the intersection, a sharp change of the crystal structure from wurtzite to perfect zinc blende is observed. The performed low-temperature conductance measurements demonstrate that the intersection does not impose an obstacle to current transport.

Group III-V coaxial core-shell semiconducting nanowire heterostructures possess unique advantages over their planar counterparts in logic, photovoltaic, and light-emitting devices. Dimensional confinement of electronic carriers and interface complexity in nanowires are known to produce local electronic potential landscapes along the radial direction that deviate from those along the normal to planar heterojunction interfaces. However, understanding of selected electronic and optoelectronic carrier transport properties and device characteristics remains lacking without a direct measurement of band alignment in individual nanowires. Here, we report on, in the GaAs/AlxGa 1-xAs and GaAs/AlAs core-shell nanowire systems, how photocurrent and photoluminescence spectroscopies can be used together to construct a band diagram of an individual heterostructure nanowire with high spectral resolution, enabling quantification of conduction band offsets.

Tunnelling between a three-dimensional emitter contact and a two-dimensional electron system (2DES) is studied in magnetic fields aligned perpendicular to the barriers of a double-barrier heterostructure. The differential conductance around the Fermi energy exhibits a magnetic-field-dependent pseudogap. This pseudogap is shown to be thermally activated and to depend on the two-dimensional electron density. We attribute this pseudogap to an extra energy that an electron tunnelling from the emitter into the 2DES has to overcome as a result of the correlated state of the 2DES.

A novel nonlocal supercurrent, carried by quartets, each consisting of four electrons, is expected to appear in a voltage-biased three-terminal Josephson junction. This supercurrent results from a nonlocal Andreev bound state (ABS), formed among three superconducting terminals. While in a two-terminal Josephson junction the usual ABS, and thus the dc Josephson current, exists only in equilibrium, the ABS, which gives rise to the quartet supercurrent, persists in the nonlinear regime. In this work, we report such resonance in a highly coherent three-terminal Josephson junction made in an InAs nanowire in proximity to an aluminum superconductor. In addition to nonlocal conductance measurements, cross-correlation measurements of current fluctuations provided a distinctive signature of the quartet supercurrent. Multiple device geometries had been tested, allowing us to rule out competing mechanisms and to establish the underlying microscopic origin of this coherent nondissipative current.

New data, backed up by simulations, support the existence of Majorana fermions in the one-dimensional topological superconductor that is induced by placing an aluminium superconductor close to an indium-arsenide nanowire.

Incorporation of catalyst atoms during the growth process of semiconductor nanowires reduces the electron mean free path and degrades their electronic properties. Aberration-corrected scanning transmission electron microscopy (STEM) is now capable of directly imaging single Au atoms within the dense matrix of a GaAs crystal, by slightly tilting the GaAs lattice planes with respect to the incident electron beam. Au doping values in the order of 10(17-18) cm(3) were measured, making ballistic transport through the nanowires practically inaccessible.

Gold-assisted vapor–liquid–solid (VLS) growth of InAs nanowires was optimized and used for growth of highly mismatched InAs/GaAs core–shell heterostructure nanowires having the wurtzite structure. The motivation is 2-fold, providing means for surface passivation of InAs nanowires (NWs) for electronic devices and ballistic transport applications on one hand and for studying the structural properties of a highly mismatched system in a core–shell, cylindrical configuration on the other hand. The misfit between the InAs core and the mostly relaxed GaAs shell was deduced from the average spacing between the edge dislocations, the periodicity of the Moire fringes resulting from the overlap between the InAs and GaAs lattices and the splitting in the electron diffraction images. Both line and loop edge dislocations are formed in the strain relaxation process. The experimental radial and axial misfits were found to be approximately 6 ± 1% and 4 ± 0.5%, respectively.

Tailoring of GaAs and InAs nanowires (NWs) to be suited for measurements of ballistic transport is discussed in this paper. Methods used to avoid imperfections most harmful for the transport properties are described. We consider the imperfections, which frequently occur in III-V NWs: occasional stacking faults, unintentional impurities (like gold atoms originating from the catalyst in the vapor-liquid-solid growth method) and imperfections associated with the NW side facets. Foremost important is obtaining GaAs and InAs NWs, in which either a pure wurtzite or pure zinc-blende structure is enforced, i.e., overcoming the inherent tendency of the two structures to intermix in III-V NWs. Next follows elimination, or at least minimization of the number of incorporated impurities. In InAs NWs, this has been achieved by using low-growth temperature combined with a low-growth rate. Finally, embedding the NWs in an in situ grown shell has provided a robust way for passivation of the surface states and keeping the electrons away from any impurities adhered to the surface.

The kinetic energy of Cooper pairs, in doubly connected superconducting cylinders, is a function of the applied flux and the ratio between the diameter of the cylinder and the zero temperature coherence length d / ξ (0). If d > ξ (0) the known Little–Parks oscillations are observed. On the other hand if d ξ (0), the superconducting state is energetically not favored around odd multiples of half flux quanta even at T ∼ 0 , resulting in the so called destructive regime [Y. Liu, et al., Science 294 (2001) 2332]. We developed a novel technique to fabricate superconducting doubly connected nanocylinders with both diameter and thickness less than 100 nm, and performed magnetoresistance measurements on such Nb and Al cylinders. In the Nb cylinders, where d > ξ (0), we observed the LP oscillations. In the Al cylinders we did not observe a transition to the superconducting state due to the proximity effect, resulted from an Au layer coating the Al. However, we did observe Altshuler–Aronov–Spivak (h/2e) oscillations in these cylinders.

We performed electronic transport measurements on one-dimensional InAs quantum wires. In sufficiently disordered wires, transport is dominated by Coulomb blockade, and the conductance can be well described by tunneling through a quantum dot embedded between two one-dimensional Luttinger liquid wires. In contrast to previous experiments in other material systems, in our system the conductance difference between peak to valley decreases with decreasing temperature for several consecutive peaks. This phenomenon is theoretically expected to occur only for strongly interacting systems with small Luttinger interaction parameter $gl1/2$; we find for our InAs wires a value of $g\ensuremath{\approx}0.4$. Possible mechanisms leading to these strong correlations are discussed.

In Coulomb drag, a current flowing in one conductor can induce a voltage across an adjacent conductor via the Coulomb interaction. The mechanisms yielding drag effects are not always understood, even though drag effects are sufficiently general to be seen in many low-dimensional systems. In this Letter, we observe Coulomb drag in a Coulomb-coupled double quantum dot and, through both experimental and theoretical arguments, identify cotunneling as essential to obtaining a correct qualitative understanding of the drag behavior., This work was supported by the Gordon and Betty Moore Foundation Grant No. GBMF3429, the U.S.-Israel BSF Grants No. 2014014 and No. 2008149, the NSF under DMR-0906062, and the MINECO Grant No. FIS2014-52564. A. J. K. acknowledges an ABB Stanford Graduate Fellowship and an IQIM Postdoctoral Scholarship from the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center (NSF Grant No. PHY-1125565).

Non-linear charge transport in SIS Josephson junctions has a unique signature in the shuttled charge quantum between the two superconductors. In the zero-bias limit Cooper pairs, each with twice the electron charge, carry the Josephson current. An applied bias $V_{SD}$ leads to multiple Andreev reflections (MAR), which in the limit of weak tunneling probability should lead to integer multiples of the electron charge $ne$ traversing the junction, with $n$ integer larger than $2{\Delta}/eV_{SD}$ and ${\Delta}$ the superconducting order parameter. Exceptionally, just above the gap, $eV_{SD}>2{\Delta}$, with Andreev reflections suppressed, one would expect the current to be carried by partitioned quasiparticles; each with energy dependent charge, being a superposition of an electron and a hole. Employing shot noise measurements in an SIS junction induced in an InAs nanowire (with noise proportional to the partitioned charge), we first observed quantization of the partitioned charge $q=e^*/e=n$, with $n=1-4$; thus reaffirming the validity of our charge interpretation. Concentrating next on the bias region $eV_{SD}{\approx}2{\Delta}$, we found a reproducible and clear dip in the extracted charge to $q{\approx}0.6$, which, after excluding other possibilities, we attribute to the partitioned quasiparticle charge. Such dip is supported by numerical simulations of our SIS structure.

The work presents an experimental demonstration of time-reversal asymmetry of electron states propagating along boundary separating areas with opposite magnetic fields. For this purpose we have fabricated a hybrid ferromagnet-semiconductor device in form of a Hall cross with two ferromagnets deposited on top. The magnets generated two narrow magnetic barriers of opposite polarity in the active Hall area. We have observed that if the signs of the barriers are reversed, the bend resistance changes its sign. Using the Landauer-Buttiker theory, we have demonstrated that this is a direct consequence of asymmetric transmission of the "snake" and the "cycloidal" trajectories formed the boundary separating the regions with opposite magnetic field directions., 8 pages, 5 figures, presented at 11th Int. Conf. on Modulated Semiconductor Structures Nara, July 2003. To be published in Physica E

We study the photoluminescence (PL) spectrum of a two-dimensional electron system at the high magnetic field limit, where all electrons reside at the lowest Landau level (ν

Single molecule magnets display fascinating quantum mechanical behavior, and may have important technological applications for information storage and quantum computation. A brief review is given of the physical properties of Mn12-acetate, one of the two prototypical molecular nanomagnets that have been most intensively investigated. Descriptions and discussions are given of the Mn12 magnetic cluster and the fundamental process of quantum tunneling of a nanoscopic spin magnetization; the distinction between thermally-assisted tunneling and pure quantum tunneling, and a study of the crossover between the two regimes; and a review of earlier investigations that suggest that the tunneling in this system is due to locally varying second-order crystal anisotropy which gives rise to a distribution of tunnel splittings. In the second part of the paper, we report results obtained by a new experimental method that confirm our earlier conclusion that the tunnel splittings in Mn12 are distributed rather than single-valued, as had been generally assumed.

We present a novel method for fabrication of contacts to nanosize particles. The method is based on conventional optical lithography of GaAs/AlGaAs molecular beam epitaxy grown structures. Taking advantage of the difference in etch rate between GaAs and AlGaAs a narrow gap is formed between metal contacts deposited on the side of a mesa structure. We demonstrate electrostatic trapping of charged metal clusters into these structures using alternating electric fields.

Mixed types I-II multiple quantum well structures consist of alternating narrow and wide GaAs wells (NW and WW), separated by AlAs barriers. The transfer of electrons from the NW to the WW results in the formation of two-dimensional electron and hole gases (2DEG and 2DHG) at the WW and NW, respectively. The present study investigated the influence of the 2DEG and the 2DHG on the optical properties of the materials. The study utilized double beam photoluminescence (PL) and microwave modulated PL, offering high-resolution spectroscopy and control of the density of the gases and their kinetic energy. The results have shown that the existence of the low-density 2DEG in the WW causes the formation of trions or plasma-like recombination. In addition, electrons transfer through the barrier leading to a barrier-NW indirect recombination emission. The latter is influenced by an electrostatic potential induced by the two-dimensional gases.

A new concept to form ballistic quantum wires based on a triple split gate structure on top of a GaAs/AlGaAs heterostructure is presented. Due to the flexibility in the design we propose this method, which would allow one to check the predictions of the Luttinger liquid model. The current–voltage characteristic of an embedded tunneling barrier in a 2 μm long ballistic quantum wire is also addressed in some detail.

We study the photoluminescence spectrum of a low density (ν

Quantized conductance in nanowires can be observed at low temperature in transport measurements; however, the observation of sub-bands at room temperature is challenging due to temperature broadening. So far, conduction band splitting at room temperature has not been observed in III–V nanowires mainly due to the small energetic separations between the sub-bands. We report on the measurement of conduction sub-bands at room temperature, in single InAs nanowires, using Kelvin probe force microscopy. This method does not rely on charge transport but rather on measurement of the nanowire Fermi level position as carriers are injected into a single nanowire transistor. As there is no charge transport, electron scattering is no longer an issue, allowing the observation of the sub-bands at room temperature. We measure the energy of the sub-bands in nanowires with two different diameters, and obtain excellent agreement with theoretical calculations based on an empirical tight-binding model.

Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of considerable technological importance but remains a challenge due to the amphoteric behavior of the dopant atoms. Here we show that placing a narrow GaAs quantum well in the AlAs shell effectively getters residual carbon acceptors leading to an \emph{unintentional} p-type doping. Magneto-optical studies of such a GaAs/AlAs core multi-shell NW reveal quantum confined emission. Theoretical calculations of NW electronic structure confirm quantum confinement of carriers at the core/shell interface due to the presence of ionized carbon acceptors in the 1~nm GaAs layer in the shell. Micro-photoluminescence in high magnetic field shows a clear signature of avoided crossings of the $n=0$ Landau level emission line with the $n=2$ Landau level TO phonon replica. The coupling is caused by the resonant hole-phonon interaction, which points to a large 2D hole density in the structure., Comment: just published in Nano Letters (http://pubs.acs.org/doi/full/10.1021/nl500818k)

In our single electron transistor (SET), a droplet of about 50 electrons is separated from two conducting leads by tunnel barriers. A set of electrodes (Fig. 1(a)), on the surface of a GaAs/AlGaAs heterostructure which contains a two-dimensional electron gas (2DEG), is used to confine the electrons and create the tunnel barriers. The 2DEG is depleted beneath the electrodes, and the narrow constrictions between electrodes form the tunnel barriers. To make our SETs smaller than earlier ones, we have fabricated shallower 2DEG heterostructures [1] as well as finer metallic gate patterns by electron-beam lithography. The smaller size of the SETs is critical to our observation of the Kondo effect (dimensions are given in Fig. 1(a)). For details of device fabrication see Ref. [2].

A high resolution scanning Hall probe microscope has been used to study the penetration of magnetic flux into thin strips of superconducting niobium as the applied field is slowly ramped. The strips, with widths w =100 μm, and thicknesses d ≈1 μm, are thick enough such that vortices are truly three dimensional ( d ≫ λ ). However, the small ratio d / w implies very strong demagnetization effects, and the relative smallness of d emphasizes the importance of the long-range force between vortex ends over the short-range force between their bulk core currents. The microscope has 1–2 μm spatial resolution and around 30 mG field sensitivity, allowing high-resolution imaging of flux features over its approx. 150×150 μm 2 scan range. At low fields of a few tens of gauss, we observe Meissner screening of the external field. As the field is increased towards several kilogauss, flux begins to enter the sample in the form of small (≈10 μm wide) dendritic fingers. These fingers persist over all temperatures investigated, from 0.3 to 0.95 T c . They appear to grow in such a way as to maximize their separation from neighbouring fingers. This suggests a growth mechanism of the flux front mediated by a competition between long-range repulsive interactions between mesoscopic flux-containing regions, and the strong pinning that maintains the stability of the flux front.

We have measured the local electronic compressibility of a two-dimensional hole gas as it crosses the $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ metal-insulator transition. In the metallic phase, the compressibility follows the mean-field Hartree-Fock (HF) theory and is found to be spatially homogeneous. In the insulating phase it deviates by more than an order of magnitude from the HF predictions and is spatially inhomogeneous. The crossover density between the two types of behavior agrees quantitatively with the transport critical density, suggesting that the system undergoes a thermodynamic change at the transition.

The current distribution in the integer quantum Hall regime was studied by inductive coupling technique and transport measurements. Using an inductive coupling method we were able to study the screening properties of a two-dimensional electron gas (2DEG). We found that having gates in the vicinity of the 2DEG influence the distribution of the non-equilibrium Hall current injected into the sample. From the inductive measurements we proved that the non-equilibrium Hall current is carried by bulk states. We also demonstrated the role of externally applied electric fields. 2DEG V-grooved samples allowed us to realize a configuration at which the normal component of the magnetic field to the plane of the 2DEG alternates in sign. We showed that for such a magnetic field configuration, the quantum Hall effect survives although the magnitude of the quantized Hall coefficient is multiplied by the number of the grooves. This observation can easily be explained by the alternation of the current direction in each subsequent sidewall of the groove, and therefore implies the current flow in the bulk of the sample.

We present a study of interacting, separately confined electron and hole layers that are photoexcited in undoped GaAs/AlAs mixed types I–II quantum wells. The low-temperature interband photoluminescence spectra and circularly polarized integrated intensity were studied as a function of a perpendicularly applied magnetic field ( B⩽7 T ) and 2DEG density ( n e ⩽8×10 11 cm −2 ). The observed rich structure of the lowest Landau index magnetoexcitons and the strong intensity oscillations at integral filling factors are interpreted as due to cooperative 2DEG–2DHG transitions and, possibly, to the formation of weakly bound inter-layer excitons.

We report on a comparative study of the photoinduced microwave absorption (PMA) (contactless photoconductivity) at 35.6 GHz and the photoluminescence (PL) in undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ quantum wells (QW's) having various well widths (50\char21{}200 \AA{}). While the PL and its excitation (PLE) spectra probe the excitonic transitions, the PMA intensity and its excitation (PMAE) spectra provide information on unbound electron-hole generation processes. The PMAE spectra show strong $(\mathrm{e}1:\mathrm{h}\mathrm{h}1)1S$ and $(\mathrm{e}1:1\mathrm{h}1)1S$ excitonic bands. Since these are bound electron-hole transitions that are observed to give rise to free carrier microwave absorption, exciton dissociation processes are involved. A model is presented that explains these bands in terms of Auger-like exciton dissociation at low temperatures and thermal exciton dissociation at high temperatures. We also discuss the effect of carrier and exciton localization in the spatially fluctuating QW potential.

We determine the exciton exchange splitting in a wide GaAs quantum well. Our method is based on applying a magnetic field parallel to the layers and measuring the oscillator strength ratio of the Zeeman split lines in two linear polarizations. We develop a theoretical model to describe the effect of the magnetic field on the exciton spectrum, and use it to determine the exchange splitting in a 22-nm quantum well to be $22\ifmmode\pm\else\textpm\fi{}3 \ensuremath{\mu}\mathrm{eV}.$ These measurements also allow us to make an accurate determination of the value of $q=0.03,$ the Luttinger parameter which appears in the cubic term of the valence band Zeeman Hamiltonian.

We studied the microwave cyclotron resonance (CR) of photoexcited free and weakly localized electrons and holes in undoped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ multiple quantum wells (MQW's) of various well widths. The photoinduced microwave absorption was measured at a frequency of ${\ensuremath{\omega}}_{\mathrm{mw}}=35.6\mathrm{}\mathrm{GHz}$ and at various lattice temperatures in the range of ${T}_{\mathrm{L}}=4.2--300\mathrm{}\mathrm{K}.$ The interband excitation intensity was very low, so that the density of photogenerated electrons and holes was of the order of $nl~{10}^{8}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}.$ In all the studied QW's, an electron CR was observed, while a heavy hole CR was measured only in narrow QW's. By model fitting the CR line shape, the electron and hole cyclotron masses and the electron scattering rate dependence on ${T}_{\mathrm{L}}$ and on the microwave power were obtained. Assuming that the electron in-plane mobility at ${\ensuremath{\omega}}_{\mathrm{mw}}$ is proportional to the inverse scattering rate, we find that it varies in the range of $(0.8--8)\ifmmode\times\else\texttimes\fi{}{10}^{5}{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{sec}}^{\ensuremath{-}1}$ for 100 \AA{} and 200 \AA{} MQW's. This is less than the mobility measured in modulation doped QW's of similar widths. We present a detailed analysis of the temperature dependence of the electron scattering rate by combining the electron-phonon, electron-impurity, and electron-interface roughness scattering rates. The latter is found to be an important scattering mechanism in undoped MQW's at low temperatures. The CR analysis also shows that the electron cyclotron mass varies (in the range of $0.055--{0.070m}_{0})$ with increasing either ${T}_{\mathrm{L}}$ or the microwave power. These variations are interpreted in terms of weak electron localization in large area, in-plane potential fluctuations arising from interface roughness.

The (3 1 1)A and (5 1 1)A planes of GaAs were used for the growth of high-quality two-dimensional hole gas (2DHG) and electron gas (2DEG) structures, respectively. A back-gated, inverted interface, AlGaAs/GaAs structure in which a 2DHG or a 2DEG was embedded was studied. This particular structure enabled the two-dimensional carrier concentration to be varied over two orders of magnitude in a single device, as well as the measurement of extremely low carrier densities in the mid 109 cm−2 range. The remarkably low carrier concentration we were able to achieve in both a 2DHG and a 2DEG opens new frontiers for the study of mesoscopic phenomena governed by Coulomb interactions between carriers, in particular, the possible existence of a Wigner crystal.

We compare the photoluminescence spectra of the negatively and positively charged excitons in GaAs quantum wells. We use a structure which enables us to observe both complexes within the same sample. We find that their binding energy and Zeeman splitting are very similar at zero magnetic field, but evolve very differently at high fields. We discuss the implications of these observations on our understanding of the charge excitons structure in high magnetic fields.

We have successfully used (311)A and (511)A GaAs for the realization of high quality two-dimensional hole gas (2DHG) and electrons gas (2DEG) structures, respectively. This study was performed mostly on a back-gated, inverted interface, GaAs/AlGaAs structure, in which a 2DHG or 2DEG is embedded. This particular structure enabled the variation of the 2D carrier concentration over two orders of magnitude in a single device, as well as measurement of extremely low carrier densities in the mid 109 cm−2 range. This remarkably low carrier concentration achieved both in a 2DHG and in a 2DEG opens new frontiers for the study of mesoscopic phenomena governed by Coulomb interactions between carriers and, in particular, the possible existence of a Wigner crystal.