arXiv subjects

Fabian O. von Rohr

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

At least 37 records · Page 2Linked to original sources

arXiv2024

Magnetism in EuAlSi and the Eu1-xSrxAlSi Solid Solution

The magnetic properties of EuAlSi, a compound comprising a honeycomb lattice of Al and Si atoms and a triangular lattice of Eu atoms, are presented. Moreover, we have prepared the Eu1-xSrxAlSi solid solution, to study the evolution of the collective quantum properties from the ferromagnetic EuAlSi towards the superconducting SrAlSi. A possible quantum critical point is suggested to exist in the vicinity of to x of 0.96, at which the suppression of ferromagnetic order is concomitant with the emergence of superconductivity.

cond-mat.str-el↗

arXiv2024

Efficient Soft-Chemical Synthesis of Large van-der-Waals Crystals of the Room-Temperature Ferromagnet 1T-CrTe2

We herein report on a fast and convenient soft-chemical synthesis approach towards large 1T-CrTe2 van-der-Waals crystals. This compound is formed X-ray diffraction pure, with a complete conversion within just over 2 h from flux-grown LiCrTe2 crystals using diluted acids. Due to the availability of high-quality single crystals, we have confirmed the crystal structure for the first time by single-crystal X-ray diffraction experiments. For the acid deintercalated 1T-CrTe2 crystals, we find long-range ferromagnetic order with a Curie temperature of $T_{\rm C}$ = 318 K. We further revealed the magnetic structure of 1T-CrTe2 using low-temperature neutron powder diffraction experiments and determine the magnetic Hamiltonian using density functional theory. X-ray diffraction experiments of post-annealed crystals suggest a thermal stability of 1T-CrTe2 up to at least 100 °C. Our findings expand the synthesis methods for 1T-CrTe2 crystals, which hold promise for integrated room-temperature spintronics applications.

cond-mat.mtrl-sci↗

arXiv2024

Controlling the Magnetic Properties of the van der Waals Multiferroic Crystals Co$_{1-x}$Ni$_{x}$I$_2$

The structurally related compounds NiI$_2$ and CoI$_2$ are multiferroic van der Waals materials, in which helimagnetic orders exist simultaneously with electric polarization. Here, we report on the evolution of the crystal structure and of the magnetic properties across the solid solution Co$_{1-x}$Ni$_{x}$I$_2$. We have successfully grown crystals of the whole range of the solid solution, i.e. $x = 0-1$, by employing the self-selecting vapor growth (SSVG) technique and by carefully tuning the synthesis conditions according to the chemical composition. Our structural investigations show that the crystal symmetry changes from $P\bar{3}m1$ to $R\bar{3}m$ when Ni substitutes for Co beyond $x = 0.2$. Both the lattice parameters and magnetic properties evolve continuously and smoothly from one end member to the other, showing that they can be finely tuned by the chemical composition. We also observe that the Ni substitution degree in the solid solution affects the metamagnetic transition typical for CoI$_2$ at high magnetic fields. In particular, we find the existence of the metamagnetic transition similar to that for CoI$_2$ in the NiI$_2$ structure. Based on magnetic measurements we construct the phase diagram of the Co$_{1-x}$Ni$_{x}$I$_2$ system. Controlling the magnetic properties by the chemical composition may open new pathways for the fabrication of electronic devices made of two-dimensional (2D) flakes of multiferroic van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2024

Influence of magnetism on vertical hopping transport in CrSBr

We investigate the c-direction conduction in CrSBr in the linear regime, not accessible in other van der Waals (vdW) magnetic semiconductors, because of the unmeasurably low current. The resistivity -- $10^8$ to $10^{11}$ times larger than in the a- and b-directions -- exhibits magnetic state dependent thermally activated and variable range hopping transport. In the spin-flip phase at 2 T, the activation energy is 20 meV lower than in the antiferromagnetic state, due to a downshift of the conduction band edge, in agreement with ab-initio calculations. In the variable range hopping regime, the average hopping length decreases from twice the interlayer distance to the interlayer distance at 2 T, because in the antiferromagnetic state the large exchange energy impedes electrons hopping between adjacent layers. Our work demonstrates that the linear transport regime provides new information about electronic processes in vdW magnetic semiconductors, and shows how magnetism influences these processes both in real and reciprocal space.

cond-mat.mtrl-sci↗

arXiv2024

Ti4Ir2O a time-reversal-invariant fully gapped unconventional superconductor

Here we report muon spin rotation (muSR) experiments on the temperature and field dependence of the effective magnetic penetration depth (lambda) in the eta-carbide-type suboxide Ti4Ir2O, a superconductor with an considerably high upper critical field. Temperature dependence of penetration depth, obtained from transverse-field (TF)-muSR measurements, is in perfect agreement with an isotropic fully gaped superconducting state. Furthermore, our ZF muSR results confirm that the time-reversal symmetry is preserved in the superconducting state. We find, however, a notably low ratio of 1.22 between the superconducting critical temperature and the superfluid density. This value is close to most unconventional superconductors, showing that a very small superfluid density is present in the superconducting state of Ti4Ir2O. The presented results will pave the way for further theoretical and experimental investigations to obtain a microscopic understanding of the origin of such a high upper critical field in an isotropic single gap superconducting system.

cond-mat.supr-con↗

arXiv2023

Structural Phase Transition and Superconductivity in 2H-BaGaGe with Buckled Honeycomb Layers

We report on the structural and superconducting properties of the intermetallic compound BaGaGe. We find that this material undergoes a structural second-order phase transition from the distorted AlB$_2$-type structure (1H, $a$ = 4.3254(2) Å, $c$ = 5.1078(3) Å, P6/mmm) into the CaIn$_2$-type structure (2H, $a$ = 4.3087(3) Å, $c$ = 10.2117(6) Å, P6$_3$/mmc) at a transition temperature of $T_{\rm S}$ = 253 K. We find that the structural phase-transition corresponds to a coherent buckling of the honeycomb layers, which we can interpret as a disorder-to-order transition of the atoms located within this layer. We show that the 2H-BaGaGe phase becomes superconducting at a critical temperature of $T_{\rm c}$ = 2.1 K. The bulk nature of the superconductivity in 2H-BaGaGe is confirmed by means of specific heat measurements, where we determine a value of $ΔC$/$γT_{\rm c}$ = 1.59, which is close to the expected BCS value in the weak coupling limit.

cond-mat.supr-con↗

arXiv2023

Synthesis and Anisotropic Magnetic Properties of LiCrTe$_2$ Single Crystals with a Triangular-Lattice Antiferromagnetic Structure

We report on the synthesis of LiCrTe$_2$ single crystals and on their anisotropic magnetic properties. We have obtained these single crystals by employing a Te/Li-flux synthesis method. We find LiCrTe$_2$ to crystallize in a TlCdS$_2$ -type structure with cell parameters of $a$ = 3.9512(5) Å and $c$ = 6.6196(7) Å at $T$ = 175 K. The content of lithium in these crystals was determined to be near stoichiometric by means of neutron diffraction. We find a pronounced magnetic transition at $T^{\rm ab}_{\rm N}$ = 144 K and $T^{\rm c}_{\rm N}$ = 148 K, respectively. These transition temperatures are substantially higher than earlier reports on polycrystalline samples. We have performed neutron powder diffraction measurements that reveal that the long-range low-temperature magnetic structure of single crystalline LiCrTe$_2$ is an A-type antiferromagnetic (AFM) structure. Our DFT calculations are in good agreement with these experimental observations. We find the system to be easy axis with moments oriented along the $c$-direction experimentally as well as in our calculations. Thereby, the magnetic Hamiltonian can be written as $H = H_{\rm Heisenberg} + \sum_i K_c (S_i^z)^2$ with $K_c=-0.34K$ (where $|S^z|=\frac{3}{2}$). We find LiCrTe$_2$ to be highly anisotropic, with a pronounced metamagnetic transition for $H \perp ab$ with a critical field of $μH_{MM}$(5 K) $\approx$ 2.5 T. Using detailed orientation-dependent magnetization measurements, we have determined the magnetic phase diagram of this material. Our findings suggest that LiCrTe$_2$ is a promising material for exploring the interplay between crystal structure and magnetism, and could have potential applications in spin-based 2D devices.

cond-mat.mtrl-sci↗

arXiv2023

Chemical Principles of Intrinsic Topological Superconductors

The concept of topological superconductivity has attracted immense interest in the physics community recently for several reasons: First, topological superconductors represent new phases of matter, topologically distinct from any other known phase of matter. Second, their discovery would present the first realization of Majorana zero modes. Third, intrinsic topological superconductors promise to become an important ingredient for next-generation quantum technologies. There are a handful of candidates to date considered as potential intrinsic topological superconductors. All of these display signs of unconventional, potentially topological superconductivity. However, the results from different experimental methods are inconclusive. One of the major challenges in the field of topological superconductivity has been the scarcity of potential materials, in contrast to the many exciting, theoretical predictions of properties that could be unraveled by such a discovery. Currently, it should be far to say that a material that convincingly displays intrinsic topological superconductivity, and Majorana zero modes, has so far not been discovered. This perspective aims at summarizing the results of the most actively discussed potential topological superconductors for chemists. But, I will also compile the essential physical and chemical design principles of these materials from a chemists' perspective in order to motivate synthetic chemists to join the quest for the discovery of the first intrinsic topological superconductor.

cond-mat.supr-con↗

arXiv2023

Observation of the metallic mosaic phase in 1$T$-TaS$_2$ at equilibrium

The transition-metal dichalcogenide tantalum disulphide (1$T$-TaS$_2$) hosts a commensurate charge density wave (CCDW) at temperatures below 165~K where it also becomes insulating. The low temperature CCDW phase can be driven into a metastable "mosaic" phase by means of either laser or voltage pulses which shows a large density of CDW domain walls as well as a closing of the electronic band gap. The exact origins of this pulse-induced metallic mosaic are not yet fully understood. Here, using scanning tunneling microscopy and spectroscopy (STM/STS), we observe the occurrence of such a metallic mosaic phase on the surface of TaS$_2$ without prior pulse excitation over continuous areas larger than $100 \times 100$~nm$^2$ and macroscopic areas on the millimetre scale. We attribute the appearance of the mosaic phase to the presence of surface defects which cause the formation of the characteristic dense domain wall network. Based on our STM measurements, we further argue how the appearance of the metallic behaviour in the mosaic phase could be explained by local stacking differences of the top layer. Thus, we provide a potential avenue to explain the origin of the pulse induced mosaic phase.

cond-mat.str-el↗

arXiv2022

Discovery of Superconductivity in Nb$_4$SiSb$_2$ with a V$_4$SiSb$_2$-Type Structure and Implications of Interstitial Doping on its Physical Properties

We report on the discovery, structural analysis, and the physical properties of Nb$_4$SiSb$_2$ -- a hitherto unknown compound crystallizing in the V$_4$SiSb$_2$-type structure with the tetragonal space group $I4/mcm$ and unit cell parameters $a$ = 10.3638(2) $\mathring{\mathrm{A}}$ and $c$ = 4.9151(2) $\mathring{\mathrm{A}}$. We find Nb$_4$SiSb$_2$ to be a metal undergoing a transition to a superconducting state at a critical temperature of $T_{\rm c} \approx$ 1.6 K. The bulk nature of the superconductivity in this material is confirmed by the observation of a well defined discontinuity in specific heat with a normalized specific heat jump of $ΔC(T_{\rm c})/γT_{\rm c} = 1.33\, {\rm mJ}\, {\rm mol}^{-1}\, {\rm K}^{-2}$. We find that for Nb$_4$SiSb$_2$, the unoccupied sites on the $4b$ Wyckoff position can be partially occupied with Cu, Pd, or Pt. Low-temperature resistivity measurements show transitions to superconductivity for all three compounds at $T_{\rm c} \approx\, 1.2\, {\rm K}$ for Nb$_4$Cu$_{0.2}$SiSb$_2$, and $T_{\rm c} \approx\, 0.8\, {\rm K}$ for Nb$_4$Pd$_{0.2}$SiSb$_2$ as well as for Nb$_4$Pt$_{0.14}$SiSb$_2$. The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.

cond-mat.supr-con↗

arXiv2022

A Sacrificial Magnet Concept for Field Dependent Surface Science Studies

We demonstrate a straightforward approach to integrating a magnetic field into a low-temperature scanning tunneling microscope (STM) by adhering an NdFeB permanent magnet to a magnetizable sample plate. To render our magnet concept compatible with high-temperature sample cleaning procedures, we make the irreversible demagnetization of the magnet a central part of our preparation cycle. After sacrificing the magnet by heating it above its Curie temperature, we use a transfer tool to attach a new magnet in-situ prior to transferring the sample into the STM. We characterize the magnetic field created by the magnet using the Abrikosov vortex lattice of superconducting NbSe2. Excellent agreement between the distance dependent magnetic fields from experiments and simulations allows us to predict the magnitude and orientation of magnetic flux at any location with respect to the magnet and the sample plate. Our concept is an accessible solution for field-dependent surface science studies that require fields in the range of up to 400 mT and otherwise detrimental heating procedures.

physics.app-ph↗

arXiv2022

Modified interlayer stacking and insulator to correlated-metal transition driven by uniaxial strain in 1$T$-TaS$_{2}$

Interlayer coupling is strongly implicated in the complex electronic properties of 1$T$-TaS$_2$ , but the interplay between this and electronic correlations remains unresolved. Here, we employ angle-resolved photoemission spectroscopy (ARPES) to reveal the effect of uniaxial strain engineering on the electronic structure and interlayer coupling in 1$T$-TaS$_2$ . The normally insulating ground state is transformed into a correlated-metal phase under strain, as evidenced by the emergence of a narrow band at the Fermi level. Temperature dependent ARPES measurements reveal that the metallic behaviour only develops below the commensurate charge density wave (CCDW) transition, where interlayer dimerization produces a band-insulator in unstrained samples. Electronic structure calculations demonstrate that the correlated metallic behaviour is stabilized by a previously predicted but unobserved bulk stacking structure with a modified interlayer coupling of the Ta d$_{z^{2}}$ electrons. Our combined approach lays bare the role of correlations and interlayer coupling in 1$T$-TaS$_2$ , providing critical input for understanding superconductivity under pressure and the metastable hidden phase induced using non-equilibrium protocols in this platform material for correlated physics.

cond-mat.str-el↗

arXiv2022

Dynamic Magnetic Crossover at the Origin of the Hidden-Order in van der Waals Antiferromagnet CrSBr

The van der Waals material CrSBr stands out as a promising two-dimensional magnet. Especially, its high magnetic ordering temperature and versatile magneto-transport properties make CrSBr an important candidate for new devices in the emergent field of two-dimensional magnetic materials. To date, the magnetic and structural properties of CrSBr have not been fully elucidated. Here, we report on the detailed temperature-dependent magnetic and structural properties of this material, by comprehensively combining neutron scattering, muon spin relaxation spectroscopy, synchrotron X-ray diffraction, and magnetization measurements. We evidence that this material undergoes a transition to an A-type antiferromagnetic state below $T_{\rm N} \approx$ 140 K, with a pronounced two-dimensional character as deduced from the determined critical exponent of $β\approx $ 0.18. In our analysis of the field-induced metamagnetic transition, we find that the ferromagnetic correlations within the monolayers persist clearly above the Néel temperature in this material. Furthermore, we unravel the low-temperature (i.e. $T < T_{\rm N}$) magnetic hidden order within the long-range magnetically ordered state. We find that it is associated to a slowing down of the magnetic fluctuations, accompanied by a continuous reorientation of the internal magnetic field. These take place upon cooling below $T_s$ $\approx$ 100 K, until a spin freezing process occurs at $T$* $\approx$ 40 K. We argue this complex dynamic behavior to reflect a magnetic crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by the mixed-anion character of the material. Our findings indicate that the magnetic and structural properties of CrSBr widen its potential application as a component for spin-based electronic devices.

cond-mat.mtrl-sci↗

arXiv2022

Quasi 1D electronic transport in a 2D magnetic semiconductor

We investigate electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor that is attracting attention because of its magnetic properties. We find an extremely pronounced anisotropy that manifests itself in qualitative and quantitative differences of all quantities measured along the in-plane \textit{a} and \textit{b} crystallographic directions. In particular, we observe a qualitatively different dependence of the conductivities $σ_a$ and $σ_b$ on temperature and gate voltage, accompanied by orders of magnitude differences in their values ($σ_b$/$σ_a \approx 3\cdot10^2-10^5$ at low temperature and large negative gate voltage). We also find a different behavior of the longitudinal magnetoresistance in the two directions, and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The observed phenomenology -- together with unambiguous signatures of a 1D van Hove singularity that we detect in energy resolved photocurrent measurements -- indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. We conclude that CrSBr is the first 2D semiconductor to show distinctly quasi 1D electronic transport properties.

cond-mat.mes-hall↗

arXiv2021

Group-9 Transition Metal Suboxides Adopting the Filled-Ti$_2$Ni Structure: A Class of Superconductors Exhibiting Exceptionally High Upper Critical Fields

The Ti$_2$Ni and the related $η$-carbide structure are known to exhibit various intriguing physical properties. The Ti$_2$Ni structure with the cubic space group $Fd\bar{3}m$ is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related $η$-carbide compounds correspond to a filled version of the Ti$_2$Ni structure. Here, we report on the structure and superconductivity in the $η$-carbide type suboxides Ti$_4$M$_2$O with M = Co, Rh, Ir. We have successfully synthesized all three compounds in single phase form. We find all three compounds to be type-II bulk superconductors with transition temperatures of $T_{\rm c}$ = 2.7, 2.8, and 5.4 K, and with normalized specific heat jumps of $ΔC/γT_{\rm c}$ = 1.65, 1.28, and 1.80 for Ti$_4$Co$_2$O, Ti$_4$Rh$_2$O, and Ti$_4$Ir$_2$O, respectively. We find that all three superconductors, exhibit high upper-critical fields. Particularly noteworthy is Ti$_4$Ir$_2$O with an upper critical field of $μ_0 H_{\rm c2}{\rm (0)}$ =~16.06~T, which exceeds by far the weak-coupling Pauli limit of 9.86~T. The role of the void filling light atom X has so far been uncertain for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti$_2$Co. In contrast to the metallic $η$-carbide type suboxides Ti$_4$M$_2$O, we find that Ti$_2$Co displays a semimetallic behavior. Hence, the octahedral void-filling oxygen plays a crucial role for the overall physical properties, even though its effect on the crystal structure is small. Our results indicate that the design of new superconductors by incorporation of electron-acceptor atoms may in the Ti$_2$Ni-type structures and other materials with crystallographic void position be a promising future approach. The remarkably high upper critical fields, in this family of compounds, may furthermore spark significant future interest.

cond-mat.supr-con↗

arXiv2021

Synthetic control over polymorph formation in the d-band semiconductor system FeS$_2$

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS$_2$. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS$_2$. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high-quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS$_2$ polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H$_2$S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 $\times$ 10$^{18}$ cm$^{-3}$. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS$_2$ phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite -- that is now synthetically available in a straightforward fashion -- is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements.

cond-mat.mtrl-sci↗

arXiv2021

Break of symmetry at the surface of IrTe$_2$ upon phase transition measured by X-ray photoelectron diffraction

IrTe$_2$ undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of $1T-$IrTe$_2$ in the first-order phase transition, from the $(1\times1)$ phase to the $(5\times1)$ phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe$_2$ trigonal to a monoclonic unit cell and confirm the occurence of the $(5\times1)$ reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.

cond-mat.mtrl-sci↗

arXiv2021

Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric $η$-Carbide Nb$_4$Rh$_2$C$_{1-δ}$

The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Copper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as $μ_0 H_{\rm Pauli} \approx 1.86{\rm [T/K]} \cdot T_{\rm c}$ for a weak-coupling BCS superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic $η$-carbide-type compound Nb$_4$Rh$_2$C$_{1-δ}$, with a normalized specific heat jump of $ΔC/γT_{\rm c} =$ 1.64. We find that this material has a remarkably high upper critical field of $μ_0 H_{\rm c2}{\rm (0)}$ =~28.5~T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of $μ_0 H_{\rm Pauli}$~=~18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.

cond-mat.supr-con↗