arXiv subjects

D. Weiss

Publications and source records attributed to D. Weiss.

At least 37 records · Page 2Linked to original sources

arXiv2018

Edge currents driven by terahertz radiation in graphene in quantum Hall regime

We observe that the illumination of unbiased graphene in the quantum Hall regime with polarized terahertz laser radiation results in a direct edge current. This photocurrent is caused by an imbalance of persistent edge currents, which are driven out of thermal equilibrium by indirect transitions within the chiral edge channel. The direction of the edge photocurrent is determined by the polarity of the external magnetic field, while its magnitude depends on the radiation polarization. The microscopic theory developed in this paper describes well the experimental data.

cond-mat.mes-hall↗

arXiv2018

Electric-field modification of interfacial spin-orbit field-vector

Current induced spin-orbit magnetic fields (iSOFs), arising either in single-crystalline ferromagnets with broken inversion symmetry1,2 or in non-magnetic metal/ferromagnetic metal bilayers3,4, can produce spin-orbit torques which act on a ferromagnet's magnetization,thus offering an efficient way for its manipulation.To further reduce power consumption in spin-orbit torque devices, it is highly desirable to control iSOFs by the field-effect, where power consumption is determined by charging/discharging a capacitor5,6. In particular, efficient electric-field control of iSOFs acting on ferromagnetic metals is of vital importance for practical applications. It is known that in single crystalline Fe/GaAs (001) heterostructures with C2v symmetry, interfacial SOFs emerge at the Fe/GaAs (001) interface due to the lack of inversion symmetry7,8. Here, we show that by applying a gate-voltage across the Fe/GaAs interface, interfacial SOFs acting on Fe can be robustly modulated via the change of the magnitude of the interfacial spin-orbit interaction. Our results show that, for the first time, the electric-field in a Schottky barrier is capable of modifying SOFs, which can be exploited for the development of low-power-consumption spin-orbit torque devices.

cond-mat.mes-hall↗

arXiv2018

Engineering and improving the magnetic properties of thin Fe layers through exchange coupling with hard magnetic Dysprosium layers

We report on a comprehensive study of the magnetic coupling between soft magnetic Fe layers and hard magnetic Dysprosium (Dy) layers at low temperatures (4.2 - 120K). For our experiments we prepared thin films of Fe and Dy and multilayers of Fe/Dy by ultra-high vacuum sputtering. The magnetic properties of each material were determined with a superconducting quantum interference device. Furthermore, we performed magnetoresistance measurements with similarly grown, microstructured devices, where the anisotropic magnetoresistance (AMR) effect was used to identify the magnetization state of the samples. By analyzing and comparing the corresponding data of Fe and Dy, we show that the presence of a Dy layer on top of the Fe layer significantly influences its magnetic properties and makes it magnetically harder. We perform a systematic evaluation of this effect and its dependence on temperature and on the thickness of the soft magnetic layer. All experimental results can consistently be explained with exchange coupling at the interface between the Fe and the Dy layer. Our experiments also yield a negative sign of the AMR effect of thin Dy films, and an increase of the Dy films' Curie temperature, which is due to growth conditions.

cond-mat.mtrl-sci↗

arXiv2017

Circular and linear magnetic quantum ratchet effects in dual-grating-gate CdTe-based nanostructures

We report on the observation and systematic study of polarization sensitive magnetic quantum ratchet effects induced by alternating electric fields in the terahertz frequency range. The effects are detected in (Cd,Mn)Te-based quantum well (QW) structures with inter-digitated dual-grating-gate (DGG) lateral superlattices. A dc electric current excited by cw terahertz laser radiation shows 1/B-periodic oscillations with an amplitude much larger than the photocurrent at zero magnetic field. Variation of gate voltages applied to individual grating gates of the DGG enables us to change the degree and the sign of the lateral asymmetry in a controllable way. The data reveal that the photocurrent reflects the degree of lateral asymmetry induced by different gate potentials. We show that the magnetic ratchet photocurrent includes the Seebeck thermoratchet effect as well as the effects of "linear" and "circular" ratchets, which are sensitive to the corresponding polarization of the driving electromagnetic force. Theoretical analysis performed in the framework of semiclassical approach and taking into account Landau quantization describes the experimental results well.

cond-mat.mes-hall↗

arXiv2017

Magneto-resistance oscillations induced by high-intensity terahertz radiation

We report on observation of pronounced terahertz radiation-induced magneto-resistivity oscillations in AlGaAs/GaAs two-dimensional electron systems, the THz analog of the microwave induced resistivity oscillations (MIRO). Applying high power radiation of a pulsed molecular laser we demonstrate that MIRO, so far observed at low power only, are not destroyed even at very high intensities. Experiments with radiation intensity ranging over five orders of magnitude from $0.1$ W/cm$^2$ to $10^4$ W/cm$^2$ reveal high-power saturation of the MIRO amplitude, which is well described by an empirical fit function $I/(1 + I/I_s)^β$ with $β\sim 1$. The saturation intensity Is is of the order of tens of W/cm$^2$ and increases by six times by increasing the radiation frequency from $0.6$ to $1.1$ THz. The results are discussed in terms of microscopic mechanisms of MIRO and compared to nonlinear effects observed earlier at significantly lower excitation frequencies.

cond-mat.mes-hall↗

arXiv2017

Magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with a lateral asymmetric superlattice

We report on the observation of magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field applied normal to the quantum well plane. A dc electric current excited by cw terahertz laser radiation shows 1/B-oscillations with an amplitude much larger as compared to the photocurrent at zero magnetic field. We show that the photocurrent is caused by the combined action of a spatially periodic in-plane potential and the spatially modulated radiation due to the near field effects of light diffraction. Magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization and, for (Cd,Mn)Te at low temperatures, from the exchange enhanced Zeeman splitting in diluted magnetic heterostructures. Theoretical analysis, considering the magnetic quantum ratchet effect in the framework of semiclassical approach, describes quite well the experimental results.

cond-mat.mes-hall↗

arXiv2016

Photogalvanic probing of helical edge channels in 2D HgTe topological insulators

We report on the observation of a circular photogalvanic current excited by terahertz (THz) laser radiation in helical edge channels of HgTe-based 2D topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from right-handed to left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a 7 $μ$m distance at CNP approaches 2e2/h, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with the experimental data.

cond-mat.mes-hall↗

arXiv2016

Low temperature dissipation scenarios in palladium nano-mechanical resonators

We study dissipation in Pd nano-mechanical resonators at low temperatures in the linear response regime. Metallic resonators have shown characteristic features of dissipation due to tunneling two level systems (TLS). This system offers a unique tunability of the dissipation scenario by adsorbing hydrogen ($H_2$) which induces a compressive stress. The intrinsic stress is expected to alter TLS behaviour. We find a sub-linear power law $\sim T^{0.4}$ in dissipation. As seen in TLS dissipation scenarios we find a logarithmic increase of frequency characteristic from the lowest temperatures till a characteristic temperature $T_{co}$ is reached. In samples without $H_2$ the $T_{co} \sim 1K$ whereas with $H_2$ it is clearly reduced to $\sim 700 mK$. Based on standard TLS phenomena we attribute this to enhanced phonon-TLS coupling in samples with compressive strain. We also find with $H_2$ there is a saturation in low temperature dissipation which may possibly be due to super-radiant interaction between TLS and phonons. We discuss the data in the scope of TLS phenomena and similar data for other systems.

cond-mat.mes-hall↗

arXiv2016

An extensive spectroscopic time-series of three Wolf-Rayet stars. I. The lifetime of large-scale structures in the wind of WR 134

During the summer of 2013, a 4-month spectroscopic campaign took place to observe the variabilities in three Wolf-Rayet stars. The spectroscopic data have been analyzed for WR 134 (WN6b), to better understand its behaviour and long-term periodicity, which we interpret as arising from corotating interaction regions (CIRs) in the wind. By analyzing the variability of the He II $λ$5411 emission line, the previously identified period was refined to P = 2.255 $\pm$ 0.008 (s.d.) days. The coherency time of the variability, which we associate with the lifetime of the CIRs in the wind, was deduced to be 40 $\pm$ 6 days, or $\sim$ 18 cycles, by cross-correlating the variability patterns as a function of time. When comparing the phased observational grayscale difference images with theoretical grayscales previously calculated from models including CIRs in an optically thin stellar wind, we find that two CIRs were likely present. A separation in longitude of $Δϕ\simeq$ 90$^{\circ}$ was determined between the two CIRs and we suggest that the different maximum velocities that they reach indicate that they emerge from different latitudes. We have also been able to detect observational signatures of the CIRs in other spectral lines (C IV $λλ$5802,5812 and He I $λ$5876). Furthermore, a DAC was found to be present simultaneously with the CIR signatures detected in the He I $λ$5876 emission line which is consistent with the proposed geometry of the large-scale structures in the wind. Small-scale structures also show a presence in the wind, simultaneously with the larger scale structures, showing that they do in fact co-exist.

astro-ph.SR↗

arXiv2016

MIRO-like oscillations of magneto-resistivity in GaAs heterostructures induced by THz radiation

We report on the study of terahertz radiation induced MIRO-like oscillations of magneto-resistivity in GaAs heterostructures. Our experiments provide an answer on two most intriguing questions - effect of radiation helicity and the role of the edges - yielding crucial information for understanding of the MIRO origin. Moreover, we demonstrate that the range of materials exhibiting radiation-induced magneto-oscillations can be largely extended by using high-frequency radiation.

cond-mat.mes-hall↗

arXiv2015

Probing quantum capacitance in a 3D topological insulator

We measure the quantum capacitance and probe thus directly the electronic density of states of the high mobility, Dirac type of two-dimensional electron system, which forms on the surface of strained HgTe. Here we show that observed magneto-capacitance oscillations probe, in contrast to magnetotransport, primarily the top surface. Capacitance measurements constitute thus a powerful tool to probe only one topological surface and to reconstruct its Landau level spectrum for different positions of the Fermi energy.

cond-mat.mes-hall↗

arXiv2015

Terahertz ratchet effects in graphene with a lateral superlattice

Experimental and theoretical studies on ratchet effects in graphene with a lateral superlattice excited by alternating electric fields of terahertz frequency range are presented. A lateral superlatice deposited on top of monolayer graphene is formed either by periodically repeated metal stripes having different widths and spacings or by inter-digitated comb-like dual-grating-gate (DGG) structures. We show that the ratchet photocurrent excited by terahertz radiation and sensitive to the radiation polarization state can be efficiently controlled by the back gate driving the system through the Dirac point as well as by the lateral asymmetry varied by applying unequal voltages to the DGG subgratings. The ratchet photocurrent includes the Seebeck thermoratchet effect as well as the effects of "linear" and "circular" ratchets, sensitive to the corresponding polarization of the driving electromagnetic force. The experimental data are analyzed for the electronic and plasmonic ratchets taking into account the calculated potential profile and the near field acting on carriers in graphene. We show that the photocurrent generation is based on a combined action of a spatially periodic in-plane potential and the spatially modulated light due to the near field effects of the light diffraction.

cond-mat.mes-hall↗

arXiv2015

Magneto-resistance quantum oscillations in a magnetic two-dimensional electron gas

Magneto-transport measurements of Shubnikov-de Haas (SdH) oscillations have been performed on two-dimensional electron gases (2DEGs) confined in CdTe and CdMnTe quantum wells. The quantum oscillations in CdMnTe, where the 2DEG interacts with magnetic Mn ions, can be described by incorporating the electron-Mn exchange interaction into the traditional Lifshitz-Kosevich formalism. The modified spin splitting leads to characteristic beating pattern in the SdH oscillations, the study of which indicates the formation of Mn clusters resulting in direct anti-ferromagnetic Mn-Mn interaction. The Landau level broadening in this system shows a peculiar decrease with increasing temperature, which could be related to statistical fluctuations of the Mn concentration.

cond-mat.mes-hall↗

arXiv2015

Cyclotron Resonance Assisted Photocurrents in Surface States of a 3D Topological Insulator Based on a Strained High Mobility HgTe Film

We report on the observation of cyclotron resonance induced photocurrents, excited by continuous wave terahertz radiation, in a 3D topological insulator (TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent formation is supported by complimentary measurements of magneto-transport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the effective masses and the mobility of 2D Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.

cond-mat.mes-hall↗

arXiv2015

Double island Coulomb blockade in (Ga,Mn)As-nanoconstrictions

We report on a systematic study of the Coulomb blockade effects in nanofabricated narrow constrictions in thin (Ga,Mn)As films. Different low-temperature transport regimes have been observed for decreasing constriction sizes: the ohmic, the single electron tunnelling (SET) and a completely insulating regime. In the SET, complex stability diagrams with nested Coulomb diamonds and anomalous conductance suppression in the vicinity of charge degeneracy points have been observed. We rationalize these observations in the SET with a double ferromagnetic island model coupled to ferromagnetic leads. Its transport characteristics are analyzed in terms of a modified orthodox theory of Coulomb blockade which takes into account the energy dependence of the density of states in the metallic islands.

cond-mat.mes-hall↗

arXiv2015

Magnetooptical Study of Zeeman Effect in Mn modulation-doped InAs/InGaAs/InAlAs Quantum Well Structures

We report on a magneto-photoluminescence (PL) study of Mn modulation-doped InAs/InGaAs/InAlAs quantum wells. Two PL lines corresponding to the radiative recombination of photoelectrons with free and bound-on-Mn holes have been observed. In the presence of a magnetic field applied in the Faraday geometry both lines split into two circularly polarized components. While temperature and magnetic field dependences of the splitting are well described by the Brillouin function, providing an evidence for exchange interaction with spin polarized manganese ions, the value of the splitting exceeds the expected value of the giant Zeeman splitting by two orders of magnitude for a given Mn density. Possible reasons of this striking observation are discussed.

cond-mat.mes-hall↗

arXiv2014

Fractional Quantum Hall Effect in a Diluted Magnetic Semiconductor

We report the observation of the fractional quantum Hall effect in the lowest Landau level of a two-dimensional electron system (2DES), residing in the diluted magnetic semiconductor Cd(1-x)Mn(x)Te. The presence of magnetic impurities results in a giant Zeeman splitting leading to an unusual ordering of composite fermion Landau levels. In experiment, this results in an unconventional opening and closing of fractional gaps around filling factor v = 3/2 as a function of an in-plane magnetic field, i.e. of the Zeeman energy. By including the s-d exchange energy into the composite Landau level spectrum the opening and closing of the gap at filling factor 5/3 can be modeled quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES provides a novel means to manipulate fractional states.

cond-mat.str-el↗

arXiv2014

Magneto-Photoluminescence of InAs/InGaAs/InAlAs quantum well structures

Photoluminescence (PL) and highly circularly-polarized magneto-PL (up to 50% at 6 T) from two-step bandgap InAs/InGaAs/InAlAs quantum wells (QWs) are studied. Bright PL is observed up to room temperature, indicating a high quantum efficiency of the radiative recombination in these QW. The sign of the circular polarization indicates that it stems from the spin polarization of heavy holes caused by the Zeeman effect. Although in magnetic field the PL line are strongly circularly polarized, no energy shift between the counter-polarized PL lines was observed. The results suggest that the electron and the hole g-factor to be of the same sign and close magnitudes.

cond-mat.mes-hall↗