arXiv subjects

S. F. Fischer

Publications and source records attributed to S. F. Fischer.

8 recordsLinked to original sources

arXiv2026

Quantum oscillations and linear magnetoresistance in ultraclean CaVO$_3$ thin films

Advances in epitaxy of transition metal oxides with perovskite structure allow novel insights into transport mechanisms of strongly correlated electron systems, which are of interest for future transparent electronics. In this study, we investigate magnetotransport properties of thin epitaxial CaVO$_3$ films, grown coherently strained on LaAlO$_3$, and demonstrate for the ultraclean limit quantum oscillations. Fermi liquid behavior is detected in the temperature-dependent resistivity $\rho(T) \sim T^2$ up to 20 K, with effective mean free paths exceeding up to 20 times the film thickness (38 nm). Shubnikov-de Haas oscillations and a non-linear Hall resistance reveal two electron-like (1 and 2) and one hole-like (h) carriers, reflecting the three-fold Fermi surface of orthorhombic CaVO$_3$, with effective charge carrier densities and mobilities at 4.2 K of: (1) $n_1 \approx 9.3 \cdot 10^{21}{}cm^{-3}$ with low mobility $\mu_1 \approx 926{}cm^{2}V^{-1}s^{-1}$, (2) $n_2 \approx 7.2 \cdot 10^{19}{}cm^{-3}$ with high mobility $\mu_2 \approx 6600 {}cm^{2}V^{-1}s^{-1}$, and (h) $n_h \approx 2.2 \cdot 10^{18}{}cm^{-3}$ with $\mu_h \approx 1500{}cm^{2}V^{-1}s^{-1}$. A non-saturating linear magnetoresistance dominates at low temperatures, exceeding the value for single crystals by 30$\%$. Our findings on epitaxial films demonstrate the delicate interplay of multiple carriers with correlations stemming from a non-spherical nested Fermi surface of a perovskite structure with orthorhombic distortion.

cond-mat.str-el↗

arXiv2025

Enhanced phonon-drag by nanoscale design of homoepitaxial \hbox{$\beta$-Ga$_2$O$_3$}

Phonon drag may be harnessed for thermoelectric generators and devices. Here, we demonstrate the geometric control of the phonon-drag contribution to the thermopower. In nanometer-thin electrically conducting $\beta$-Ga$_2$O$_3$ films homoepitaxially-grown on insulating substrates it is enhanced from -0,4 mV/K to up to -3 mV/K at 100 K by choice of the film thickness. Analysis of the temperature-dependent Seebeck coefficients reveal that a crossover from three-dimensional to quasi-two-dimensional electron-phonon interaction occurs for film thicknesses below 75~nm. The ratio of phonon-phonon to electron-phonon relaxation times in these confined structures is $10$ times larger than that of bulk. Generally the phonon drag can be tuned depending on the relations between the phonon-drag interaction length $\lambda_\text{PD}$, the phonon mean free path $\lambda$ and the film thickness $d$. Phonon drag can be enhanced for $\lambda_\text{PD}\gg\lambda>d$.

cond-mat.mes-hall↗

arXiv2024

Resistive and ballistic phonon transport in $\beta$-Ga$_2$O$_3$

The anisotropic thermal conductivity and the phonon mean free path (mfp) in monoclinic $\beta$-Ga$_2$O$_3$ single crystals and homoepitaxial films of several $\mu$m were determined using the 3$\omega$-method in the temperature range from 10K-300 K. The measured effective thermal conductivity of both, single crystal and homoepitaxial films are in the order of 20 W/(mK) at room temperature, below 30 K it increases with a maximum of 1000 to 2000 W/(mK) and decreases with T$^3$ below 25 K. Analysis of the phonon mfp shows a dominance of phonon-phonon-Umklapp scattering above 80 K, below which the influence of point-defect scattering is observed. Below 30 K the phonon mfp increases until it is limited by the total $\beta$-Ga$_2$O$_3$ sample size. A crossover from resistive to ballistic phonon transport is observed below 20 K and boundary effects of the total sample size become dominant. This reveals that the homoepitaxial film-substrate interface is highly phonon-transparent. The resistive and ballistic phonon transport regimes in $\beta$-Ga$_2$O$_3$ are discussed corresponding to the models of Callaway and Majumdar, respectively.

cond-mat.mtrl-sci↗

arXiv2020

Semimetal to semiconductor transition in $\text{Bi}/\text{TiO}_{2}$ core/shell nanowires

We demonstrate the full thermoelectric and structural characterization of individual bismuth-based (Bi-based) core/shell nanowires. The influence of strain on the temperature dependence of the electrical conductivity, the absolute Seebeck coefficient and the thermal conductivity of bismuth/titanium dioxide ($\text{Bi}/\text{TiO}_{2}$) nanowires with different diameters is investigated and compared to bismuth (Bi) and bismuth/tellurium (Bi/Te) nanowires and bismuth bulk. Scattering at surfaces, crystal defects and interfaces between the core and the shell reduces the electrical conductivity to less than $5\,\%$ and the thermal conductivity to less than $25\,\%$ to $50\,\%$ of the bulk value at room temperature. On behalf of a compressive strain, $\text{Bi}/\text{TiO}_{2}$ core/shell nanowires show a decreasing electrical conductivity with decreasing temperature opposed to that of Bi and Bi/Te nanowires. We find that the compressive strain induced by the $\text{TiO}_{2}$ shell can lead to a band opening of bismuth increasing the absolute Seebeck coefficient by $10\,\%$ to $30\,\%$ compared to bulk at room temperature. In the semiconducting state, the activation energy is determined to $\left|41.3\pm0.2\right|\,\text{meV}$. We show that if the strain exceeds the elastic limit the semimetallic state is recovered due to the lattice relaxation.

physics.app-ph↗

arXiv2019

Absolute Seebeck coefficient of thin platinum films

The influence of size effects on the thermoelectric properties of thin platinum films is investigated and compared to the bulk. Structural properties, like the film thickness and the grain size, are varied. We correlate the electron mean free path with the temperature dependence of the electrical conductivity and the absolute Seebeck coefficient $S_{\text{Pt}}$ of platinum. We use a measurement platform as a standardized method to determine $S_{\text{Pt}}$ and show that $S_{\text{Pt,film}}$ is reduced compared to $S_{\text{Pt,bulk}}$. Boundary and surface scattering reduce the thermodiffusion and the phonon drag contribution to $S_{\text{Pt,film}}$ by nearly the same factor. A detailed discussion and a model to describe the temperature dependence of the absolute Seebeck coefficient and the influence of size effects of electron-phonon and phonon-phonon interaction on $S_{\text{Pt}}$ is given.

physics.app-ph↗

arXiv2008

Non-local Aharonov-Bohm conductance oscillations in an asymmetric quan-tum ring

We investigate ballistic transport and quantum interference in a nanoscale quantum wire loop fab-ricated as a GaAs/AlGaAs field-effect heterostructure. Four-terminal measurements of current and voltage characteristics as a function of top gate voltages show negative bend resistance as a clear signature of ballistic transport. In perpendicular magnetic fields phase-coherent transport leads to Aharonov-Bohm (AB) conductance oscillations which show equal amplitudes in the local and the non-local measurement at a temperature of 1.5 K and above. We attribute this novel observation to the symmetry of the orthogonal cross junctions connecting the four quantum wire leads with the asymmetric quantum wire ring.

cond-mat.mes-hall↗

arXiv2004

Optimization of nanostructured permalloy electrodes for a lateral hybrid spin-valve structure

Ferromagnetic electrodes of a lateral semiconductor-based spin-valve structure are designed to provide a maximum of spin-polarized injection current. A single-domain state in remanence is a prerequisite obtained by nanostructuring Permalloy thin film electrodes. Three regimes of aspect ratios $m$ are identified by room temperature magnetic force microscopy: (i) high-aspect ratios of $m \ge 20$ provide the favored remanent single-domain magnetization states, (ii) medium-aspect ratios $m \sim 3$ to $m \sim 20$ yield highly remanent states with closure domains and (iii) low-aspect ratios of $m \le 3$ lead to multi-domain structures. Lateral kinks, introduced to bridge the gap between micro- and macroscale, disturb the uniform magnetization of electrodes with high- and medium-aspect ratios. However, vertical flanks help to maintain a uniformly magnetized state at the ferromagnet-semiconcuctor contact by domain wall pinning.

cond-mat.other↗

arXiv1997

Charge separation in photosynthesis via a spin exchange coupling mechanism

A new mechanism for the primary photoinduced charge separation in photosynthesis is proposed. It involves as real intermediate between the excited special pair state P* and the primary charge separated state P+HL- a trip-trip-singlet PTBLT, which consists of a triplet on the dimer P and a further triplet on the monomer BL. Both combine to a singlet. The electron transfer is caused by spin exchange couplings. The transient spectrum of the short lived intermediate, formerly taken as evidence for the charge transfer state P+BL-, is reinterpreted as a transient excitation of this trip-trip singlet.

cond-mat.soft↗