arXiv subjects

R. Baumbach

Publications and source records attributed to R. Baumbach.

5 recordsLinked to original sources

arXiv2025

Epitaxial Stabilization and Emergent Charge Order in Copper Selenide Thin Films

We demonstrate epitaxial growth of copper selenide (Cu$_{2-x}$Se) thin films in both cubic and rhombohedral phases, achieved via molecular beam epitaxy on Al$_2$O$_3$ (001) substrates. Remarkably, the high-temperature cubic phase -- which in bulk transforms into the rhombohedral structure below 400 K -- is stabilized at room temperature and below, well outside its bulk equilibrium stability range. In the cubic phase films, temperature-dependent electrical transport reveals a pronounced, hysteretic resistivity peak near 140 K, accompanied by unit cell doubling along the [111] direction, as observed by x-ray diffraction, which are hallmarks of a charge density wave (CDW) transition. First-principles calculations show strong Fermi surface nesting in the cubic phase, consistent with the observed CDW instability. In contrast, the rhombohedral films exhibit suppressed nesting and no structural modulation. These results not only unambiguously identify a previously unreported CDW in Cu$_{2-x}$Se thin films, but also establish an epitaxial platform for tuning emergent electronic phases via strain and interface engineering.

cond-mat.mtrl-sci↗

arXiv2021

Collinear antiferromagnetic order in URu$_2$Si$_{2-x}$P$_x$ revealed by neutron diffraction

The hidden order phase in URu$_2$Si$_2$ is highly sensitive to electronic doping. A special interest in silicon-to-phosphorus substitution is due to the fact that it may allow one, in part, to isolate the effects of tuning the chemical potential from the complexity of the correlated $f$ and $d$ electronic states. We investigate the new antiferromagnetic phase that is induced in URu$_2$Si$_{2-x}$P$_x$ at $x\gtrsim0.27$. Time-of-flight neutron diffraction of a single crystal ($x=0.28$) reveals $c$-axis collinear $\mathbf{q}_\mathrm{m}=(\frac12,\frac12,\frac12)$ magnetic structure with localized magnetic moments ($\approx2.1\,μ_\mathrm{B}$). This points to an unexpected analogy between the (Si,P) and (Ru,Rh) substitution series. Through further comparisons with other tuning studies of URu$_2$Si$_2$, we are able to delineate the mechanisms by which silicon-to-phosphorus substitution affects the system. In particular, both the localization of itinerant 5$f$ electrons as well as the choice of $\mathbf{q}_m$ appears to be consequences of the increase in chemical potential. Further, enhanced exchange interactions are induced by chemical pressure and lead to magnetic order, in which an increase in inter-layer spacing may play a special role.

cond-mat.str-el↗

arXiv2020

Layer- and Gate-tunable Spin-Orbit Coupling in a High Mobility Few-Layer Semiconductor

Spin-orbit coupling (SOC) is a relativistic effect, where an electron moving in an electric field experiences an effective magnetic field in its rest frame. In crystals without inversion symmetry, it lifts the spin degeneracy and leads to many magnetic, spintronic and topological phenomena and applications. In bulk materials, SOC strength is a constant that cannot be modified. Here we demonstrate SOC and intrinsic spin-splitting in atomically thin InSe, which can be modified over an unprecedentedly large range. From quantum oscillations, we establish that the SOC parameter αis thickness-dependent; it can be continuously modulated over a wide range by an out-of-plane electric field, achieving intrinsic spin splitting tunable between 0 and 20 meV. Surprisingly, αcould be enhanced by an order of magnitude in some devices, suggesting that SOC can be further manipulated. Our work highlights the extraordinary tunability of SOC in 2D materials, which can be harnessed for in operando spintronic and topological devices and applications.

cond-mat.mes-hall↗

arXiv2019

Measurement of Two Low-Temperature Energy Gaps in the Electronic Structure of Antiferromagnetic USb2 Using Ultrafast Optical Spectroscopy

Ultrafast optical spectroscopy is used to study the antiferromagnetic f-electron system USb2. We observe the opening of two charge gaps at low temperatures (<45 K), arising from renormalization of the electronic structure. Analysis of our data indicates that one gap is due to hybridization between localized f-electron and conduction electron bands, while band renormalization involving magnons leads to the emergence of the second gap. These experiments thus enable us to shed light on the complex electronic structure emerging at the Fermi surface in f-electron systems.

cond-mat.str-el↗

arXiv2011

Andreev reflection spectroscopy on the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$: Microscopic evidence for multiple superconducting order parameters

Point-contact Andreev reflection spectroscopy was performed on single crystals of the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$, down to 90mK and up to 3Tesla. The conductance spectra showed multiple structures, including zero-bias peaks and spectral dips, which are interpreted as signatures of multiple pairing gaps, one clearly having nodes. Samples with 2% Ru replacing Os were also measured, showing pronounced spectral humps consistent with the emergence of a non-nodal gap. Detailed analysis of the spectral evolution revealed a magnetic field-\emph{vs.}-temperature phase diagram for PrOs$_{4}$Sb$_{12}$ characterized by two distinct superconducting order parameters.

cond-mat.supr-con↗