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.•Accessible magnetic field generation.•Selectable field strength and orientation.•Compatible with high-temperature sample preparation.

Discovery of superconductivity in Nb4SiSb2 with a V4SiSb2-type structure and implications of interstitial doping on its physical properties.

We report on the discovery, structural analysis, and the physical properties of Nb4SiSb2 - a hitherto unknown compound crystallizing in the V4SiSb2-type structure with the tetragonal space group I4/mcm and unit cell parameters a = 10.3638(2) Å and c = 4.9151(2) Å. We find Nb4SiSb2 to be a metal undergoing a transition to a superconducting state at a critical temperature of T c ≈ 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 c)/γT c = 1.33 mJ mol-1 K-2. We find that for Nb4SiSb2, 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 c ≈ 1.2 K for Nb4Cu0.2SiSb2, and T c ≈ 0.8 K for Nb4Pd0.2SiSb2 as well as for Nb4Pt0.14SiSb2. The addition of electron-donor atoms into these void positions, henceforth, lowers the superconducting transition temperature in comparison to the parent compound.

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. Here, we report on its detailed magnetic and structural characteristics. We evidence that it undergoes a transition to an A-type antiferromagnetic state below TN ≈ 140 K with a pronounced two-dimensional character, preceded by ferromagnetic correlations within the monolayers. Furthermore, we unravel the low-temperature 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 field. These take place upon cooling below Ts ≈ 100 K, until a spin freezing process occurs at T* ≈ 40 K. We argue this complex behavior to reflect a crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by its mixed-anion character. Our findings reinforce CrSBr as an important candidate for devices in the emergent field of two-dimensional magnetic materials.

Quasi-1D Electronic Transport in a 2D Magnetic Semiconductor.

Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qualitative and quantitative differences of all quantities measured along the in-plane a and b crystallographic directions is found. In particular, a qualitatively different dependence of the conductivities σa and σb on temperature and gate voltage, accompanied by orders of magnitude differences in their values (σb /σa  ≈ 3 × 102  to 105 at low temperature and negative gate voltage) are observed, together with 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-and unambiguous signatures of a 1D van Hove singularity detected 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. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.

Quantum Materials Discovery by Combining Chemical and Physical Design Principles.

Exploratory quantum materials discovery remains crucial to progress in material science. Due to the grand challenges that we are facing in predicting these materials and their properties from scratch, chemical design principles remain a key ingredient for the discovery of new materials. Chemical heuristics, structure, bonding, as well as global and local symmetries are at the very foundation of materials properties. In this regard, in this research, we aim to identify functional materials by composition-structure-property understanding. Materials discovery consists of a subset of methods and design principles that go hand in hand until a desired material or property is realized. However, materials synthesis is still far from a rational design approach. Rather, materials, and especially metastable materials, have to be accessed and synthesized in an exploratory, laboratory-intensive fashion. At the same time, quantum materials discovery is a vibrant highly active field of research that has seen various leaps of progress in recent years, and that holds the promise for many more in the coming years. Here, we lay out how we are discovering new materials and new materials physics in our and other chemical physics, or physical chemistry research groups, and how chemistry and chemical synthesis play a crucial role in this process.

Synthetic control over polymorph formation in the d-band semiconductor system FeS2.

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS2. 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 FeS2. 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 FeS2 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 H2S 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 × 1018 cm-3. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS2 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.

Break of symmetry at the surface of IrTe2upon phase transition measured by x-ray photoelectron diffraction.

IrTe2undergoes 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-IrTe2in the first-order phase transition, from the (1 × 1) phase to the (5 × 1) 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 IrTe2trigonal to a monoclinic unit cell and confirm the occurrence of the (5 × 1) reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.

Polytypism and superconductivity in the NbS2 system.

We report on the phase formation and the superconducting properties in the NbS2 system. Specifically, we have performed a series of standardized solid-state syntheses in this system, which allow us to establish a comprehensive synthesis map for the formation of the two polytypes 2H-NbS2 and 3R-NbS2, respectively. We show that the identification of two polytypes by means of X-ray diffraction is not always unambiguous. Our physical property measurements on a phase-pure sample of 3R-NbS2, on a phase-pure sample of 2H-NbS2, and a mixed phase sample confirm earlier reports that 2H-NbS2 is a bulk superconductor and that 3R-NbS2 is not a superconductor above T = 1.75 K. Our results clearly show that specific heat measurements, as true bulk measurements, are crucial for the identification of superconducting materials in this and related systems. Our results indicate that for the investigation of van der Waals materials great care has to be taken on choosing the synthesis conditions for obtaining phase pure samples.

Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric η-Carbide Nb4Rh2C1-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 Cooper 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 µ0 H Pauli ≈ 1.86[T/K]·T c for a weak-coupling Bardeen-Schrieffer-Cooper (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 Nb4Rh2C1-δ, with a normalized specific heat jump of ΔC/γT c = 1.64. We find that this material has a remarkably high upper critical field of µ0 H c2(0) = 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of µ0 H 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.

The h-Sbx WO3+2x Oxygen Excess Antimony Tungsten Bronze.

We describe the previously unreported oxygen excess hexagonal antimony tungsten bronze is reported, with a composition of Sb0.5 W3 O10 , in the following denoted as h-Sbx WO3+2x with x=0.167, to demonstrate its analogy to classical Ax WO3 tungsten bronzes. This compound forms in a relatively narrow temperature range between 580 °C

High-Pressure Synthesis and Characterization of ß-GeSe-A Six-Membered-Ring Semiconductor in an Uncommon Boat Conformation.

Two-dimensional materials have significant potential for the development of new devices. Here we report the electronic and structural properties of ß-GeSe, a previously unreported polymorph of GeSe, with a unique crystal structure that displays strong two-dimensional structural features. ß-GeSe is made at high pressure and temperature and is stable under ambient conditions. We compare it to its structural and electronic relatives α-GeSe and black phosphorus. The ß form of GeSe displays a boat conformation for its Ge-Se six-membered ring ("six-ring"), while the previously known α form and black phosphorus display the more common chair conformation for their six-rings. Electronic structure calculations indicate that ß-GeSe is a semiconductor, with an approximate bulk band gap of Δ ≈ 0.5 eV, and, in its monolayer form, Δ ≈ 0.9 eV. These values fall between those of α-GeSe and black phosphorus, making ß-GeSe a promising candidate for future applications. The resistivity of our ß-GeSe crystals measured in-plane is on the order of ρ ≈ 1 Ω·cm, while being essentially temperature independent.