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Stereochemically active lone-pair (SCALP) cations are attractive units for realizing optical anisotropy. Antimony (III) chloride perovskites with SCALP have remained largely unknown till date. We synthesized vacancy ordered Cs3Sb2Cl9 perovskite single crystals with SbCl6 octahedral linkage containing SCALP. Remarkably, Cs3Sb2Cl9 single crystals exhibit an exceptional birefringence of 0.12 +(-) 0.01 at 550 nm, which is the largest among pristine all-inorganic halide perovskites. The SCALP brings a large local structural distortion of the SbCl6 octahedra promoting birefringence optical responses in Cs3Sb2Cl9 single crystals. Theoretical calculations reveal that the considerable hybridization of Sb 5s with Sb 5p and Cl 3p states largely contribute to the SCALP. Furthermore, the change in the Sb-Cl-Sb bond angle creates distortion in the SbCl6 octahedral arrangement in the apical and equatorial directions within the crystal structure incorporating the required anisotropy for the birefringence. This work explores pristine inorganic halide perovskite single crystals as a potential birefringent material with prospects in integrated optical devices.

We report the synthesis, characterization, low-temperature magnetic, and thermodynamic measurements of the novel milli-Kelvin adiabatic demagnetization refrigeration (mK-ADR) candidate material NaYbGeO$_4$ which exhibits a distorted square lattice arrangement of YbO$_{6}$ magnetic units. Magnetization and specific heat indicate weakly interacting effective spin-1/2 moments below 10~K, with a Curie-Weiss temperature of only 15~mK, that can be polarized by magnetic fields of order 1~T. For the ADR performance test, we start the demagnetization from 5~T at a temperature of $\sim 2$~K and reach a minimum temperature of 150~mK at zero field. The warming curve indicates a sharp magnetic transition in the heat capacity at 210~mK, implying only weak magnetic frustration. The entropy density of $S_{\rm GS}\simeq 101$ mJ K$ ^{-1}$cm$^{-3}$ and hold time below 2~K of 220~min are competitive while the minimal temperature is higher compared to frustrated Ytterbium-oxide ADR materials studied under similar conditions., Comment: 9 pages, 7 figures

The occurrence of a noncollinear magnetic structure at a Mn monolayer grown epitaxially on Fe(100) is predicted theoretically, using spinor density-functional theory, and observed experimentally, using x-ray magnetic circular dichroism (XMCD) and linear dichroism (XMLD) spectroscopies. The combined use of XMCD and XMLD at the Mn-absorption edge allows us to assess the existence of ferromagnetic and antiferromagnetic order at the interface, and also to determine the moment orientations with element specificity. The experimental results thus obtained are in excellent agreement with the magnetic structure determined theoretically.

Motivated by experimental suggestions of anionic redox processes helping to design higher energy lithium ion-battery cathode materials, we investigate this effect using first-principles electronic structure calculations for Li-rich layered sulfides. We identify the determination of the energetic contribution of intra-atomic Hund's exchange coupling as a major obstacle to a reliable theoretical description. We overcome this challenge by developing a particularly efficient flavor of charge-self-consistent combined density functional + dynamical mean-field theory (DFT+DMFT) calculations. Our scheme allows us to describe the spin ground states of the transition metal d shell, the electronic structure of the materials, and its energetics. As a result of the high-spin to low-spin transition the average intercalation voltage shows intriguing non-monotonic behavior. We rationalize these findings by an analysis of the fluctuations of spin and charge degrees of freedom. Our work demonstrates the relevance of most recent insights into correlated electron materials for the physics of functional materials such as Li-ion battery compounds.

Attaining milli-Kelvin temperatures is often a prerequisite for the study of novel quantum phenomena and the operation of quantum devices. Adiabatic demagnetization refrigeration (ADR) is an effective, easy and sustainable alternative to evaporation or dilution cooling with the rare and super-expensive $^3$He. Paramagnetic salts, traditionally used for mK-ADR, suffer from chemical instability related to water of crystallization. We report synthesis, characterization as well as low-temperature magnetization and specific heat measurements of two new UHV compatible candidate materials NaYbP$_2$O$_7$ and KYbP$_2$O$_7$. Utilizing the PPMS at 2 K, the ADR of sintered pellets with Ag powder admixture starting at 5 T yields base temperatures (warm-up times) of 45 mK (55 min) and 37 mK (35 min) for NaYbP$_2$O$_7$ and KYbP$_2$O$_7$, respectively, slightly advantageous to KBaYb(BO$_3$)$_2$ (45 mK and 40 min) studied under similar conditions., Comment: 9 pages, 7 figures

In this article we employ density functional theory in the generalized gradient approximation to investigate the structural and electronic properties of the solid solution alloy $\text{Zn}\text{Se}_x\text{S}_{1-x}$ in the wurtzite structure. We analyzed the character of the bond lengths and angles at the atomic scale, using a supercell approach that does not impose any constraint on the crystal potential. We show that the bond lengths of pristine ZnS and ZnSe compounds are almost preserved between nearest neighbors, which is different from what would be anticipated if Vegard's law were valid at the atomic level. We also show that bond lengths start behaving in accordance to Vegard's law from the third shell of nearest neighbors onward, which in turn determines the average lattice parameters of the alloys determined by diffraction experiments. Fundamental building blocks around the anions are identified and are shown to be non-rigid but still volume preserving. Finally, the geometrical analysis is connected to the trend exhibited by the electronic structure, and in particular by the band gap. The latter is found to exhibit a small deviation from the linearity with respect to the Se concentration, in accordance to available experimental data. By assuming a quadratic dependence, we can extract a bowing parameter and analyze various contributions to it with various calculations under selected constraints. The structural deformation in response to the doping process is shown to be the driving force behind the deviation from linearity. The difference in stiffness between ZnS and ZnSe is showed to play a key role in the asymmetric behavior of the bowing parameter observed in the S-rich and Se-rich regions.

The optoelectronic properties and ultimately photovoltaic performance of hybrid lead halide perovskites, is inherently related to the dynamics of the organic cations. Here we report on the dynamics of the formamidinium (FA) cation in FAPbX3 perovskites for chloride and bromide varieties, as studied by neutron spectroscopy. Elastic fixed window scan measurements showed the onset of reorientational motion of FA cations in FAPbCl3 to occur at a considerably higher temperature compared to that in FAPbBr3. In addition, we observed two distinct dynamical transitions only in the chloride system, suggesting a significant variation in the reorientational motions of the FA cation with temperature. Quasielastic neutron scattering data analysis of FAPbCl3 showed that in the low temperature orthorhombic phase, FA cations undergo 2-fold jump reorientations about the C-H axis which evolve into an isotropic rotation in the intermediate tetragonal and high temperature cubic phases. Comparing the results with those from FAPbBr3, reveal that the time scale, barrier to reorientation and the geometry of reorientational motion of the FA cation are significantly different for the two halides. We note that this dependence of the dynamic properties of the A-site cation on the halide, is unique to the FA series; the geometry of methylammonium (MA) cation dynamics in MAPbX3 is known to be insensitive to different halides.

Two seemingly similar crystal structures of the low-temperature (~100 K) MAPbX$_3$ (X=I,Br,Cl) perovskites, but with different relative Methylammonium (MA) ordering, have appeared as representatives of this orthorhombic phase. Distinguishing them by X-ray diffraction experiments is difficult and conventional first-principles based molecular-dynamics approaches are often too computationally intensive to be feasible. Therefore, to determine the thermodynamically stable structure, we use a recently introduced on-the-fly Machine-Learning Force Field method, which reduces the computation time from years to days. The molecules exhibit a large degree of anharmonic motion depending on temperature: i.e. rattling, twisting and tumbling. We observe the crystal's 'librational pathways' while slowly heating it in isothermal-isobaric simulations. Marked differences in the thermal evolution of structural parameters allow us to determine the real structure of the system via a comparison with experimentally determined crystal structures.

We present a combination of thermodynamic and dynamic experimental signatures of a disorder driven dynamic cooperative paramagnet in a 50% site diluted triangular lattice spin-1/2 system, Y$_2$CuTiO$_6$. Magnetic ordering and spin freezing are absent down to 50 mK, far below the Curie Weiss scale of ~-134 K. We observe scaling collapses of the magnetic field- and temperature-dependent magnetic heat capacity and magnetisation data, respectively, in conformity with expectations from the random singlet physics. Our experiments establish the suppression of any freezing scale, if at all present, by more than three orders of magnitude, opening a plethora of interesting possibilities such as disorder-stabilized long range quantum entangled ground states., Comment: 18 pages, 9 figures, published in Physical Review Letters

Chemical exfoliation of MoS$_2$ via Li-intercalation route has led to many desirable properties and spectacular applications due to the presence of a metastable state in addition to the stable H phase. However, the nature of the specific metastable phase formed, and its basic charge conduction properties have remained controversial. Using spatially resolved Raman spectroscopy (~1 micrometer resolution) and photoelectron spectroscopy (~120 nm resolution), we probe such chemically exfoliated MoS$_2$ samples in comparison to a mechanically exfoliated H phase sample and confirm that the dominant metastable state formed by this approach is a distorted T' state with a small semiconducting gap. Investigating two such samples with different extents of Li residues present, we establish that Li+ ions, not only help to exfoliate MoS$_2$ into few layer samples, but also contribute to enhancing the relative stability of the metastable state as well as dope the system with electrons, giving rise to a lightly doped small bandgap system with the T' structure, responsible for its spectacular properties., Comment: 34 pages, Main manuscript + Supplementary Material

MoS_2 in its two-dimensional (2D) form is known to exhibit many fundamentally interesting and technologically important properties. One of the most popular routes to form extensive amount of such 2D samples is the chemical exfoliation route. However, the nature and origin of the specific polymorph of MoS_2 primarily responsible for such spectacular properties has remained controversial with claims of both T and T' phases as well as metallic and semiconducting natures. We show that a comprehensive scrutiny of the available literature data of Raman spectra from such samples allow little scope for such ambiguities, providing overwhelming evidence for the formation of the T' phase as the dominant metastable state in all such samples. We also explain that this small band-gap T' phase may attain substantial conductivity due to thermal and chemical doping of charge-carriers, explaining the contradictory claims of metallic and semiconducting nature of such samples, thereby attaining a consistent view of all reports available so far., Comment: 22 pages, 6 Figures. Published in APL Materials

On a lattice composed of triangular plaquettes where antiferromagnetic exchange interactions between localized spins cannot be simultaneously satisfied, the system becomes geometrically frustrated with magnetically disordered phases remarkably different from a simple paramagnet. Spin liquid belongs to one of these exotic states, in which a macroscopic degeneracy of the ground state gives rise to the rich spectrum of collective phenomena. Here, we report on the discovery of a new magnetic state in the heterostructures derived from a single unit cell (111)-oriented spinel CoCr2O4 sandwiched between nonmagnetic Al2O3 spacers. The artificial quasi-two-dimensional material composed of three triangle and one kagome atomic planes shows a degree of magnetic frustration which is almost two orders of magnitude enlarged compared to the bulk crystals. Combined resonant X-ray absorption and torque magnetometry measurements confirm that the designer system exhibits no sign of spin ordering down to 30 mK, implying a possible realization of a quantum spin liquid state in the two dimensional limit.

Tailoring transport properties of strongly correlated electron systems in a controlled fashion counts among the dreams of materials scientists. In copper oxides, varying the carrier concentration is a tool to obtain high-temperature superconducting phases. In manganites, doping results in exotic physics such as insulator-metal transitions (IMT), colossal magnetoresistance (CMR), orbital- or charge-ordered (CO) or charge-disproportionate (CD) states. In most oxides, antiferromagnetic order and charge-disproportionation are asssociated with insulating behavior. Here we report the realization of a unique physical state that can be induced by Mo doping in LaFeO$_3$: the resulting metallic state is a site-selective Mott insulator where itinerant electrons evolving in low-energy Mo states coexist with localized carriers on the Fe sites. In addition, a local breathing-type lattice distortion induces charge disproportionation on the latter, without destroying the antiferromagnetic order. A state, combining antiferromangetism, metallicity and CD phenomena is rather rare in oxides and may be of utmost significance for future antiferromagnetic memory devices., Comment: 8 pages, 4 figures

We study the optical properties of the Ruddlesden-Popper series of iridates Sr$_{n+1}$Ir$_n$O$_{3n+1}$ ($n$=1, 2 and $\infty$) by solving the Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and screened interactions $W$ are obtained by the $GW$ approximation including spin-orbit coupling. The computed optical conductivity spectra show strong excitonic effects and reproduce very well the experimentally observed double-peak structure, in particular for the spin-orbital Mott insulators Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$. However, $GW$ does not account well for the correlated metallic state of SrIrO$_3$ owing to a much too small band renormalization, and this affects the overall quality of the optical conductivity. Our analysis describes well the progressive redshift of the main optical peaks as a function of dimensionality ($n$), which is correlated with the gradual decrease of the electronic correlation (quantified by the constrained random phase approximation) towards the metallic $n=\infty$ limit. We have also assessed the quality of a computationally cheaper BSE approach that is based on a model dielectric function and conducted on top of DFT+$U$ one-electron energies. Unfortunately, this model BSE approach does not accurately reproduce the outcome of the full $GW$+BSE method and leads to larger deviations to the measured spectra., Comment: 13 pages, 8 figures

What is so unique in TlCuCl3 which drives so many unique magnetic features in this compound? To study these properties, here we employ a combination of ab-initio band structure, tight-binding model, and an effective quantum field theory. Within a density-functional theory (DFT) calculation, we find an unexpected bulk Dirac cone without spin-orbit coupling (SOC). Tracing back to its origin, we identify, for the first time, the presence of a Su-Schrieffer-Heeger (SSH) like dimerized Cu chain lying in the 3D crystal structure. The SSH chain, combined with SOC, stipulates an anisotropic 3D Dirac cone where chiral and helical states are intertwined. As a Heisenberg interaction is introduced, we show that the dimerized Cu sublattices of the SSH chain condensate into spin-singlet, dimerized magnets. In the magnetic ground state, we also find a topological phase, distinguished by the axion angle. Finally, to study how the topological axion term couples to magnetic excitations, we derive a Chern-Simons-Ginzburg-Landau action from the 3D SSH Hamiltonian. We find that axion term provides an additional mass term to the Higgs mode, and a lifetime to paramagnons, which are independent of the quantum critical physics. The axion-Higgs interplay can be probed with electric and magnetic field applied parallel or anti-parallel to each other., Comment: 10 pages, 5 figures

Extremely large magnetoresistance (XMR), observed in transition metal dichalcogendies, WTe$_2$, has attracted recently a great deal of research interests as it shows no sign of saturation up to the magnetic field as high as 60 T, in addition to the presence of type-II Weyl fermions. Currently, there has been a lot of discussion on the role of band structure changes on the temperature dependent XMR in this compound. In this contribution, we study the band structure of WTe$_2$ using angle-resolved photoemission spectroscopy (ARPES) and first-principle calculations to demonstrate that the temperature dependent band structure has no substantial effect on the temperature dependent XMR as our measurements do not show band structure changes on increasing the sample temperature between 20 and 130 K. We further observe an electronlike surface state, dispersing in such a way that it connects the top of bulk holelike band to the bottom of bulk electronlike band. Interestingly, similar to bulk states, the surface state is also mostly intact with the sample temperature. Our results provide invaluable information in shaping the mechanism of temperature dependent XMR in WTe$_2$., Comment: 7 pages, 3 figures. arXiv admin note: text overlap with arXiv:1705.07217