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We present a comprehensive study of the structural, magnetic, and thermodynamic properties, as well as the adiabatic demagnetization refrigeration (ADR) performance of NaGdP$_2$O$_7$. Although NaGdP$_2$O$_7$ exhibits antiferromagnetic ordering at a N\'eel temperature of $T_{\rm N} = 570$ mK in zero field, ADR experiments achieved a minimum temperature of 220 mK starting from $T = 2$ K under an applied magnetic field of $\mu_0H = 5$ T. The warm-up time back to $T = 2$ K exceeds 60 hours, which is roughly 50 times longer than that of its Yb-based analogue, underscoring the potential of NaGdP$_2$O$_7$ as an efficient precooling stage in double-stage ADR systems. We show that NaGdP$_2$O$_7$ can be seen as a network of ferromagnetic spin chains with antiferromagnetic interchain couplings and also investigate the influence of antiferromagnetic ordering on the magnetic entropy. We find that the temperature dependence of the entropy plays a more dominant role than its magnetic field dependence in the magnetically ordered state., Comment: 10 pages, 6 figures

The synergy of strong spin-orbit coupling and electron-electron interactions gives rise to unconventional topological states, such as topological Mott insulator, Weyl semimetal, and quantum spin liquid. In this study, we have grown single crystals of the pyrochlore iridate Pr$_2$Ir$_2$O$_7$ and explored its magnetic, lattice dynamical, and electronic properties. While Raman spectroscopy data reveal six phonon modes confirming the cubic \textit{Fd$\bar{3}$m} crystal symmetry, dc magnetic susceptibility data show no anomalies and hence indicate the absence of magnetic phase transitions down to 2~K. Both temperature-dependent electric transport and optical conductivity data reveal the metallic character of Pr$_2$Ir$_2$O$_7$. The optical conductivity spectrum contains a mid-infrared absorption band, which becomes more pronounced with decreasing temperature due to spectral weight transfer from high to low energies. The presence of the mid-infrared band hints at the importance of correlation physics. The optical response furthermore suggests that Pr$_2$Ir$_2$O$_7$ is close to the Weyl semimetal phase., Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev. B

The manifold of energetically degenerate configurations arising from competing interactions in frustrated magnets gives rise to an enhanced entropy at lowest temperatures, which can be utilized for adiabatic demagnetization refrigeration (ADR). We review structural and magnetic properties of various Yb- and Gd-based oxides featuring frustration related to different triangular moment configurations and (in some cases) structural randomness. In comparison to paramagnetic hydrated salts, which have traditionally been employed for mK-ADR, these novel ADR materials enable cooling to temperatures several times lower than the magnetic interaction strength, significantly enhancing the entropy density and cooling power at a given target temperature. A further advantage is their chemical stability, allowing for a much simpler ADR pill design and ultra-high vacuum applications. For the temperature range between 0.02 and 2 K, a systematic comparison of the field-induced entropy density change is provided, that illustrates the advantages of frustrated magnets for low-temperature ADR., Comment: 19 pages, 8 figures

The performance of permanent magnets is intricately tied to their magnetic hysteresis loop. In this study, we investigate the heavy-fermion ferromagnet CeAgSb$_2$ through magnetization measurements under uniaxial stress. We observe a 2400 % increase in magnetic coercivity with just a modest stress of approximately 1 kbar. This effect persists even after pressure release, attributable to stress-induced defects that efficiently pin domain walls. Other magnetic properties such as ordering temperature and saturation moment exhibit only weak pressure dependencies and display full reversibility. Our findings offer a promising route for increasing coercive field strength and enhancing the energy product in ferromagnetic materials and are potentially applicable to a broad spectrum of commercial or emerging magnetic applications., Comment: Main text: 7 pages, 3 figures. Supplemental material: 3 pages, 4 figures

We use magnetometry, calorimetry, and high-resolution capacitive dilatometry, as well as single-crystal neutron diffraction to explore temperature-field phase diagram of the anisotropic honeycomb magnet BaCo$_2$(AsO$_4)_2$. Our data reveal four distinct ordered states observed for in-plane magnetic fields. Of particular interest is the narrow region between 0.51 and 0.55 T that separates the up-up-down order from the fully polarized state and coincides with the field range where signatures of the spin-liquid behavior have been reported. We show that magnetic Bragg peaks persist in this intermediate phase, thus ruling out its spin-liquid nature. However, the simultaneous nonmonotonic evolution of nuclear Bragg peaks suggests the involvement of the lattice, witnessed also in other regions of the phase diagram where large changes in the sample length are observed upon entering the magnetically ordered states. Our data highlight the importance of lattice effects in BaCo$_2$(AsO$_4)_2$.

We present electrical resistivity measurements on the prototypical heavy-fermion metal YbRh$_{2}$Si$_{2}$ (YRS) under $a$-axis tensile and compressive strain and focus on the evolution of the resistivity maximum near 136~K that arises from the interplay of the Kondo effect and the crystal electric field (CEF) splitting. While compressive strain reduces $T_{\rm max}$, similar as previously reported for hydrostatic pressure, $T_{\rm max}$ is enhanced up to 145~K for 0.13\% tensile strain. Model calculations for the strain effect on CEF splitting in YRS reveal a negligible shift of the levels. Instead, the enhancement of the resistivity maximum indicates a 20\% increase of the Kondo temperature. This opens the perspective to access the hidden zero-field QCP in pure YRS.

Honeycomb Na$_2$IrO$_3$ is a prototype spin-orbit Mott insulator and Kitaev magnet. We report a combined structural and electrical resistivity study of Na$_2$IrO$_3$ single crystals. Laue back-scattering diffraction indicates twinning with $\pm 120^\circ$ rotation around the $c^*$-axis while scanning electron microscopy displays nanothin lines parallel to all three b-axis orientations of twin domains. Energy dispersive x-ray analysis line-scans across such domain walls indicate no change of the Ir signal intensity, i.e. intact honeycomb layers, while the Na intensity is reduced down to $\sim 2/3$ of its original value at the domain walls, implying significant hole doping. Utilizing focused-ion-beam micro-sectioning, the temperature dependence of the electrical resistance of individual domain walls is studied. It demonstrates the tuning through the metal-insulator transition into a correlated-metal ground state by increasing hole doping., Comment: 8 pages, 9 figures

Manipulating the size and orientation of quantum materials is often used to tune emergent phenomena, but precise control of these parameters is also necessary from an experimental point of view. Various synthesis techniques already exist, such as epitaxial thin film growth and chemical etching, that are capable of producing specific sample dimensions with high precision. However, certain materials exist as single crystals that are often difficult to manipulate, thereby limiting their studies to a certain subset of experimental techniques. One particular class of these materials are the lithium and sodium iridates that are promising candidates for hosting a Kitaev quantum spin liquid state. Here we present a controlled method of using a focused ion beam at grazing incidence to reduce the size of a $\beta$-Li$_2$IrO$_3$ single crystal to a thickness of 1 $\mu m$. Subsequent x-ray diffraction measurements show the lattice remains intact, albeit with a larger mosaic spread. The integrity of the magnetic order is also preserved as the temperature dependent magnetic diffraction peak follows the same trend as its bulk counterpart with a transition temperature at TN = 37.5 K. Our study demonstrates a technique that opens up the possibility of nonequilibrium experiments where submicron thin samples are often essential.

The spin-reversal in dilute Li$_2$(Li$_{1-x}$Fe$_{x}$)N with $x < 1$ % is dominated by resonant quantum tunneling of spatially well-separated states. We report on the effect of finite couplings between those states that give rise to cooperative, simultaneous quantum tunneling of two spins. This phenomenon, known as spin-spin cross relaxation, effectively elucidates the fine-structure observed in isothermal magnetization loops, a previously unresolved aspect. Temperature and field-dependent magnetization measurements were conducted over a range from T = 2 K to 300 K in applied fields of up to $\mu_0H$ = 7 T. Magnetic dipole fields are computed numerically. Our findings affirm the absence of stoichiometric defects in Li$_2$(Li$_{1-x}$Fe$_{x}$)N and underscore its exemplary suitability as a model system for investigating spin-reversal processes at the microscopic level. This is attributed to its comparatively simple crystal structure, the availability of large single crystals, elevated characteristic energies, and well-defined energy levels, Comment: 9 pages, 7 figures

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

We delineate quantum magnetism in the strongly spin-orbit coupled, distorted honeycomb-lattice antiferromagnet BiYbGeO$_{5}$. Our magnetization and heat capacity measurements reveal that its low-temperature behavior is well described by an effective $J_{\rm eff}=1/2$ Kramers doublet of Yb$^{3+}$. The ground state is nonmagnetic with a tiny spin gap. Temperature-dependent magnetic susceptibility, magnetization isotherm, and heat capacity could be modeled well assuming isolated spin dimers with anisotropic exchange interactions $J_{\rm Z} \simeq 2.6$~K and $J_{\rm XY} \simeq 1.3$~K. Heat capacity measurements backed by muon spin relaxation suggest the absence of magnetic long-range order down to at least 80\,mK both in zero field and in applied fields. This sets BiYbGeO$_5$ apart from Yb$_2$Si$_2$O$_7$ with its unusual regime of magnon Bose-Einstein condensation and suggests negligible interdimer couplings, despite only a weak structural deformation of the honeycomb lattice., Comment: 6 pages, 5 figures

We explore the effect of heat treatment in argon atmosphere under various temperatures up to $500^\circ$C on single crystals of $\alpha$-RuCl$_3$ by study of the mass loss, microprobe energy dispersive x-ray spectroscopy, powder x-ray diffraction, electrical resistance as well as low-temperature magnetic susceptibility and specific heat. Clear signatures of dechlorination and oxidation of Ru appear for annealing temperatures beyond $300^\circ$C. Analysis of the specific heat below 2~K reveals a RuO$_2$ mass fraction of order $1\%$ for pristine $\alpha$-RuCl$_3$ which increases up to $20\%$ after thermal annealing, fully consistent with mass-loss analysis. The small RuO$_2$ inclusions drastically reduce the global electrical resistance and may thus significantly affect low-temperature thermal transport and Hall effect., Comment: 7 pages, 7 figures

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

Crystal structure, thermodynamic properties, and adiabatic demagnetization refrigeration (ADR) effect in the spin-7/2 triangular antiferromagnet KBaGd(BO3)2 are reported. With the average nearest-neighbor exchange coupling of 44 mK, this compound shows magnetic order below $T_N = 263$\,mK in zero field. The ADR tests reach the temperature of $T_{min} = 122$ mK, more than twice lower than $T_N$, along with the entropy storage capacity of 192 mJK$^{-1}$cm$^{-3}$ and the hold time of more than 8 hours in the PPMS setup, both significantly improved compared to the spin-1/2 Yb$^{3+}$ analog. We argue that KBaGd(BO3)2 shows a balanced interplay of exchange and dipolar couplings that together with structural randomness and geometrical frustration shift $T_{min}$ to well below the ordering temperature $T_N$, therefore facilitating the cooling., Comment: 8 pages, 6 figures, minor changes to the previous version, accepted for publication in Phys. Rev. B

Spin-orbit coupling in heavy 5$d$ metal oxides, in particular, iridates have received tremendous interest in recent years due to the realization of exotic electronic and magnetic phases. Here, we report the synthesis, structural, magnetic, thermodynamic, and optical properties of the ternary iridate Pr$_3$IrO$_7$. Single crystals of Pr$_3$IrO$_7$ have been grown by the KF flux method. Structural analysis shows that Pr$_3$IrO$_7$ crystallizes in an orthorhombic phase with $Cmcm$ symmetry. The electron energy loss spectroscopy study indicates that Pr is in a 3+ valence state, which implies a 5+ oxidation state of Ir. Magnetization data measured at high and low magnetic fields do not exhibit any bifurcation between $M_{ZFC}$ and $M_{FC}$, however, a weak hump in $M(T)$ is observed at $T^*$$\sim$10.4~K. The specific heat data reveal two maxima at $\sim$253 K and $\sim$4.8 K. The optical conductivity $\sigma_1(\omega)$ spectrum shows 24 infrared-active phonon modes and reveals an insulating behavior with an optical gap $\Delta_{OP}$ of size $\sim$500~meV. During cooling down, the temperature-dependent reflectivity spectrum reveals eight extra phonon modes below the structural phase transition ($\sim$ 253 K). An anomaly is observed at around $T^*$ in the temperature evolution of infrared-active mode frequencies suggesting the presence of significant spin-phonon coupling in the system., Comment: 18 pages, 8 figures, to appear in J. Phys.: Condens. Matter

Case studies of $1T$-TiSe$_2$ and YBa$_2$Cu$_3$O$_{7-\delta}$ have demonstrated that x-ray diffraction (XRD) studies can be used to trace even subtle structural phase transitions which are inherently connected with the onset of superconductivity in these benchmark systems. Yet, the utility of XRD in the investigation of superconductors like MgB$_2$ lacking an additional symmetry-breaking structural phase transition is not immediately evident. Even though, high-resolution powder XRD experiments on MgB$_2$ in combination with maximum entropy method (MEM) analyses hinted at differences between the electron density distributions at room temperature and 15K, i.e. below the $T_c$ of approx. 39K. The high-resolution single-crystal XRD experiments in combination with multipolar refinements presented here can reproduce these results, but show that the observed temperature-dependent density changes are almost entirely due to a decrease of atomic displacement parameters as a natural consequence of reduced thermal vibration amplitude with decreasing temperature. Our investigations also shed new light on the presence or absence of magnesium vacancies in MgB$_2$ samples $-$ a defect type claimed to control the superconducting properties of the compound. We propose that previous reports on the tendency of MgB$_2$ to form non-stoichiometric Mg$_{1-x}$B$_2$ phases ($1 - x \sim$0.95) during high-temperature (HT) synthesis might result from the interpretation of XRD data of insufficient resolution and/or usage of inflexible refinement models., Comment: 41 pages, 5 figures, Supporting Information available

Geometric frustration inhibits magnetic systems from ordering, opening a window to unconventional phases of matter. The paradigmatic frustrated lattice in three dimensions to host a spin liquid is the pyrochlore, although there remain few experimental compounds thought to realize such a state. Here we go beyond the pyrochlore via molecular design in the metal-azolate framework [Mn(II)(ta)$_2$], which realizes a closely related centred pyrochlore lattice of Mn-spins with $S=5/2$. Despite a Curie-Weiss temperature of $-21$ K indicating the energy scale of magnetic interactions, [Mn(II)(ta)$_2$] orders at only 430 mK, putting it firmly in the category of highly frustrated magnets. Comparing magnetization and specific heat measurements to numerical results for a minimal Heisenberg model, we predict that this material displays distinct features of a classical spin liquid with a structure factor reflecting Coulomb physics in the presence of charges., Comment: Main: 6 pages, 3 figures. Supplementary: 15 pages, 12 figures. Updated Version

We present magnetization measurements carried out on polycrystalline and single-crystalline samples of $\alpha$-Li$_2$IrO$_3$ under hydrostatic pressures up to 2 GPa and establish the temperature-pressure phase diagram of this material. The N\'eel temperature ($T_{\rm{N}}$) of $\alpha$-Li$_2$IrO$_3$ is slightly enhanced upon compression with $dT_{\rm{N}}/dp$ = 1.5 K/GPa. Above 1.2 GPa, $\alpha$-Li$_2$IrO$_3$ 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 $T_{\rm{N}}$ 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 $\Gamma$. Our study reveals a common thread in the interplay of magnetism and dimerization in pressured Kitaev materials., Comment: 8 pages, 7 figures

We present magnetization measurements on polycrystalline $\beta$-Li$_2$IrO$_3$ under hydrostatic pressures up to 3~GPa and construct the temperature-pressure phase diagram of this material. Our data confirm that magnetic order breaks down in a first-order phase transition at $p_{\rm{c}}$ $\approx$ 1.4~GPa and additionally reveal a step-like feature -- magnetic signature of structural dimerization -- that appears at $p_{\rm{c}}$ and shifts to higher temperatures upon further compression. Following the structural study by L. S. I. Veiga et al. [Phys. Rev. B 100, 064104 (2019)], we suggest that a partially dimerized phase with a mixture of magnetic and non-magnetic Ir$^{4+}$ sites develops above $p_{\rm{c}}$. This phase is thermodynamically stable between 1.7 and 2.7~GPa according to our ab initio calculations. It confines the magnetic Ir$^{4+}$ sites to weakly coupled tetramers with a singlet ground state and no long-range magnetic order. Our results rule out the formation of a pressure-induced spin-liquid phase in $\beta$-Li$_2$IrO$_3$ and reveal peculiarities of the magnetism collapse transition in a Kitaev material. We also show that a compressive strain imposed by the pressure treatment of $\beta$-Li$_2$IrO$_3$ enhances signatures of the 100~K magnetic anomaly at ambient pressure., Comment: 10 pages, 10 figures

Perovskite SrRuO$_3$ is a prototypical itinerant ferromagnet which allows interface engineering of its electronic and magnetic properties. We report synthesis and investigation of atomically flat artificial multilayers of SrRuO$_3$ with the spin-orbit semimetal SrIrO$_3$ in combination with band-structure calculations with a Hubbard $U$ term and topological analysis. They reveal an electronic reconstruction and emergence of flat Ru-4d$_{xz}$ bands near the interface, ferromagnetic interlayer coupling and negative Berry-curvature contribution to the anomalous Hall effect. We analyze the Hall effect and magnetoresistance measurements as a function of the field angle from out of plane towards in-plane orientation (either parallel or perpendicular to the current direction) by a two-channel model. The magnetic easy direction is tilted by about $20^\circ$ from the sample normal for low magnetic fields, rotating towards the out-of-plane direction by increasing fields. Fully strained epitaxial growth enables a strong anisotropy of magnetoresistance. An additional Hall effect contribution, not accounted for by the two-channel model is compatible with stable skyrmions only up to a critical angle of roughly $45^\circ$ from the sample normal. Within about $20^\circ$ from the thin film plane an additional peak-like contribution to the Hall effect suggests the formation of a non-trivial spin structure., Comment: to be published in Phys. Rev. B

Li$_2$Sr[MnN]$_2$ single crystals were successfully grown out of Li rich flux. The crystal structure was determined by single crystal X-ray diffraction and revealed almost linear $-$N$-$Mn$-$N$-$Mn$-$ chains as central structural motif. Tetragonal columns of this air and moisture sensitive nitridomanganate were employed for electrical transport, heat capacity, and anisotropic magnetization measurements. Both the electronic and magnetic properties are most remarkable, in particular the linear increase of the magnetic susceptibility with temperature that is reminiscent of underdoped cuprate and Fe-based superconductors. Clear indications for antiferromagnetic ordering at $T_{\rm N} = 290$ K were obtained. Metallic transport behavior is experimentally observed in accordance with electronic band structure calculations., Comment: 12 pages, 7 figures

Generation of very low temperatures has been crucially important for applications and fundamental research, as low-temperature quantum coherence enables operation of quantum computers and formation of exotic quantum states, such as superfluidity and superconductivity. One of the major techniques to reach milli-Kelvin temperatures is adiabatic demagnetization refrigeration (ADR). This method uses almost non-interacting magnetic moments of paramagnetic salts where large distances suppress interactions between the magnetic ions. The large spatial separations are facilitated by water molecules, with a drawback of reduced stability of the material. Here, we show that an H$_2$O-free frustrated magnet KBaYb(BO$_3$)$_2$ can be ideal refrigerant for ADR, achieving at least 22\,mK upon demagnetization under adiabatic conditions. Compared to conventional refrigerants, KBaYb(BO$_3)_2$ does not degrade even under high temperatures and ultra-high vacuum conditions. Further, its frustrated magnetic network and structural randomness enable cooling to temperatures several times lower than the energy scale of magnetic interactions, which is the main limiting factor for the base temperature of conventional refrigerants., Comment: accepted for publication in Communications Materials

We investigate pressure-induced structural changes to the Peierls-type distorted low-temperature phase of the low-dimensional Sc$_3$CoC$_4$ as a possible origin of its pressure-enhanced superconductivity. By means of cryogenic high-pressure x-ray diffraction experiments we could reveal subtle, but significant structural differences between the low-temperature phase at ambient and elevated pressures. We could thus establish the structure of the superconducting phase of the title compound which interestingly still shows the main features of the Peierls-type distorted low-temperature phase. This indicates that in contrast to other low-dimensional materials a suppression of periodic structural distortions is no prerequisite for superconducitivity in the transition metal carbide.

Thermodynamics of the Kitaev honeycomb magnet $\alpha$-RuCl$_3$ is studied for different directions of in-plane magnetic field using measurements of the magnetic Gr\"uneisen parameter $\Gamma_B$ and specific heat $C$. We identify two critical fields $B_c^{\rm AF1}$ and $B_c^{\rm AF2}$ corresponding, respectively, to a transition between two magnetically ordered states and the loss of magnetic order toward a quantum paramagnetic state. The $B_c^{AF2}$ phase boundary reveals a narrow region of magnetic fields where inverse melting of the ordered phase may occur. No additional transitions are detected above $B_c^{\rm AF2}$ for any direction of the in-plane field, although a shoulder anomaly in $\Gamma_B$ is observed systematically at $8-10$ T. Large field-induced entropy effects imply additional low-energy excitations at low fields and/or strongly field-dependent phonon entropies. Our results establish universal features of $\alpha$-RuCl$_3$ in high magnetic fields and challenge the presence of a field-induced Kitaev spin liquid in this material., Comment: 4.5 pages + Supplemental Material

In the iron-pnictide material CeFeAsO not only the Fe moments, but also the local 4f moments of the Ce order antiferromagnetically at low temperatures. We elucidate on the peculiar role of the Ce on the emergence of superconductivity. While application of pressure suppresses the iron SDW ordering temperature monotonously up to 4 GPa, the Ce-4f magnetism is stabilized, until both types of magnetic orders disappear abruptly and a narrow SC dome develops. With further increasing pressure characteristics of a Kondo-lattice system become more and more apparent in the electrical resistivity. This suggests a connection of the emergence of superconductivity with the extinction of the magnetic order and the onset of Kondo-screening of the Ce-4f moments., Comment: 6 pages, 3 figures + supplemental material

We investigated the static and dynamic magnetic properties of the polar ferrimagnet Mn$_2$Mo$_3$O$_8$ in three magnetically ordered phases via magnetization, magnetic torque, and THz absorption spectroscopy measurements. The observed magnetic field dependence of the spin-wave resonances, including Brillouin zone-center and zone-boundary excitations, magnetization, and torque, are well described by an extended two-sublattice antiferromagnetic classical mean-field model. In this orbitally quenched system, the competing weak easy-plane and easy-axis single-ion anisotropies of the two crystallographic sites are determined from the model and assigned to the tetra- and octahedral sites, respectively, by ab initio calculations.

We report the observation of an antipolar phase in cubic GaNb$_4$S$_8$ driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb$_4$S$_4$ molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at $T_\mathrm{JT}=31$\,K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above $T_\mathrm{JT}$. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb$_4$S$_8$ may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferrolectric.

New determination of the magnetic anisotropy from single crystals of (Fe$_{1-x}$Co$_x$)$_2$B alloys are presented. The anomalous temperature dependence of the anisotropy constant is discussed using the standard Callen-Callen theory, which is shown to be insufficient to explain the experimental results. A more material specific study using first-principles calculations with disordered moments approach gives a much more consistent interpretation of the experimental data. Since the intrinsic properties of the alloys with $x=0.3-0.35$ are promising for permanent magnets applications, initial investigation of the extrinsic properties are described, in particular the crystallization of melt spun ribbons with Cu, Al, and Ti additions. Previous attempts at developing a significant hysteresis have been unsuccessful in this system. Our melt-spinning experiment indicates that this system shows rapid crystallization.

Measurements of the magnetic Gr\"uneisen parameter ($\Gamma_B$) and specific heat on the Kitaev material candidate $\alpha$-RuCl$_3$ are used to access in-plane field- and temperature-dependence of the entropy up to 12 T and down to 1 K. No signatures corresponding to phase transitions are detected beyond the boundary of the magnetically ordered region, but only a shoulder-like anomaly in $\Gamma_B$, involving an entropy increment as small as $10^{-5} R\log 2$. These observations put into question the presence of a thermodynamic phase transition between the purported quantum spin liquid and the field-polarized state of $\alpha$-RuCl$_3$. We show theoretically that at low temperatures $\Gamma_B$ is sensitive to crossings in the lowest excitations within gapped phases, and identify the measured shoulder-like anomaly as being of such origin. Exact diagonalization calculations demonstrate that the shoulder-like anomaly can be reproduced in extended Kitaev models that gain proximity to an additional phase at finite field without entering it. We discuss manifestations of this proximity in other measurements., Comment: published version (incl. Supplemental Material)

The excitations in honeycomb $\alpha$-Li$_2$IrO$_3$ have been investigated with high-resolution resonant inelastic x-ray scattering (RIXS) at the O K edge. The low-energy response is dominated by a fully resolved ladder of excitations, which we interpret as being due to multi-phonon processes in the presence of strong electron-phonon coupling (EPC). At higher energies, the orbital excitations are shown to be dressed by phonons. The high quality of the data permits a quantitative test of the analytical model for the RIXS cross-section, which has been proposed to describe EPC in transition metal oxides (TMOs). We find that the magnitude of the EPC is comparable to that found for a range of 3d TMOs. This indicates that EPC may be of equal importance in determining the phenomenology displayed by corresponding 5d based systems., Comment: Accepted in Physical Review B

A remarkably large magnetic anisotropy energy of 305 K is computed by quantum chemistry methods for divalent Fe$^{2+}$ $d^6$ substitutes at Li-ion sites with $D_{6h}$ point-group symmetry within the solid-state matrix of Li$_3$N. This is similar to values calculated by the same approach and confirmed experimentally for linearly coordinated monovalent Fe$^{1+}$ $d^7$ species, among the largest so far in the research area of single-molecule magnets. Our ab initio results therefore mark a new exciting exploration path in the search for superior single-molecule magnets, rooted in the $d_{xy}^{1.5} d_{x^2-y^2}^{1.5} d_{z^2}^1 d_{yz}^1 d_{zx}^1$ configuration of $d^6$ transition-metal ions with linear or quasilinear nearest-neighbor coordination. This $d^6$ axial anisotropy may be kept robust even for symmetries lower than $D_{6h}$, provided the ligand and farther-neighbor environment is engineered such that the $d_{xy}^{1.5} d_{x^2-y^2}^{1.5} d_{z^2}^1 d_{yz}^1 d_{zx}^1$ - $d_{xy}^{1} d_{x^2-y^2}^{1} d_{z^2}^2 d_{yz}^1 d_{zx}^1$ splitting remains large enough., Comment: 5 pages, 1 figure

$^7$Li nuclear magnetic resonance (NMR) and terahertz (THz) spectroscopies are used to probe magnetic excitations and their field dependence in the hyperhoneycomb Kitaev magnet $\beta$-Li$_2$IrO$_3$. Spin-lattice relaxation rate ($1/T_1$) measured down to 100\,mK indicates gapless nature of the excitations at low fields (below $H_c\simeq 2.8$\,T), in contrast to the gapped magnon excitations found in the honeycomb Kitaev magnet $\alpha$-RuCl$_3$ at zero applied magnetic field. At higher temperatures in $\beta$-Li$_2$IrO$_3$, $1/T_1$ passes through a broad maximum without any clear anomaly at the N\'eel temperature $T_N\simeq 38$\,K, suggesting the abundance of low-energy excitations that are indeed observed as two peaks in the THz spectra, both correspond to zone-center magnon excitations. At higher fields (above $H_c$), an excitation gap opens, and a re-distribution of the THz spectral weight is observed without any indication of an excitation continuum, in contrast to $\alpha$-RuCl$_3$ where an excitation continuum was reported., Comment: 6 pages, 4 figures

The state of a sample during crystal growth from high temperature solutions is not accessible in conventional furnace systems. An optimization of the growth parameters often requires arduous trial and error procedures in particular in case of novel multicomponent systems with unknown phase diagrams. Here we present a measurement technique based on lock-in amplification that allows for in-situ detection of the liquidus and solidus temperatures as well as structural phase transitions. A thin, metallic measurement wire is mounted in close vicinity to the melt. Characteristic anomalies in the time-dependent electrical resistivity of this wire allow for the detection of latent heat release without using a reference crucible. The method is implemented in a 'feedback furnace' and enables an adjustment of the temperature profile based on the occurrence or absence of phase transitions. The absolute temperature serves as an additional source of information. Obtained phase transition temperatures are in good agreement with differential thermal analysis (DTA)., Comment: 9 pages, 12 figures, accepted for publication in Crystal Research & Technology

We present a systematic 57Fe-Moessbauer study on highly diluted Fe centers in Li2(Li1-xFex)N single-crystals as a function of temperature and magnetic field applied transverse and longitudinal with respect to the single-ion anisotropy axis. Below 30 K the Fe centers exhibit a giant magnetic hyperfine field of E_A = 70.25(2)T parallel to the axis of strongest electric field gradient Vzz = -154.0(1) V/A2. Fluctuations of the magnetic hyperfine field are observed between 50K and 300K and described by the Blume two-level relaxation model. From the temperature dependence of the uctuation rate an Orbach spin-lattice relaxation process is deduced. An Arrhenius analysis yields a single thermal activation barrier of E_A = 570(6)K and an attempt frequency nu_0 = 309(10) GHz. Moessbauer spectroscopy studies with applied transverse magnetic fields up to 5T reveal a large increase of the uctuation rate by more than one order of magnitude. In longitudinal magnetic fields a splitting of the uctuation rate into two branches is observed consistent with a Zeeman induced modifcation of the energy levels. The experimental observations are qualitatively reproduced by a single-ion effective spin Hamiltonian analysis assuming a Fe1+ d7 charge state with unquenched orbital moment and a J = 7=2 ground state. It is demonstrated that a weak axial single-ion anisotropy D of the order of a few Kelvin can cause a two orders of magnitude larger energy barrier for longitudinal spin fluctuations., Comment: 19 pages, 17 figures

Detailed magnetization, specific heat, and $^7$Li nuclear magnetic resonance (NMR) measurements on single crystals of the hyperhoneycomb Kitaev magnet $\beta$-Li$_2$IrO$_3$ are reported. At high temperatures, {\cred anisotropy of the magnetization is reflected by the different Curie-Weiss temperatures for different field directions}, in agreement with the combination of a ferromagnetic Kitaev interaction ($K$) and a negative off-diagonal anisotropy ($\Gamma$) as two leading terms in the spin Hamiltonian. At low temperatures, magnetic fields applied along $a$ or $c$ have only a weak effect on the system and reduce the N\'eel temperature from 38 K at 0 T to about 35.5 K at 14 T, with no field-induced transitions observed up to 58 T on a powder sample. In contrast, the field applied along $b$ causes a drastic reduction in the $T_N$ that vanishes around $H_c=2.8$ T giving way to a crossover toward a quantum paramagnetic state. $^7$Li NMR measurements in this field-induced state reveal a gradual line broadening and a continuous evolution of the line shift with temperature, suggesting the development of local magnetic fields. The spin-lattice relaxation rate shows a peak around the crossover temperature 40 K and follows power-law behavior below this temperature., Comment: 10 pages, 9 figures; published version

In the quest for realizations of quantum spin liquids, the exploration of Kitaev materials - spin-orbit entangled Mott insulators with strong bond-directional exchanges - has taken center stage. However, in these materials the local spin-orbital j=1/2 moments typically show long-range magnetic order at low temperature, thus defying the formation of a spin-liquid ground state. Using resonant inelastic x-ray scattering (RIXS), we here report on a proximate spin liquid regime with clear fingerprints of Kitaev physics in the magnetic excitations of the honeycomb iridates alpha-Li2IrO3 and Na2IrO3. We observe a broad continuum of magnetic excitations that persists up to at least 300K, more than an order of magnitude larger than the magnetic ordering temperatures. We prove the magnetic character of this continuum by an analysis of the resonance behavior. RIXS measurements of the dynamical structure factor for energies within the continuum show that dynamical spin-spin correlations are restricted to nearest neighbors. Notably, these spectroscopic observations are also present in the magnetically ordered state for excitation energies above the conventional magnon excitations. Phenomenologically, our data agree with inelastic neutron scattering results on the related honeycomb compound RuCl3, establishing a common ground for a proximate Kitaev spin-liquid regime in these materials., Comment: 13 pages, 14 figures

We studied the effect of external pressure on the electrodynamic properties of $\alpha$-Li$_2$IrO$_3$ single crystals in the frequency range of the phonon modes and the Ir $d$-$d$ transitions. The abrupt hardening of several phonon modes under pressure supports the onset of the dimerized phase at the critical pressure $P_c$=3.8 GPa. With increasing pressure an overall decrease in spectral weight of the Ir $d$-$d$ transitions is found up to $P_c$. Above $P_c$, the local (on-site) $d$-$d$ excitations gain spectral weight with increasing pressure, which hints at a pressure-induced increase in the octahedral distortions. The non-local (intersite) Ir $d$-$d$ transitions show a monotonic blue-shift and decrease in spectral weight. The changes observed for the non-local excitations are most prominent well above $P_c$, namely for pressures $\geq$12 GPa, and only small changes occur for pressures close to $P_c$. The profile of the optical conductivity at high pressures ($\sim$20 GPa) appears to be indicative for the dimerized state in iridates., Comment: 10 pages, 6 figures; accepted for publication in Phys. Rev. B

We use powder x-ray diffraction to study the effect of pressure on the crystal structure of the honeycomb rhodate Li$_2$RhO$_3$. We observe low-pressure ($P$$<$$P_{c1}$ = 6.5 GPa) and high-pressure ($P$$>$$P_{c2}$ = 14 GPa) regions corresponding to the monoclinic $C2/m$ symmetry, while a phase mixture is observed at intermediate pressures. At $P$$>$$P_{c2}$, the honeycomb structure becomes distorted and features short Rh--Rh bonds forming zigzag chains stretched along the crystallographic $a$ direction. This is in contrast to dimerized patterns observed in triclinic high-pressure polymorphs of $\alpha$-Li$_2$IrO$_3$ and $\alpha$-RuCl$_3$. Density-functional theory calculations at various pressure conditions reveal that the observed rhodium zigzag-chain pattern is not expected under hydrostatic pressure but can be reproduced by assuming anisotropic pressure conditions., Comment: 11 pages, 11 figures

LiSr$_2$[CoN$_2$] single crystals were successfully grown out of Li-rich flux. Temperature- and field-dependent measurements of the magnetization in the range of $T = 2 - 300$ K and up to $\mu_{0}\textit{H} = 7$ T as well as measurements of the heat capacity are presented. Ferromagnetic ordering emerges below $T_C = 44$ K and comparatively large coercivity fields of $\mu_0H = 0.3$ T as well as pronounced anisotropy are observed upon cooling. Polycrystalline samples of the Ca analog LiCa$_2$[CoN$_2$] were obtained and investigated in a similar way. In both compounds Co manifests orbital contributions to the magnetic moment and large single-ion anisotropy that is caused by second-order Spin-orbit coupling. Quantum chemistry calculations reveal a magnetic anisotropy energy of 7 meV, twice as large as the values reported for similar Co $d^{8}$ systems., Comment: 21 pages, 6 figures, 5 tables

We present a detailed investigation of the electronic properties of CeFeAsO under chemical (As by P substitution) and hydrostatic pressure by means of in-house and synchrotron M\"ossbauer spectroscopy. The Fe magnetism is suppressed due to both pressures and no magnetic order was observed above a P-substitution level of 40% or 5.2 GPa hydrostatic pressure. We compared both pressures and found that the isovalent As by P substitution change the crystallographic and electronic properties differently than hydrostatic pressure., Comment: supplement is included in the pdf file

The magnetic properties of dilute Li$_2$(Li$_{1-x}$Fe$_x$)N with $x \sim 0.001$ are dominated by the spin of single, isolated Fe atoms. Below $T = 10$ K the spin-relaxation times become temperature-independent indicating a crossover from thermal excitations to the quantum tunneling regime. We report on a strong increase of the spin-flip probability in $\textit{transverse}$ magnetic fields that proves the resonant character of this tunneling process. $\textit{Longitudinal}$ fields, on the other hand, lift the ground-state degeneracy and destroy the tunneling condition. An increase of the relaxation time by four orders of magnitude in applied fields of only a few milliTesla reveals exceptionally sharp tunneling resonances. Li$_2$(Li$_{1-x}$Fe$_x$)N represents a comparatively simple and clean model system that opens the possibility to study quantum tunneling of the magnetization at liquid helium temperatures., Comment: 6 pages, 3 figures, accepted for Phys. Rev. Lett

Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate $\beta$-Li$_2$IrO$_3$ is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation ($\mu$SR) measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The Neel temperature of $\beta$-Li$_2$IrO$_3$ increases with the slope of 0.9 K/GPa upon initial compression, but the reduction in the polarization field $H_c$ reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange $\Gamma$ under pressure., Comment: 4 pages + Supplemental Material (published version)

We have synthesized single crystals of Dirac semimetal candidates AZnBi2 with A=Ba and Sr. In contrast to A=Sr, the Ba material displays a novel local Zn vacancy ordering, which makes the observation of quantum oscillations in out-of-plane magnetic fields possible. As a new Dirac semimetal candidate, BaZnBi2 exhibits small cyclotron electron mass, high quantum mobility, and non-trivial Berry phases. Three Dirac dispersions are observed by ARPES and identified by first- principles band-structure calculations. Compared to AMn(Bi/Sb)2 systems which host Mn magnetic moments, BaZnBi2 acts as non-magnetic analogue to investigate the intrinsic properties of Dirac fermions in this structure family., Comment: To be published in PRB

\alpha-Fe single crystals of rhombic dodecahedral habit were grown from a melt of Li$_{84}$N$_{12}$Fe$_{\sim 3}$. Crystals of several millimeter along a side form at temperatures around $T \approx 800^\circ$C. Upon further cooling the growth competes with the formation of Fe-doped Li$_3$N. The b.c.c. structure and good sample quality of \alpha-Fe single crystals were confirmed by X-ray and electron diffraction as well as magnetization measurements and chemical analysis. A nitrogen concentration of 90\,ppm was detected by means of carrier gas hot extraction. Scanning electron microscopy did not reveal any sign of iron nitride precipitates., Comment: 13 pages, 4 figures

B-site ordered thin films of double perovskite Sr$_2$CoIrO$_6$ were epitaxially grown by a metal-organic aerosol deposition technique on various substrates, actuating different strain states. X-ray diffraction, transmission electron microscopy and polarized far-field Raman spectroscopy confirm the strained epitaxial growth on all used substrates. Polarization dependent Co $L_{2,3}$ X-ray absorption spectroscopy reveals a change of the magnetic easy axis of the antiferromagnetically ordered (high-spin) Co$^{3+}$ sublattice within the strain series. By reversing the applied strain direction from tensile to compressive, the easy axis changes abruptly from in-plane to out-of-plane orientation. The low-temperature magnetoresistance changes its sign respectively and is described by a combination of weak anti-localization and anisotropic magnetoresistance effects., Comment: 19 pages, 29 figures (including supplemental material)

We report on isothermal magnetization, M\"ossbauer spectroscopy, and magnetostriction as well as temperature-dependent alternating-current (ac) susceptibility, specific heat, and thermal expansion of single crystalline and polycrstalline Li$_2$(Li$_{1-x}$Fe$_x$)N with $x = 0$ and $x \approx 0.30$. Magnetic hysteresis emerges at temperatures below $T \approx 50\,$K with coercivity fields of up to $\mu_0H = 11.6\,$T at $T = 2\,$K and magnetic anisotropy energies of $310\,$K ($27\,$meV). The ac susceptibility is strongly frequency dependent ($f\,=\,10$--$10,000\,$Hz) and reveals an effective energy barrier for spin reversal of $\Delta E \approx 1100\,$K. The relaxation times follow Arrhenius behavior for $T > 25\,$K. For $T < 10\,$K, however, the relaxation times of $\tau \approx 10^{10}\,$s are only weakly temperature-dependent indicating the relevance of a quantum tunneling process instead of thermal excitations. The magnetic entropy amounts to more than $25\,$J mol$^{-1}_{\rm Fe}\,$K$^{-1}$ which significantly exceeds $R$ln2, the value expected for the entropy of a ground state doublet. Thermal expansion and magnetostriction indicate a weak magneto-elastic coupling in accordance with slow relaxation of the magnetization. The classification of Li$_2$(Li$_{1-x}$Fe$_x$)N as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior., Comment: 12 pages, 10 figures

Sc$_3$CoC$_4$ is the only superconductor in the group of the isotypic carbides Sc$_3M$C$_4$ ($M$ = Mn, Fe, Ru, Os, Co, Rh, Ir, Ni), rendering it into an ideal benchmark system to systematically study the prerequisites and mechanism of superconductivity in such quasi one-dimensional structures. To investigate the isotope effect, the substitution series Sc$_3$Co($^{12}$C$_{1-x}$$^{13}$C$_x$)$_4$ with $x$ = 0, 0.5 and 1 was synthesized by arc melting. The sample homogeneity was confirmed by powder X-ray diffraction and NMR spectroscopy. The resulting isotope coefficient based on magnetization studies on polycrystalline samples and electrical resistivity measurements on single crystals of $\alpha$ = 0.58 lies close to the value of 0.5 predicted by BCS theory., Comment: 16 pages, 5 figures, 2 tables

Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate $\alpha$-Li$_{2}$IrO$_{3}$ reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of $P_{c}$ = 3.8 GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the $ab$ plane and thereby the Ir hexagon network, leading to the formation of Ir--Ir dimers. These observations are independently predicted and corroborated by our \textit{ab initio} density functional theory calculations where we find that the appearance of Ir--Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at $P_{c}$ the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices $A_{2}$$M$O$_{3}$ with $A$= Li, Na and $M$ = Ir, Ru., Comment: 6 pages, 2 figures