Thermodynamic evidence of fractionalized excitations in {\alpha}-RuCl3

Fractionalized excitations are of considerable interest in recent condensed-matter physics. Fractionalization of the spin degrees of freedom into localized and itinerant Majorana fermions are predicted for the Kitaev spin liquid, an exactly solvable model with bond-dependent interactions on a two-dimensional honeycomb lattice. As function of temperature, theory predicts a characteristic two-peak structure of the heat capacity as fingerprint of these excitations. Here we report on detailed heat-capacity experiments as function of temperature and magnetic field in high-quality single crystals of {\alpha}-RuCl3 and undertook considerable efforts to determine the exact phonon background. We measured single-crystalline RhCl3 as non-magnetic reference and performed ab-initio calculations of the phonon density of states for both compounds. These ab-initio calculations document that the intrinsic phonon contribution to the heat capacity cannot be obtained by a simple rescaling of the nonmagnetic reference using differences in the atomic masses. Sizable renormalization is required even for non-magnetic RhCl3 with its minute difference from the title compound. In {\alpha}-RuCl3 in zero magnetic field, excess heat capacity exists at temperatures well above the onset of magnetic order. In external magnetic fields far beyond quantum criticality, when long-range magnetic order is fully suppressed, the excess heat capacity exhibits the characteristic two-peak structure. In zero field, the lower peak just appears at temperatures around the onset of magnetic order and seems to be connected with canonical spin degrees of freedom. At higher fields, beyond the critical field, this peak is shifted to 10 K. The high-temperature peak located around 50 K is hardly influenced by external magnetic fields, carries the predicted amount of entropy, R/2 ln2, and may resemble remnants of Kitaev physics

Momentum-resolved lattice dynamics of parent and electron-doped Sr2_{2}IrO4_{4}

The mixing of orbital and spin character in the wave functions of the $5d$ iridates has led to predictions of strong couplings among their lattice, electronic and magnetic degrees of freedom. As well as realizing a novel spin-orbit assisted Mott-insulating ground state, the perovskite iridate Sr$_{2}$IrO$_{4}$ has strong similarities with the cuprate La$_{2}$CuO$_{4}$, which on doping hosts a charge-density wave that appears intimately connected to high-temperature superconductivity. These phenomena can be sensitively probed through momentum-resolved measurements of the lattice dynamics, made possible by meV-resolution inelastic x-ray scattering. Here we report the first such measurements for both parent and electron-doped Sr$_{2}$IrO$_{4}$. We find that the low-energy phonon dispersions and intensities in both compounds are well described by the same nonmagnetic density functional theory calculation. In the parent compound, no changes of the phonons on magnetic ordering are discernible within the experimental resolution, and in the doped compound no anomalies are apparent due to charge-density waves. These measurements extend our knowledge of the lattice properties of (Sr$_{1-x}$La$_{x}$)$_{2}$IrO$_{4}$ and constrain the couplings of the phonons to magnetic and charge order.Comment: 8 pages, 6 figures (+ 12 pages, 6 figures of supplemental material

Coulomb interactions and screening effects in few-layer black phosphorus: A tight-binding consideration beyond the longwavelength limit

Coulomb interaction and its screening play an important role in many physical phenomena of materials ranging from optical properties to many-body effects including superconductivity. Here, we report on a systematic study of dielectric screening in few-layer black phosphorus (BP), a twodimensional material with promising electronic and optical characteristics. We use a combination of a tight-binding model and rigorously determined bare Coulomb interactions, which allows us to consider relevant microscopic effects beyond the long-wavelength limit. We calculate the dielectric function of few-layer BP in the random phase approximation and show that it exhibits strongly anisotropic behavior even in the static limit. We also estimate the strength of effective local and non-local Coulomb interactions and determine their doping dependence. We find that the pz states responsible for low-energy excitations in BP provide a moderate contribution to the screening, weakening the on-site Coulomb interaction by less that a factor of two. Finally, we calculate the full plasmon spectrum of few-layer BP and discuss the effects beyond long-wavelengths. © 2017 IOP Publishing Ltd

Towards Cubic Symmetry for Ir4+: Structure and Magnetism of the Antifluorite K2IrBr6

Crystal structure, electronic state of Ir4+, and magnetic properties of the antifluorite compound K2IrBr6 are studied using high-resolution synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS), thermodynamic and transport measurements, and ab initio calculations. The crystal symmetry is reduced from cubic at room temperature to tetragonal below 170 K and eventually to monoclinic below 122 K. These changes are tracked by the evolution of the noncubic crystal-field splitting Δ measured by RIXS. Nonmonotonic changes in Δ are ascribed to the competing effects of the tilt, rotation, and deformation of the IrBr6 octahedra as well as tetragonal strain on the electronic levels of Ir4+. The Néel temperature of TN=11.9 K exceeds that of the isostructural K2IrCl6, and the magnitude of frustration on the fcc spin lattice decreases. We argue that the replacement of Cl by Br weakens electronic correlations and enhances magnetic couplings. © 2021 American Physical Society.N.K. thanks Somnath Ghara for his help with resistivity measurements. A.A.T. thanks Adam Aczel and Anna Efimenko for fruitful discussions on the antifluorites, and Yurii Skourski for performing the high-field magnetization measurements. The work in Augsburg was supported by the Federal Ministry for Education and Research through the Sofja Kovalevskaya Award of Alexander von Humboldt Foundation (A.A.T.). The work was partially supported by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ-2020-0060). We acknowledge ESRF and APS for providing synchrotron beamtime for this project, and thank Andy Fitch for his technical support during the experiment at ID22, ESRF. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility, operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Extraordinary facility operations were supported in part by the DOE Office of Science through the National Virtual Biotechnology Laboratory, a consortium of DOE national laboratories focused on the response to COVID-19, with funding provided by the Coronavirus CARES Act. We also acknowledge the support of the HLD at HZDR, member of European Magnetic Field Laboratory (EMFL)

USING THE MODERN THEORY OF MAGNETIC SUSCEPTIBILITY APPROACH TO DESCRIBE THE LOW-DIMENSIONAL MATERIALS: TWO-DIMENSIONAL ANTIMONY

Antimonene is a recently discovered two-dimensional material with high environmental stability and great electronic properties. We report on a computational study of magnetic response of antimonene focusing on the effect of gate voltage, playing an important role due to the structure of material.This work was supported by the Russian Science Foundation Grant № 21-12-00338

Pressure-Induced Dimerization and Collapse of Antiferromagnetism in the Kitaev Material α-Li2IrO3

We present magnetization measurements carried out on polycrystalline and single-crystalline samples of α-Li2IrO3 under hydrostatic pressures up to 2 GPa and establish the temperature-pressure phase diagram of this material. The Néel temperature (TN) of α-Li2IrO3 is slightly enhanced upon compression with dTN/dp = 1.5 K/GPa. Above 1.2 GPa, α-Li2IrO3 undergoes a first-order phase transition toward a nonmagnetic dimerized phase, with no traces of the magnetic phase observed above 1.8 GPa at low temperatures. The critical pressure of the structural dimerization is strongly temperature dependent. This temperature dependence is well reproduced on the ab initio level by taking into account lower phonon entropy in the nonmagnetic phase. We further show that the initial increase in TN of the magnetic phase is due to a weakening of the Kitaev interaction K along with the enhancement of the Heisenberg term J and off-diagonal anisotropy Γ. Our study reveals a common thread in the interplay of magnetism and dimerization in pressured Kitaev materials. © 2022 American Physical Society.This work was funded by the German Research Foundation (DFG) via Project No. 107745057 (TRR80) and via the Sino-German Cooperation Group on Emergent Correlated Matter. D.P. acknowledges financial support by the Russian Science Foundation, Grant No. 21-72-10136

Anisotropic Two-Dimensional Screening at the Surface of Black Phosphorus

Electronic screening can have direct consequences for structural arrangements on the nanoscale, such as on the periodic ordering of adatoms on a surface. So far, such ordering phenomena have been explained in terms of isotropic screening of free electronlike systems. Here, we directly illustrate the structural consequences of anisotropic screening, making use of a highly anisotropic two-dimensional electron gas (2DEG) near the surface of black phosphorous. The presence of the 2DEG and its filling is controlled by adsorbed potassium atoms, which simultaneously serve to probe the electronic ordering. Using scanning tunneling microscopy, we show that the anisotropic screening leads to the formation of potassium chains with a well-defined orientation and spacing. We quantify the mean interaction potential utilizing statistical methods and find that the dimensionality and anisotropy of the screening is consistent with the presence of a band bending-induced 2DEG near the surface. The electronic dispersion of the 2DEG inferred by electronic ordering is consistent with that measured by angle-resolved photoemission spectroscopy. © 2019 American Physical Society

Momentum-resolved lattice dynamics of parent and electron-doped Sr_2IrO_4

The mixing of orbital and spin character in the wave functions of the 5d iridates has led to predictions of strong couplings among their lattice, electronic, and magnetic degrees of freedom. As well as realizing a novel spin-orbit assisted Mott-insulating ground state, the perovskite iridate Sr_2IrO_4 has strong similarities with the cuprate La_2CuO_4, which on doping hosts a charge-density wave that appears intimately connected to high-temperature superconductivity. These phenomena can be sensitively probed through momentum-resolved measurements of the lattice dynamics, made possible by meV-resolution inelastic x-ray scattering. Here we report the first such measurements for both parent and electron-doped Sr_2IrO_4. We find that the low-energy phonon dispersions and intensities in both compounds are well described by the same nonmagnetic density functional theory calculation. In the parent compound, no changes of the phonons on magnetic ordering are discernible within the experimental resolution, and in the doped compound no anomalies are apparent due to charge-density waves. These measurements extend our knowledge of the lattice properties of (Sr_(1−x)La_x)_2IrO_4 and constrain the couplings of the phonons to magnetic and charge order