arXiv subjects

G. Springholz

Publications and source records attributed to G. Springholz.

47 records · Page 3Linked to original sources

arXiv2013

Photoemission of Bi$_2$Se$_3$ with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation?

Topological insulators are characterized by Dirac cone surface states with electron spins aligned in the surface plane and perpendicular to their momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy. We solve this puzzle and show that vacuum ultraviolet photons (50-70 eV) with linear or circular polarization probe indeed the initial state spin texture of Bi$_2$Se$_3$ while circularly polarized 6 eV low energy photons flip the electron spins out of plane and reverse their spin polarization. Our photoemission calculations, considering the interplay between the varying probing depth, dipole selection rules and spin-dependent scattering effects involving initial and final states explain these findings, and reveal proper conditions for light-induced spin manipulation. This paves the way for future applications of topological insulators in opto-spintronic devices.

cond-mat.mes-hall↗

arXiv2012

Absence of nonlocal resistance in microstructures of PbTe quantum wells

We report on experiments allowing to set an upper limit on the magnitude of the spin Hall effect and the conductance by edge channels in quantum wells of PbTe embedded between PbEuTe barriers. We reexamine previous data obtained for epitaxial microstructures of n-type PbSe and PbTe, in which pronounced nonlocal effects and reproducible magnetoresistance oscillations were found. Here we show that these effects are brought about by a quasi-periodic network of threading dislocations adjacent to the BaF$_2$ substrate, which give rise to a p-type interfacial layer and an associated parasitic parallel conductance. We then present results of transport measurements on microstructures of modulation doped PbTe/(Pb,Eu)Te:Bi heterostructures for which the influence of parasitic parallel conductance is minimized, and for which quantum Hall transport had been observed, on similar samples, previously. These structures are of H-shaped geometry and they are patterned of 12 nm thick strained PbTe quantum wells embedded between Pb$_{0.92}$Eu$_{0.08}$Te barriers. The structures have different lateral sizes corresponding to both diffusive and ballistic electron transport in non-equivalent L valleys. For these structures no nonlocal resistance is detected confirming that PbTe is a trivial insulator. The magnitude of spin Hall angle gamma is estimated to be smaller than 0.02 for PbTe/PbEuTe microstructures in the diffusive regime.

cond-mat.mes-hall↗

arXiv2010

Contact superconductivity in In-PbTe junctions

The authors report on electron transport studies on superconductor-semiconductor hybrid structures of indium and n-type lead telluride, either in the form of quantum wells or bulk crystals. In-PbTe contacts form by spontaneous alloying, which occurs already at room temperature. The alloyed phase penetrates deeply into PbTe and forms metallic contacts even in the presence of depletion layers at the semiconductor surface. Although the detailed structure of this phase is unknown, we observe that it exhibits a superconducting transition at temperatures below 10 K. This causes such substantial reduction of the contact resistances that they even become comparable to those predicted for ideal superconductor-normal conductor contacts. Most importantly, this result indicates that the interface phase in the superconducting state becomes nearly homogeneous - in contrast to the structure expected for alloyed contacts. We suggest that the unusual interface superconductivity is linked to the unique properties of PbTe, namely, its huge static dielectric constant. Apparently the alloyed interface phase contains superconducting precipitates randomly distributed within the depletion layers, and their Coulomb charging energies are extremely small. According to the existing models of the granular superconductivity, even very weak Josephson coupling between the neighboring precipitates gives rise to the formation of a global superconducting phase which explains our observations.

cond-mat.supr-con↗

arXiv2009

Exchange interactions in europium monochalcogenide magnetic semiconductors and their dependence on hydrostatic strain

The classical Heisenberg model is applied in a Monte Carlo study to investigate the distance dependence of the indirect nearest neighbor (NN) exchange and next-nearest neighbor (NNN) superexchange interaction in EuO, EuS, EuSe and EuTe. For this purpose, first, the dependence of the magnetic ordering temperature, i.e., Curie, respectively, Néel temperature for ferromagnetic and antiferromagnetic ordering on the exchange constants was determined. This was then employed for the analysis of experimental data of hydrostatic pressure experiments. It is shown that all experimental findings, i.e., the strong increase of the critical temperatures, as well as the transition from antiferromagnetic to ferromagnetic ordering for EuTe and EuSe with decreasing lattice parameter is well described by the magnetic Grüneisen law, in which the exchange constants depend on the interatomic distances of the Eu ions in the form of a power law. According to these calculations, the indirect NN exchange is characterized by a Grüneisen exponent of approximately 20 and the NNN superexchange by an exponent of about 10 for all four europium monochalcogenides. The latter agrees with Bloch's empirical 10/3 law for the volume dependence of superexchange interactions in insulating magnetic materials. The Monte Carlo calculations also yield significantly revised exchange constants for unstrained bulk material because spin fluctuations at non-zero temperatures are taken into account. The strong increase of the exchange constants with decreasing lattice parameter provides room for increasing the Curie temperatures in strained epitaxial structures, which is important for device applications.

cond-mat.mtrl-sci↗

arXiv2009

Spin-induced optical second harmonic generation in the centrosymmetric magnetic semiconductors EuTe and EuSe

Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe reveals spin-induced optical second harmonic generation (SHG) in the band gap vicinity at 2.1-2.4eV. The magnetic field and temperature dependence demonstrates that the SHG arises from the bulk of the materials due to a novel type of nonlinear optical susceptibility caused by the magnetic dipole contribution combined with spontaneous or induced magnetization. This spin-induced susceptibility opens access to a wide class of centrosymmetric systems by harmonics generation spectroscopy.

cond-mat.mtrl-sci↗

arXiv2006

Centrosymmetric PbTe/CdTe quantum dots coherently embedded by epitaxial precipitation

A concept for the fabrication of highly symmetric quantum dots that are coherently embedded in a single crystalline matrix is demonstrated. In this approach, the formation of the quantum dots is induced by a transformation of an epitaxial 2D quantum well into an array of isolated precipitates with dimensions of about 25 nm. The formation process is driven by the immiscibility of the constituent materials resulting from their different lattice structures. The investigated PbTe/CdTe heterosystem combines two different cubic lattices with almost identical lattice constants. Therefore, the precipitated quantum dots are almost strain free and near thermodynamic equilibrium they exhibit the shape of small-rhombo-cubo-octahedrons. The PbTe/CdTe quantum dots, grown on GaAs substrates, display intense room temperature luminescence at wavelength around 3.2 micrometer, which makes them auspicious for applications in mid-infrared photonic devices.

cond-mat.mtrl-sci↗

arXiv2005

Disorder suppression and precise conductance quantization in constrictions of PbTe quantum wells

Conductance quantization was measured in submicron constrictions of PbTe, patterned into narrow,12 nm wide quantum wells deposited between Pb$_{0.92}$Eu$_{0.08}$Te barriers. Because the quantum confinement imposed by the barriers is much stronger than the lateral one, the one-dimensional electron energy level structure is very similar to that usually met in constrictions of AlGaAs/GaAs heterostructures. However, in contrast to any other system studied so far, we observe precise conductance quantization in $2e^2/h$ units, {\it despite of significant amount of charged defects in the vicinity of the constriction}. We show that such extraordinary results is a consequence of the paraelectric properties of PbTe, namely, the suppression of long-range tails of the Coulomb potentials due to the huge dielectric constant.

cond-mat.mes-hall↗

arXiv2004

IV-VI resonant cavity enhanced photodetectors for the midinfrared

A resonant-cavity enhanced detector operating in the mid-infrared at a wavelength around 3.6 micron is demonstrated. The device is based on a narrow-gap lead salt heterostructure grown by molecular beam epitaxy. Below 140 K, the photovoltage clearly shows a single narrow cavity resonance, with a relative line width of only 2 % at 80 K.

cond-mat.mtrl-sci↗

arXiv2003

Magnetic interactions in EuTe epitaxial layers and EuTe/PbTe superlattices

The magnetic properties of antiferromagnetic (AFM) EuTe epitaxial layers and short period EuTe/PbTe superlattices (SLs), grown by molecular beam epitaxy on (111) BaF$_2$ substrates, were studied by magnetization and neutron diffraction measurements. Considerable changes of the Néel temperature as a function of the EuTe layer thickness as well as of the strain state were found. A mean field model, taking into account the variation of the exchange constants with the strain-induced lattice distortions, and the nearest neighbor environment of a Eu atoms, was developed to explain the observed $T_{\text N}$ changes in wide range of samples. Pronounced interlayer magnetic correlations have been revealed by neutron diffraction in EuTe/PbTe SLs with PbTe spacer thickness up to 60 Å. The observed diffraction spectra were analyzed, in a kinematical approximation, assuming partial interlayer correlations characterized by an appropriate correlation parameter. The formation of interlayer correlations between the AFM EuTe layers across the nonmagnetic PbTe spacer was explained within a framework of a tight-binding model. In this model, the interlayer coupling stems from the dependence of the total electronic energy of the EuTe/PbTe SL on the spin configurations in adjacent EuTe layers. The influence of the EuTe and PbTe layer thickness fluctuations, inherent in the epitaxial growth process, on magnetic properties and interlayer coupling is discussed.

cond-mat.mtrl-sci↗

arXiv2002

Step bunching during Si(001) homoepitaxy caused by the surface diffusion anisotropy

Scanning tunneling microscopy experiments show that the unstable growth morphology observed during molecular beam homoepitaxy on slightly vicinal Si(001) surfaces consists of straight step bunches. The instability occurs under step- flow growth conditions and vanishes both during low-temperature island growth and at high temperatures. An instability with the same characteristics is observed in a 2D Kinetic Monte Carlo model of growth with incorporated Si(001)- like diffusion anisotropy. This provides strong evidence that the diffusion anisotropy destabilizes growth on Si(001) and similar surfaces towards step bunching. This new instability mechanism is operational without any additional step edge barriers.

cond-mat.mtrl-sci↗

arXiv1999

Quantum ballistic transport in constrictions of n-PbTe

Conductance of submicron constrictions of PbTe:Bi was studied up to 8T and between 4.2K and 50mK. The structures were fabricated by electron beam lithography and chemical etching of high--electron mobility films grown by MBE on BaF_2. In the moderately strong magnetic fields perpendicular to the current, B>1T, the conductance shows accurate quantization in the units of 1e^2/h as a function of the side-gate voltage. In the absence of the field, a temperature-independent step structure, with an average step height approx. 1e^2/h, is observed. It is suggested that such a quantization may reflect the lifting of the Kramers degeneracy by the exchange interaction among the electrons, effective despite a large dielectric constant of bulk PbTe.

cond-mat.mes-hall↗