arXiv subjects

L. Smrcka

Publications and source records attributed to L. Smrcka.

At least 19 recordsLinked to original sources

arXiv2013

Magnetotransport in graphene on silicon side of SiC

We have studied the transport properties of graphene grown on silicon side of SiC. Samples under study have been prepared by two different growth methods in two different laboratories. Magnetoresistance and Hall resistance have been measured at temperatures between 4 and 100 K in resistive magnet in magnetic fields up to 22 T. In spite of differences in sample preparation, the field dependence of resistances measured on both sets of samples exhibits two periods of magneto-oscillations indicating two different parallel conducting channels with different concentrations of carriers. The semi-quantitative agreement with the model calculation allows for conclusion that channels are formed by high-density and low-density Dirac carriers. The coexistence of two different groups of carriers on the silicon side of SiC was not reported before.

cond-mat.mes-hall↗

arXiv2012

Infrared magneto-spectroscopy of graphite in tilted fields

The electronic structure of Bernal-stacked graphite subject to tilted magnetic fields has been investigated using infrared magneto-transmission experiments. With the increasing in-plane component of the magnetic field B, we observe significant broadening and partially also splitting of interband inter-Landau level transitions, which originate at the H point of the graphite Brillouin zone, where the charge carriers behave as massless Dirac fermions. The observed behavior is attributed to the lifting of the twofold degeneracy of Landau levels at the H point - a degeneracy which in graphite complements the standard spin and valley degeneracies typical of graphene.

cond-mat.mes-hall↗

arXiv2011

From laterally modulated two-dimensional electron gas towards artificial graphene

Cyclotron resonance has been measured in far-infrared transmission of GaAs/Al$_x$Ga$_{1-x}$As heterostructures with an etched hexagonal lateral superlattice. Non-linear dependence of the resonance position on magnetic field was observed as well as its splitting into several modes. Our explanation, based on a perturbative calculation, describes the observed phenomena as a weak effect of the lateral potential on the two-dimensional electron gas. Using this approach, we found a correlation between parameters of the lateral patterning and the created effective potential and obtain thus insights on how the electronic miniband structure has been tuned. The miniband dispersion was calculated using a simplified model and allowed us to formulate four basic criteria that have to be satisfied to reach graphene-like physics in such systems.

cond-mat.mes-hall↗

arXiv2010

K-edge XANES of substitutional and interstitial Mn atoms in (Ga,Mn)As

This work reports theoretical and experimental study of the X-ray absorption near-edge structure (XANES) at the Mn K-edge in (Ga,Mn)As diluted magnetic semiconductors. The spectra have been calculated from the first-principles using FLAPW including the core-hole effect, a special attention has been paid to consequences of coexistence of Mn impurities in substitutional and tetrahedral interstitial positions. We have performed quantitative component analysis of experimental spectra collected on the (Ga,Mn)As samples before/after annealing and etching, with the aim to determine the proportion of Mn impurity configurations. Comparison of the experimental data with theoretical computations indicates that even after annealing and etching some Mn atoms still reside in interstitial sites, although the concentration of interstitial defects has been reduced by annealing.

cond-mat.mtrl-sci↗

arXiv2010

Pre-edge XANES structure of Mn in (Ga,Mn)As

The X-ray absorption near-edge structure (XANES) at the Mn K-edge in the (Ga,Mn)As magnetic semiconductor was simulated using the full potential linearized augmented plane wave (FLAPW) method including the core-hole effect. The calculations were performed in the supercell scheme for a substitutional and two tetrahedral interstitial Mn sites. The resulting pre-edge absorption structures show sharp distinction between the spectra simulated for the substitutional and interstitial Mn defects which is determined mainly by the second nearest neighbor ligands. A single feature is obtained for the substitutional Mn impurity, whereas two peaks appear for both types of interstitial defects. An interpretation of the pre-edge features was proposed.

cond-mat.mes-hall↗

arXiv2010

Gate-induced magneto-oscillation phase anomalies in graphene bilayers

The magneto-oscillations in graphene bilayers are studied in the vicinity of the K and K' points of the Brillouin zone within the four-band continuum model ased on the simplest tight-binding approximation involving only the nearest neighbor interactions. The model is employed to construct Landau plots for a variety of carrier concentrations and bias strengths between the graphene planes. The quantum-mechanical and quasiclassical approaches are compared. We found that the quantum magneto-oscillations are only asymptotically periodic and reach the frequencies predicted quasiclassically for high indices of Landau levels. In unbiased bilayers the phase of oscillations is equal to the phase of massive fermions. Anomalous behavior of oscillation phases was found in biased bilayers with broken inversion symmetry. The oscillation frequencies again tend to quasiclassically predicted ones, which are the same for $K$ and $K'$, but the quantum approach yields the gate-tunable corrections to oscillation phases, which differ in sign for K and K'. These valley-dependent phase corrections give rise, instead of a single quasiclassical series of oscillations, to two series with the same frequency but shifted in phase.

cond-mat.mes-hall↗

arXiv2009

On the phase of magneto-oscillations in graphite

The problem of Dirac fermions in graphite subject to a perpendicular magnetic field is studied. We show analytically that the weak inter-layer interaction between the graphene sheets leads to anomalies in the Shubnikov-de Haas and de Haas-van Alphen magneto-oscillations governed by the orbits around extremal cross-sections of the graphite Fermi surface. The calculation of the Landau plot performed within a four band continuum model reveals that magneto-oscillations are aperiodic, except of the case of vanishing inter-layer interaction at the H point of the graphite Brillouin zone. Also for all other orbits along the H-K-H edge the magneto-oscillations are only asymptotically periodic in the quasi-classical limit, with the phase corresponding to massive fermions.

cond-mat.mtrl-sci↗

arXiv2007

Electron magnetotransport in GaAs/AlGaAs superlattices with weak and strong inter-well coupling

We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs superlattices formed by wide and narrow quantum wells and thin Si-doped barriers subject to tilted magnetic fields. It has been shown that illumination of the strongly coupled superlattice with narrow wells leads to reduction of its dimensionality from the 3D to 2D. The illumination-induced transition is revealed by remarkable change of magnetoresistance curves as compared to those measured before illumination. The experimental data along with tight-binding model calculations indicate that the illumination not only enhances the electron concentration but also suppresses the electron tunneling through the barriers.

cond-mat.mes-hall↗

arXiv2007

Electronic structure of unidirectional superlattices in crossed electric and magnetic fields and related terahertz oscillations

We have studied Bloch electrons in a perfect unidirectional superlattice subject to crossed electric and magnetic fields, where the magnetic field is oriented ``in-plane'', i.e. in parallel to the sample plane. Two orientation of the electric field are considered. It is shown that the magnetic field suppresses the intersubband tunneling of the Zener type, but does not change the frequency of Bloch oscillations, if the electric field is oriented perpendicularly to both the sample plane and the magnetic field. The electric field applied in-plane (but perpendicularly to the magnetic field) yields the step-like electron energy spectrum, corresponding to the magnetic-field-tunable oscillations alternative to the Bloch ones.

cond-mat.mtrl-sci↗

arXiv2007

Illumination-induced changes of the Fermi surface topology in three-dimensional superlattices

The magnetoresistance of the MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been measured in tilted magnetic fields in the as-grown state, and after brief illumination by a red-light diode at low temperature, T is approximately 0.3 K. A remarkable illumination-induced modification of magnetoresistance curves has been observed, which indicates a significant change of the superlattice Fermi surface topology. Analysis of magnetoresistance data in terms of the tight-binding model reveals that not only electron concentration and mobility have been increased by illumination, but also the coupling among 2D electron layers in neighboring quantum wells has been reduced.

cond-mat.mes-hall↗

arXiv2007

Substrate temperature changes during MBE growth of GaMnAs

Remarkably big increase of the substrate temperature during the low-temperature MBE growth of GaMnAs layers is observed by means of band gap spectroscopy. It is explained and simulated in terms of changes in the absorption/emission characteristics of the growing layer. Options for the temperature variation damping are discussed.

cond-mat.mtrl-sci↗

arXiv2006

Tunable terahertz oscillations in superlattices subject to an in-plane magnetic field

We present a concept of terahertz oscillations in superlattices that are generated under conditions apparently different from standard Bloch oscillations. These oscillations are induced when crossed magnetic and electric fields are applied to the superlattice, both in the in-plane direction. The frequency of these oscillations is tunable by the applied fields.

cond-mat.other↗

arXiv2005

Electronic Structure of Three-Dimensional Superlattices Subject to Tilted Magnetic Fields

Full quantum-mechanical description of electrons moving in 3D structures with unidirectional periodic modulation subject to tilted magnetic fields requires an extensive numerical calculation. To understand magneto-oscillations in such systems it is in many cases sufficient to use the quasi-classical approach, in which the zero-magnetic-field Fermi surface is considered as a magnetic-field-independent rigid body in k-space and periods of oscillations are related to extremal cross-sections of the Fermi surface cut by planes perpendicular to the magnetic-field direction. We point out cases where the quasi-classical treatment fails and propose a simple tight-binding fully-quantum-mechanical model of the superlattice electronic structure.

cond-mat.mes-hall↗

arXiv2003

In-plane Magnetic Field Dependent Magnetoresistance of Gated Asymmetric Double Quantum Wells

We have investigated experimentally the magnetoresistance of strongly asymmetric double-wells. The structures were prepared by inserting a thin Al$_{0.3}$Ga$_{0.7}$As barrier into the GaAs buffer layer of a standard modulation-doped GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure. The resulting double-well system consists of a nearly rectangular well and of a triangular well coupled by tunneling through the thin barrier. With a proper choice of the barrier parameters one can control the occupancy of the two wells and of the two lowest (bonding and antibonding) subbands. The electron properties can be further influenced by applying front- or back-gate voltage.

cond-mat.mes-hall↗

arXiv2003

Magnetoresistance of Si(001) MOSFETs with high concentration of electrons

We present an experimental study of electron transport in inversion layers of high-mobility Si(001) samples with occupied excited subbands. The second series of oscillations, observed in addition to the main series of Shubnikov-de Hass oscillations, is tentatively attributed to the occupation of a subband associated with the $E_{0'}$ level. Besides, a strong negative magnetoresistance and nonlinear field dependence of the Hall resistance accompany the novel oscillations at high carrier concentrations. The heating of the 2D electron layers leads to suppression of the observed anomalies.

cond-mat.mes-hall↗

arXiv2002

Cyclotron Resonance in Two-Dimensional Electron Single-Layers and Double-Layers in Tilted Magnetic Fields

The far-infrared absorption in two-dimensional electron layers subject to magnetic field of general orientation was studied theoretically. The Kubo formula was employed to derive diagonal components of the magneto-conductivity tensor of two-dimensional electron single-layers and double-layers. A parabolic quantum well was used to model single-layer systems. Bilayer systems were represented by a pair of tunnel-coupled, strictly two-dimensional quantum wells. Obtained results were compared to experimental data.

cond-mat.mes-hall↗

arXiv2001

Inter-layer Hall effect in double quantum wells subject to in-plane magnetic fields

We report on a theoretical study of the transport properties of two coupled two-dimensional electron systems subject to in-plane magnetic fields. The charge redistribution in double wells induced by the Lorenz force in crossed electric and magnetic fields has been studied. We have found that the redistribution of the charge and the related inter-layer Hall effect originate in the chirality of diamagnetic currents and give a substantial contribution to the conductivity.

cond-mat.mes-hall↗

arXiv2001

Longitudinal conductivity and transverse charge redistribution in coupled quantum wells subject to in-plane magnetic fields

In double quantum wells electrons experience a Lorentz force oriented perpendicular to the structure plane when an electric current is driven perpendicular to the direction of an in-plane magnetic field. Consequently, the excess charge is accumulated in one of the wells. The polarization of a bilayer electron system and the corresponding Hall voltage are shown to contribute substantially to the in-plane conductivity.

cond-mat.mes-hall↗