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G. Bauer

Publications and source records attributed to G. Bauer.

At least 73 records · Page 4Linked to original sources
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↗
arXiv2003

The CDFII Time-Of-Flight Detector and Impact on Beauty Flavor Tagging

The new CDFII detector incorporates a Time-of-Flight detector (TOF), employing plastic scintillator bars and fine-mesh photomultipliers. Since August 2001 the TOF system has been fully instrumented and integrated into the CDFII data acquisition system. With a design goal of $100 ps$ resolution the TOF system will provide at least two standard deviations separation between $K^{\pm}$ and $π^{\pm}$ for momenta $p<1.6 GeV/c$, complementing low momentum particle identification by means of the specific ionization energy loss measured in the drift chamber. We describe the design of the TOF detector and discuss the current status of its calibration and initial performances. Finally we review the expected impact of the TOF detector in the flavor tagging of neutral $B_S$ meson.

hep-ex↗
arXiv2003

Positioning of self-assembled Ge islands on stripe-patterned Si (001) substrates

Self-assembled Ge islands were grown on stripe-patterned Si (001) substrates by solid source molecular beam epitaxy. The surface morphology obtained by atomic force microscopy (AFM) and cross-sectional transmission electron microscopy images (TEM) shows that the Ge islands are preferentially grown at the sidewalls of pure Si stripes along [-110] direction at 650o C or along the trenches, whereas most of the Ge islands are formed on the top terrace when the patterned stripes are covered by a strained GeSi buffer layer. Reducing the growth temperature to 600oC results in a nucleation of Ge islands both on the top terrace and at the sidewall of pure Si stripes. A qualitative analysis, based on the growth kinetics, demonstrates that the step structure of the stripes, the external strain field and the local critical wetting layer thickness for the islands formation contribute to the preferential positioning of Ge islands on the stripes.

cond-mat.mtrl-sci↗
arXiv2002

Interaction Effects in Conductivity of Si Inversion Layers at Intermediate Temperatures

We compare the temperature dependence of resistivity ρ(T) of Si MOSFETs with the recent theory by Zala et al. This comparison does not involve any fitting parameters: the effective mass m* and g*-factor for mobile electrons have been found independently. An anomalous increase of ρwith temperature, which has been considered a signature of the "metallic" state, can be described quantitatively by the interaction effects in the ballistic regime. The in-plane magnetoresistance ρ(B) is qualitatively consistent with the theory; however, the lack of quantitative agreement indicates that the magnetoresistance is more susceptible to the sample-specific effects than ρ(T).

cond-mat.str-el↗
arXiv2001

The low-density spin susceptibility and effective mass of mobile electrons in Si inversion layers

We studied the Shubnikov-de Haas (SdH) oscillations in high-mobility Si-MOS samples over a wide range of carrier densities $n\simeq (1-50) \times 10^{11}$cm$^{-2}$, which includes the vicinity of the apparent metal-insulator transition in two dimensions (2D MIT). Using a novel technique of measuring the SdH oscillations in superimposed and independently controlled parallel and perpendicular magnetic fields, we determined the spin susceptibility $χ^*$, the effective mass $m^*$, and the $g^*$-factor for mobile electrons. These quantities increase gradually with decreasing density; near the 2D MIT, we observed enhancement of $χ^*$ by a factor of $\sim 4.7$.

cond-mat.mes-hall↗
arXiv2001

Exclusion of quantum coherence as the origin of the 2D metallic state in high-mobility silicon inversion layers

The temperature and density dependence of the phase coherence time $τ_ϕ$ in high-mobility silicon inversion layers was determined from the magnetoresistivity due to weak localization. The upper temperature limit for single-electron quantum interference effects was delineated by comparing $τ_ϕ$ with the momentum relaxation time $τ$. A comparison between the density dependence of the borders for quantum interference effects and the strong resistivity drop reveals that theses effects are not related to each other. As the strong resistivity drop occurs in the Drude regime, the apparent metallic behavior can not be caused by quantum coherent effects.

cond-mat.mes-hall↗
arXiv2001

Weak anisotropy and disorder dependence of the in-plane magnetoresistance in high mobility (100) Si-inversion layers

We report studies of the magnetoresistance (MR) in a two-dimensional electron system in (100) Si-inversion layers, for perpendicular and parallel orientations of the current with respect to the magnetic field in the 2D-plane. The magnetoresistance is almost isotropic; this result does not support the suggestion of the orbital origin of the MR in Si-inversion layer. In the hopping regime, however, the MR contains a weak anisotropic component that is non-monotonic in magnetic field. We found that the field, at which the MR saturates, for different samples varies by a factor of two, being lower or higher than the field of complete spin polarization of free carriers. Therefore, the saturation of the MR can not be identified with the spin polarization of free carriers.

cond-mat.str-el↗
arXiv2001

Effect of the In-Plane Magnetic Field on Conduction of the Si-inversion Layer: Magnetic Field Driven Disorder

We compare the effects of temperature, disorder and parallel magnetic field on the metallic-like temperature dependence of the resistivity. We found a similarity between the effects of disorder and parallel field: the parallel field weakens the metallic-like conduction in high mobility samples and make it similar to that for low-mobility samples. We found a smooth continuous effect of the in-plane field on conduction, without any threshold. While conduction remains non-activated, the parallel magnetic field restores the same resistivity value as the high temperature does. This matching sets substantial constraints on the choice of the theoretical models developed to explain the mechanism of the metallic conduction and parallel field magnetoresistance in 2D carrier systems. We demonstrate that the data for magneto- and temperature dependence of the resistivity of Si-MOS samples in parallel field may be well described by a simple model of the magnetic field dependent disorder.

cond-mat.str-el↗
arXiv2001

Valley Splitting in Si-Inversion Layers at Low Magnetic Fields

We report novel manifestation of the valley splitting for the two valley electron system in (100) Si-inversion layers at low carrier density. We found that valley splitting causes almost 100% modulation of the Shubnikov de Haas oscillations in very low magnetic fields, almost on the bound of the quantum interference peak of the negative magnetoresistance. From the interference pattern of oscillations we determined the valley splitting in the B=0 limit which appears to vary only within a factor of 1.3 over the density range (3-7)x10^{11}/cm^2. We found also that level broadenings in both electron valleys differ only by < 3%. The latter result shows that the inter-valley scattering is not responsible for the strong (six fold) `metallic-like' changes of the resistivity with temperature.

cond-mat.str-el↗
arXiv2000

Weak-localization type description of conduction in the "anomalous" metallic state

This paper is devoted to the temperature dependence of the resistivity in Si- MOS samples over the wide range of densities in the ``metallic phase'' (n>n_c) but not too close to the critical density n_c. Three domains of different behavior in ρ(T) are identified. These are: [i] quantum domain of `low-temperatures', where a logarithmic T-dependence of ρ(with $dρ/dT<0$) dominates; [ii] semi-classical domain of `high-temperatures', in which Drude resistivity strongly varies with T (with dρ/dT>0); and [ii] crossover between the former two, where a linear T-dependence dominates (with dρ/dT>0). In the crossover regime and at higher densities (n>20x10^{11}/cm^2), ρ(T) goes through a minimum at temperature T_{min}. Both the absolute value of T_{min} and its dependence on density are found to be in an agreement with the conventional weak-localization theory. For n smaller than \sim 20x10^{11}/cm^2, the theoretical estimate for T_{min} falls outside the experimentally accessible temperature range. This explains the absence of the minimum at these densities in the data. In total, over the two decades in the temperature (domains [ii] and [iii]), the two semiclassical effects mimic the metallic like transport properties. Our analysis shows that the behaviour of ρ(T) in the region of ρ<< h/e^2 can be described phenomenologically in terms of the conventional weak-localization theory.

cond-mat.str-el↗
arXiv1999

Weak localization in the 2D metallic regime of Si-MOS

The negative magnetoresistance due to weak localization is investigated in the two-dimensional metallic state of Si-MOS structures for high conductance values between 35 and 120 e^2/h. The extracted phase coherence time is equal to the momentum relaxation time at 10 K but nearly 100 times longer at the lowest temperature. Nevertheless, only weak logarithmic corrections to the conductivity are present in the investigated temperature and concentration range thus proving the absence of strong quantum effects due to electron-electron interaction. From saturation effects of the phase coherence time a lower boundary for spin-orbit scattering of about 200 ps is estimated.

cond-mat.mes-hall↗
arXiv1999

CDF B spectroscopy results: B^** and B_c

We report on two spectroscopy results from CDF. First, we observe the orbitally excited $B^{**}$ mesons in $B \to \ell D^{(*)}X$ events. We find $28 \pm 6 \pm 3$% of light $B$ mesons produced are $B^{**}$ states. A collective mass fit results in a $B_1$ mass of $5.71 \pm 0.02$ GeV/$c^2$. Secondly, we observe $20.4^{+6.2}_{-5.5}$ decays of $B^+_c \to J/ψ\ell^+X$, with a $ 6.40 \pm 0.39 \pm 0.13 {\rm GeV}/c^2$ mass and $ 0.46 ^{+0.18}_{-0.16} \pm 0.03 $ps lifetime. The production rate is in reasonable accordance with expectations.

hep-ex↗
arXiv1999

Time-dependent Effects in the Metallic Phase in Si-MOS: Evidence for Non-Diffusive Transport

We have found that the conduction in Si-MOS structures has a substantial imaginary component in the metallic phase for the density range 6 \times n_c > n > n_c, where n_c is the critical density of the metal-insulator transition. For high mobility samples, the corresponding delay (or advance) time equals approximately to 0.1 - 10ms and increases exponentially as density and temperature decrease. In very low mobility samples, at temperature of 0.3K, the time-lag in establishing the equilibrium resistance reaches hundreds of seconds. The delay (advance) times are approximately 10^2-10^8 times larger than the overall RC-time of the gated structure. These results give evidence for a non-Boltzmann character of the transport in the low-density metallic phase. We relate the time-dependent effects to tunneling of carries between the 2D bulk and localized states.

cond-mat.str-el↗
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↗