arXiv subjects

Fabian O. von Rohr

Publications and source records attributed to Fabian O. von Rohr.

At least 19 recordsLinked to original sources

arXiv2026

Quasi-one-dimensional topological band structure and van Hove singularities in monolayer TaIrTe$_4$ from laser $μ$-ARPES

Recent transport experiments reported a quantum spin Hall insulator phase in monolayer 1T-TaIrTe$_4$ gated away from charge neutrality. This phase is not predicted by band structure calculations and has been attributed to an electronic instability induced by strong correlations at a putative van Hove singularity. Here, we investigate the electronic structure of exfoliated monolayer 1T-TaIrTe$_4$ using micro-focus laser angle resolved photoemission. We find a strongly anisotropic band structure susceptible to density wave instabilities. Our data further reveal a saddle point singularity in the density of states. However, we find that the saddle point lies at a density for which transport experiments found no anomalies. Moreover, the quasiparticle line widths suggest weak electron correlations only. This points to a secondary role of van Hove singularities and electron correlations in the transport phase diagram of monolayer 1T-TaIrTe$_4$.

cond-mat.mtrl-sci↗

arXiv2026

Structure, Composition, and High-Field Superconductivity in Metal-Rich $\mathrmη$-Carbide-Type Compounds

$\mathrmη$-Carbide-type compounds have recently emerged as a diverse class of materials in the study of superconductivity. These phases contribute to a growing family of metal-rich quantum materials that exhibit unusual superconducting properties emerging from complex metallic bonding. Several members of the $\mathrmη$-carbide-type phases have been found to be bulk superconductors -- such as Nb$_4$Rh$_2$C$_{1-δ}$, Ta$_4$Rh$_2$C$_{1-δ}$, Ti$_4$Ir$_2$O$_{1-δ}$, and Ti$_4$Co$_2$O$_{1-δ}$ -- with transition temperatures up to $T_{\rm c} \approx$ 10 K and upper critical fields as high as $μ_0 H_{\rm c2}(0) \approx$ 30 T. Whereas the transition temperatures may fall within the range typical for intermetallic superconductors, the pronounced violation of the weak-coupling Pauli limit in many of these crystallographically high-symmetry materials is noteworthy. Here, we review recent progress on superconducting $\mathrmη$-carbide-type phases, emphasizing how crystal symmetry, synthetic challenges, transition-metal composition, and electronic structure govern their superconducting properties. Furthermore, we outline open questions and future directions, including the possible discovery of new $\mathrmη$-carbide-type materials.

cond-mat.supr-con↗

arXiv2026

Ultrafast dynamics of excitons in black phosphorus

Excitons are key quasiparticles determining the optical properties of solids. As such, they can be utilized to coherently control the electronic structure of materials using optical femtosecond pulses. Identifying the decoherence mechanism during the early non-equilibrium dynamics is crucial to achieve light-induced band-structure engineering in semiconductors. Here, we generate excitons in the direct band gap semiconductor black phosphorus with a resonant mid-infrared photoexcitation. Using time- and angle-resolved photoemission spectroscopy, we track their complex ultrafast dynamics on the few-picosecond time scale. We develop a quantum-kinetic theoretical framework to model the decoherence of excitons into dark excitons via phonon scattering. By combining simulation and experiment, we quantify key parameters describing the early dynamics of the excitons. Our work highlights phonon-mediated intravalley scattering as a fundamental limitation for coherent exciton phenomena in single-valley semiconductors.

cond-mat.mtrl-sci↗

arXiv2026

In-Plane Ferromagnetism and Critical Dynamics in Alkali-Deficient K$_{1-x}$CrTe$_2$ (with $x \approx$ 0.3) Single Crystals

Layered chromium tellurides are model systems for studying low-dimensional magnetism in van der Waals materials. We report the synthesis and characterization of K$_{1-x}$CrTe$_2$ single crystals ($x \approx 0.3$), which crystallize in the $Cm$ space group with trigonal prismatic K$^+$ coordination, unlike the octahedral environments of more stoichiometric ACrX$_2$ compounds. Magnetization measurements show a sharp ferromagnetic transition at $T_{\rm C}=117$ K and in-plane magnetic anisotropy, supported by density functional theory. Neutron diffraction reveals ferromagnetic alignment of Cr spins within and between layers. This contrasts with the out-of-plane A-type antiferromagnetism in LiCrTe$_2$ and NaCrTe$_2$, but resembles CrTe$_2$. These differences likely arise from changes in interlayer spacing, Cr oxidation state, or stacking. AC susceptibility and $μ$SR indicate short-range order above $T_{\rm C}$ and dynamic behavior below. Overall, K$_{1-x}$CrTe$_2$ provides a tunable platform for studying spin orientation and dimensionality in two-dimensional magnets.

cond-mat.mtrl-sci↗

arXiv2026

Metastable Cu$_{1-x}$CrTe$_2$ -- Completing the copper chromium delafossite series through soft chemistry

In the layered copper chromium dichalcogenide series CuCr$X_2$, the oxide, the sulfide, and the selenide analogues have been reported, but the telluride CuCrTe$_2$ has remained unsynthesized so far. Here, we report the synthesis of Cu$_{1-x}$CrTe$_2$ ($x \approx 0.3$), which forms only within a narrow temperature window near 90 °C by solvothermal cation exchange of K$_{1-x}$CrTe$_2$ ($x \approx 0.3$) and CuBr. Cu$_{1-x}$CrTe$_2$ undergoes a magnetostructural transition to an antiferromagnetic state at $T_\mathrm{N}=239$ K, a Néel temperature that is high relative to other $A$CrTe$_2$ phases and comparable to that of ferromagnetic, fully-deintercalated CrTe$_2$. Cu$_{1-x}$CrTe$_2$ is metastable and decomposes at temperatures as low as 250 °C to form spinel CuCr$_2$Te$_4$. These results highlight the importance of low-temperature topochemical routes for accessing metastable Cu(I)-containing tellurides that are inaccessible by conventional solid-state synthesis.

cond-mat.mtrl-sci↗

arXiv2026

Reversible nanoscale patterning of WTe$_2$ with a scanning tunneling microscope

Manipulating the lattice structure of ferroelectric quantum materials enables their use in low-power electronic devices, including field-effect transistors. WTe$_2$ is a Weyl-semimetal candidate and ferroelectric, both properties arising from the reduced crystal symmetry of its T$_\mathrm{d}$ ground state. The T$_\mathrm{d}$ crystal phase results from a Peierls distortion of the 1T parent structure and an interlayer shift. While experiments in WTe$_2$ have established ferroelectric switching and transient control of the predicted topological phase via ultrafast excitations, persistent electronic changes on the nanometer scale remain elusive. Here, we demonstrate that current pulses applied via scanning tunneling microscopy can both write and erase persistent nanometer-scale patterns on the surface of WTe$_2$. These patterns consist of apparent picometer in-plane and out-of-plane atomic displacements, accompanied by changes to the local density of states. The out-of-plane displacements further modulate the Peierls-like distortion present in WTe$_2$, while the in-plane displacements are indicative of ferroelectric switching. The induced patterns can be repositioned and erased, suggesting a nanoscale handle on the ferroelectric properties of WTe$_2$.

cond-mat.mtrl-sci↗

arXiv2026

Two distinct superconducting regimes in Ti4Co2O under pressures

We report on the pressure dependence of superconducting transition temperature Tc and upper critical field Bc2(0) through electrical transport of the Ti4Co2O superconductor (eg.,the superconducting transition temperature Tc = 2.5 K and the Bc2(0)=7.2T=2.9Tc). We find that the Tc exhibits non-monotonic pressure dependence:it rises monotonically at first with a pressure coefficient of dTc/dP=0.034 K/GPa, but rapidly decreases around 10-20 GPa, and then increases with the dTc/dP = 0.023 K/GPa, up to= 4.31 K at 69.7 GPa. Concurrently, the Bc2(0)exhibits a dome shaped pressure dependence, with its maximum at 5 GPa of almost twice the value at ambient pressure, exceeding the weak-coupling Pauli paramagnetic limit Bp throughout the whole pressure range. By comparing the normal-state and superconducting properties, we identify two distinct superconducting regimes, with a low-pressure superconducting phase characterized by an enhanced Bc2(0)values and Fermi-liquid normal-state electrical transport (the exponent n = 2), and a high-pressure superconducting phase with a monotonically increased Tc and an enhancement in phonon scatterings (the exponent n = 4). Room-temperature synchrotron X-ray diffraction indicates that there is no structural transition up to 55.8 GPa, which gives a relatively large bulk modulus of 192 GPa in comparison with other alloy superconductors. First-principles calculations suggest that the nonmonotonic Tc maybe closely related to the evolution of the density of states of Ti4Co2O upon compression, which is different from those of isostructural superconductors Ti4Ir2O and Nb4Rh2C. Our results show that even in the Ti4Co2O with weak spin-orbit coupling, superconductivity remains highly sensitive to the external stimuli such as pressure.

cond-mat.supr-con↗

arXiv2026

Resolving growth-induced off-stoichiometry in AgCrSe$_2$ single crystals

The layered delafossite-like antiferromagnet AgCrSe$_2$ is a superionic conductor at high temperatures and has been reported to exhibit anomalous Hall behavior and Kondo physics at low temperatures. These extraordinary transport properties have been established almost exclusively on single crystals grown by chemical vapor transport, raising questions about the role of growth-induced off-stoichiometry. Using elemental analysis, single-crystal X-ray diffraction, and magnetization measurements, we show that such crystals are indeed systematically off-stoichiometric, with a general composition of Ag$_{1-x}$Cr(Se$_{2-y}$Cl$_y$) ($x \approx y \approx 0.08$) arising from the use of CrCl$_3$ as a transport agent. This off-stoichiometry manifests in altered magnetic properties, most notably a suppressed Néel temperature of 46\,K compared to 58\,K in stoichiometric polycrystalline samples prepared by solid-state synthesis. By optimizing an Ag/Se self-flux growth method, we obtained large single crystals of AgCrSe$_2$ that recover the magnetic transition temperature and saturation field of stoichiometric powder samples. These results establish self-flux growth as a route to high-quality stoichiometric AgCrSe$_2$ single crystals and provide a reliable platform for reassessing whether the reported anomalous transport phenomena are intrinsic or arise from off-stoichiometry.

cond-mat.mtrl-sci↗

arXiv2026

Ladder-like Structural Architecture of Layered Magnetic $A_{2.4}$Cr$_8$Te$_{14}$ ($A$ = Rb, Cs) Compounds by Self-flux Synthesis

The discovery and control of intergrowth structures represent an important avenue for the targeted synthesis of new, more complex structure types. When including magnetic framework metal atoms, this enhanced complexity can transfer to rich magnetic ground states. Here, we show that the subtle adjustment of the composition of alkali-tellurium fluxes enables the synthesis of a new family of alkali chromium tellurides, $A_{2.4}$Cr$_8$Te$_{14}$ ($A$ = Rb, Cs). Their ladder-like crystal structures integrate the two-dimensional character of delafossite-like $A$CrTe$_2$ with the tunnel motifs of hollandite-like $A_{x}$Cr$_5$Te$_8$ phases. This results in a previously unobserved unique hybrid framework. Direction-dependent magnetization measurements on oriented single crystals reveal distinct magnetic ground states: Rb$_{2.4}$Cr$_8$Te$_{14}$ is antiferromagnetic with $T_{\rm N}$ = 114.5 K, while Cs$_{2.4}$Cr$_8$Te$_{14}$ is ferrimagnetic with $T_{\rm C}$ = 125.0 K. This work underscores the simplicity and effectiveness of flux growth as a design strategy for discovering low-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2026

Type-II superconductivity in the Dirac semimetal PdTe2

We report on the microscopic superconducting properties of the Dirac semimetal PdTe2. In this study, we have focused on mosaic crystals of PdTe2, and used detailed zero field and transverse field muon spin relaxation/rotation ($μ$SR), ac-magnetic susceptibility, and resistivity measurements to investigate their superconducting properties. The magnetic susceptibility measurements reveal two superconducting transition temperatures at 1.8 and 1.6~K, respectively, in agreement with earlier reports. In contrary to these reports, we find that these mosaic PdTe2 crystals, are not type-I, but rather type-II superconductors. In fact, we observe the clear manifestation of a flux line lattice through a clear diamagnetic shift and Gaussian broadening of the Fourier spectra in the superconducting state. This behavior is likely caused by the disorder in the mosaic crystals of PdTe2 studied here. Our analysis of the superconducting order parameter by the means of temperature dependent magnetic penetration depth $λ(T)$ reveals a fully gapped superconducting state that can be well-fitted using an s-wave symmetric gap. We find that PdTe2 is a promising model system for the investigation and interplay of non-trivial topology, surface superconductivity, and type-II bulk superconductivity in a van-der-Waals material. Moreover, our results indicate that the superconductivity in this material can be easily modified from type-I to type-II by disorder in the system.

cond-mat.supr-con↗

arXiv2026

Growth of Large Crystals of Janus Phase RhSeCl Using Self-Selecting Vapour Growth

In recent years, interest in 2D Janus materials has grown exponentially, particularly with regard to their applications in spintronics and optoelectronic devices. The defining feature of Janus materials is the ordered arrangement of different layer terminations - creating chemically distinct surfaces and an inherent out-of-plane polarity. Among the few known Janus materials, RhSeCl is particularly intriguing as a rare example of an intrinsic Janus compound. Owing to its exceptional chemical stability, RhSeCl offers a promising platform for exploring the physics related to the Janus-structure. However, synthesising large, high-quality crystals of this compound remains a significant challenge. Here, we report a novel synthetic pathway for growing crystals up to 6 mm in lateral size via a two-step self-selecting vapour growth reaction. We further present a comprehensive comparison of newly developed synthesis routes with all previously reported methods for RhSeCl. During these investigations, we identified a previously unreported impurity that forms in specific growth pathways and demonstrate how it can be avoided to obtain phase-pure few- and monolayer flakes. We showcase the reproducibility of the process to obtain high-quality, large single-crystals and flakes.

cond-mat.mtrl-sci↗

arXiv2025

Suppression of stripe-ordered structural phases in monolayer IrTe$_2$ by a gold substrate

Metal-assisted exfoliation of two-dimensional (2D) materials has emerged as an efficient route to isolating large-area monolayer crystals, yet the influence of the supporting metal substrate on their intrinsic properties remains poorly understood. Here, we demonstrate successful gold-assisted exfoliation of monolayer IrTe$_2$ up to the millimeter scale. Angle-resolved photoemission spectroscopy (ARPES), combined with first-principles calculations, reveals that the low-energy electronic structure closely resembles that of a freestanding monolayer 1T-IrTe$_2$. We find that quasi-covalent hybridization together with substrate-induced strain leads to only modest modifications of the electronic bands. Although strain contributes to phase stability, it is essentially hybridization that drives the stabilization of the 1T-phase of the monolayer IrTe$_2$ by suppressing stripe-ordered phase transitions. These results establish gold-assisted exfoliation as a robust route to prepare a large-area monolayer IrTe$_2$ and highlight the role of metal-substrate interaction in engineering 2D materials with tailored structural phases.

cond-mat.mtrl-sci↗

arXiv2025

Stoichiometry and Phase Control in K$_{1-x}$CrSe$_2$ via Self-Flux Synthesis

Layered delafossite-type magnetic materials, such as KCrSe$_2$, are promising platforms for studying magnetic systems and potential frustration on triangular lattices. Synthesis, structure-type control, and off-stoichiometries remain major challenges in the investigation of these delafossite-type magnets. Starting from the same self-flux composition (K:Cr:Se = 8:1:8), we isolated three distinct K$_{1-x}$CrSe$_2$ phases with $x$ = 0, 0.13--0.17, and 0.32--0.35, each adopting a different structure type depending on the quenching temperature applied. The phase evolution indicates a sequence of transformations during synthesis between compounds with varying degrees of potassium deficiency. Building on these insights into phase stability and crystal growth, we successfully grew single crystals of full-stoichiometric KCrSe$_2$ -- enabling direction-dependent magnetization measurements. These measurements reveal a pronounced field dependence of the Néel temperature at low external fields, as well as a weak metamagnetic transition. Our findings demonstrate that even a simple parameter -- such as quenching temperature -- can be used to control stoichiometry, direct phase formation, and ultimately tune the magnetic properties of delafossite-type materials.

cond-mat.mtrl-sci↗

arXiv2025

Theory of unconventional magnetism in a Cu-based kagome metal

Kagome metals have established a new arena for correlated electron physics. To date, the predominant experimental evidence centers around unconventional charge order, nematicity, and superconductivity, while magnetic fluctuations due to electronic interactions, i.e., beyond local atomic magnetism, have largely been elusive. We find the challenge of locating the appropriate parameter regime for such exotic order to center around two aspects. First, the correlations implied by low-energy orbitals have to be sufficiently large to yield a dominance of magnetic fluctuations and weak to retain an itinerant parent state. Second, the kinematic kagome profile at the Fermi level demands an efficient mitigation of sublattice interference causing the suppression of magnetic fluctuations descending from electronic on-site repulsion. We elucidate our methodology by analyzing the potential copper-based kagome compound CsCu$_3$Cl$_5$: From ab initio design and many-body analysis, we develop a model framework of realistic Cu-based kagome materials the simulations of which reveal unconventional magnetic order in a kagome metal.

cond-mat.str-el↗

arXiv2025

Charting the landscape of Bardeen-Cooper-Schrieffer superconductors in experimentally known compounds

We perform a high-throughput computational search for novel phonon-mediated superconductors, starting from the Materials Cloud 3-dimensional structure database of experimentally known inorganic stoichiometric compounds. We first compute the Allen-Dynes critical temperature (T$_c$) for 4533 non-magnetic metals using a direct and progressively finer sampling of the electron-phonon couplings. For the candidates with the largest T$_c$, we use automated Wannierizations and electron-phonon interpolations to obtain a high-quality dataset for the most promising 250 dynamically stable structures, for which we calculate spectral functions, superconducting bandgaps, and isotropic Migdal-Eliashberg critical temperatures. For 140 of these, we also provide anisotropic Migdal-Eliashberg superconducting gaps and critical temperatures. The approach is remarkably successful in finding known superconductors, and we find 24 unknown ones with a predicted anisotropic T$_{\rm c}$ above 10~K. Among them, we identify a possible double gap superconductor (p-doped BaB$_2$), a non-magnetic half-Heusler ZrRuSb, and the perovskite TaRu$_3$C, all exhibiting significant T$_{\rm c}$. Finally, we introduce a sensitivity analysis to estimate the robustness of the predictions.

cond-mat.supr-con↗

arXiv2025

Discovery of the Type-II Superconductor Ta$_4$Rh$_2$C$_{1-δ}$ with a High Upper Critical Field

We report on the discovery of superconductivity in the previously unknown compound Ta$_4$Rh$_2$C$_{1-δ}$. Ta$_4$Rh$_2$C$_{1-δ}$ crystallizes in the $η$-carbide structure type, in the cubic space group $Fd\bar{3}m$ (No.227) with a unit cell parameter of $a = $ 11.7947 Å. Temperature-dependent magnetic susceptibility, resistivity, and specific heat capacity measurements reveal that Ta$_4$Rh$_2$C$_{1-δ}$ is a type-II bulk superconductor with a critical temperature of $T_{\rm c}$ = 6.4 K, and a normalized specific heat jump $ΔC/γT_{\rm c}$ = 1.56. Notably, we find Ta$_4$Rh$_2$C$_{1-δ}$ has a high upper critical field of $μ_0 H_{\rm c2}{\rm (0)}$ = 17.4 T, which is exceeding the BCS weak coupling Pauli limit of $μ_0 H_{\rm Pauli}$ = 11.9 T.

cond-mat.supr-con↗

arXiv2025

Structural Modulation and Enhanced Magnetic Ordering in Incommensurate K$_{1-{x}}$CrSe$_2$ Crystals

Layered delafossite-type compounds and related transition metal dichalcogenides, characterized by their triangular net structures, serve as prototypical systems for exploring the intricate interplay between crystal structure and magnetic behavior. Herein, we report on the discovery of the compound K$_{1-x}$CrSe$_2$ ($x \approx$ 0.13), an incommensurately modulated phase. Single crystals of this compound were grown for the first time using a K/Se self-flux. We find a monoclinic crystal structure with incommensurate modulation, that can be rationalized by a 3+1 dimensional model. This modulation compensates for the under-stoichiometry of K cations, creating pronounced undulations in the CrSe$_2$ layers. Our anisotropic magnetization measurements reveal that K$_{1-x}$CrSe$_2$ undergoes a transition to a long-range magnetically ordered state below $T_{\mathrm N}$ = 133 K, a temperature 1.6 to 3.3 times higher than in earlier reported KCrSe$_2$ compounds. Our findings open new avenues for tuning the magnetic properties of these layered materials through structural modulation.

cond-mat.mtrl-sci↗

arXiv2025

New phase space of hardness materials and synergic enhancement of hardness and toughness in superconducting Ti2Co and Ti4Co2X (X = B, C, N, O)

Compared to traditional superhard materials with high electron density and strong covalent bonds, alloy materials mainly composed of metallic bonding structures typically have great toughness and lower hardness. Breaking through the limits of alloy materials is a preface and long term topic, which is of great significance and value for improving the comprehensive mechanical properties of alloy materials. Here, we report on the discovery of a cubic alloy semiconducting material Ti2Co with large Vickers of hardness Hvexp = 6.7 GPa and low fracture toughness of KICexp =1.51 MPa m0.5. Unexpectedly, the former value is nearly triple of the Hvcal = 2.66 GPa predicted by density functional theory (DFT) calculations and the latter value is about one or two orders of magnitude smaller than that of ordinary titanium alloy materials (KICexp = 30-120 MPa m0.5).These specifications place Ti2Co far from the phase space of the known alloy materials, but close to medium hardness materials such as MgO or TiO2. Upon incorporation of oxygen into structural void positions, both values were simultaneously improved for Ti4Co2O to = 9.7 GPa and 2.19 MPa m0.5, respectively. Further DFT calculations on the electron localization function of Ti4Co2X (X = B, C, N, O) vs. the interstitial elements indicate that these simultaneous improvements originate from the coexistence of Ti-Co metallic bonds, the emergence of newly oriented Ti-X covalent bonds, and the increase of electron concentration. Moreover, the large difference between Hvexp and Hvcal of Ti2Co suggests underlying mechanism concerning the absence of the O(16d) or Ti2-O bonds in the O-(Ti2)6 octahedron.Our discovery expands the phase space of alloy materials and illuminates the path of exploring superconducting materials with excellent mechanical performances.

cond-mat.mtrl-sci↗