Spin-orbit exciton-induced phonon chirality in a quantum magnet.

The interplay of charge, spin, lattice, and orbital degrees of freedom in correlated materials often leads to rich and exotic properties. Recent studies have brought new perspectives to bosonic collective excitations in correlated materials. For example, inelastic neutron scattering experiments revealed non-trivial band topology for magnons and spin-orbit excitons (SOEs) in a quantum magnet CoTiO3 (CTO). Here, we report phonon properties resulting from a combination of strong spin-orbit coupling, large crystal field splitting, and trigonal distortion in CTO. Specifically, the interaction between SOEs and phonons endows chirality to two [Formula: see text] phonon modes and leads to large phonon magnetic moments observed in magneto-Raman spectra. The remarkably strong magneto-phononic effect originates from the hybridization of SOEs and phonons due to their close energy proximity. While chiral phonons have been associated with electronic topology in some materials, our work suggests opportunities may arise by exploring chiral phonons coupled to topological bosons.

High ambipolar mobility in cubic boron arsenide.

Semiconductors with high thermal conductivity and electron-hole mobility are of great importance for electronic and photonic devices as well as for fundamental studies. Among the ultrahigh-thermal conductivity materials, cubic boron arsenide (c-BAs) is predicted to exhibit simultaneously high electron and hole mobilities of >1000 centimeters squared per volt per second. Using the optical transient grating technique, we experimentally measured thermal conductivity of 1200 watts per meter per kelvin and ambipolar mobility of 1600 centimeters squared per volt per second at the same locations on c-BAs samples at room temperature despite spatial variations. Ab initio calculations show that lowering ionized and neutral impurity concentrations is key to achieving high mobility and high thermal conductivity, respectively. The high ambipolar mobilities combined with the ultrahigh thermal conductivity make c-BAs a promising candidate for next-generation electronics.

Linear-in temperature resistivity from an isotropic Planckian scattering rate.

A variety of 'strange metals' exhibit resistivity that decreases linearly with temperature as the temperature decreases to zero1-3, in contrast to conventional metals where resistivity decreases quadratically with temperature. This linear-in-temperature resistivity has been attributed to charge carriers scattering at a rate given by h/τ = αkBT, where α is a constant of order unity, h is the Planck constant and kB is the Boltzmann constant. This simple relationship between the scattering rate and temperature is observed across a wide variety of materials, suggesting a fundamental upper limit on scattering-the 'Planckian limit'4,5-but little is known about the underlying origins of this limit. Here we report a measurement of the angle-dependent magnetoresistance of La1.6-xNd0.4SrxCuO4-a hole-doped cuprate that shows linear-in-temperature resistivity down to the lowest measured temperatures6. The angle-dependent magnetoresistance shows a well defined Fermi surface that agrees quantitatively with angle-resolved photoemission spectroscopy measurements7 and reveals a linear-in-temperature scattering rate that saturates at the Planckian limit, namely α = 1.2 ± 0.4. Remarkably, we find that this Planckian scattering rate is isotropic, that is, it is independent of direction, in contrast to expectations from 'hotspot' models8,9. Our findings suggest that linear-in-temperature resistivity in strange metals emerges from a momentum-independent inelastic scattering rate that reaches the Planckian limit.

Unraveling the Orbital Physics in a Canonical Orbital System KCuF_{3}.

We explore the existence of the collective orbital excitations, orbitons, in the canonical orbital system KCuF_{3} using the Cu L_{3}-edge resonant inelastic x-ray scattering. We show that the nondispersive high-energy peaks result from the Cu^{2+} dd orbital excitations. These high-energy modes display good agreement with the ab initio quantum chemistry calculation, indicating that the dd excitations are highly localized. At the same time, the low-energy excitations present clear dispersion. They match extremely well with the two-spinon continuum following the comparison with Müller ansatz calculations. The localized dd excitations and the observation of the strongly dispersive magnetic excitations suggest that the orbiton dispersion is below the resolution detection limit. Our results can reconcile with the strong local Jahn-Teller effect in KCuF_{3}, which predominantly drives orbital ordering.

Dimer rattling mode induced low thermal conductivity in an excellent acoustic conductor.

A solid with larger sound speeds usually exhibits higher lattice thermal conductivity. Here, we report an exception that CuP2 has a quite large mean sound speed of 4155 m s-1, comparable to GaAs, but single crystals show very low lattice thermal conductivity of about 4 W m-1 K-1 at room temperature, one order of magnitude smaller than GaAs. To understand such a puzzling thermal transport behavior, we have thoroughly investigated the atomic structures and lattice dynamics by combining neutron scattering techniques with first-principles simulations. This compound crystallizes in a layered structure where Cu atoms forming dimers are sandwiched in between P atomic networks. In this work, we reveal that Cu atomic dimers vibrate as a rattling mode with frequency around 11 meV, which is manifested to be remarkably anharmonic and strongly scatters acoustic phonons to achieve the low lattice thermal conductivity.

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-Tc superconductivity. RE trihydrides (REH3) generally undergo a phase transition from ambient P63/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH3 (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH4, YH6, YH9, and CeH9 with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH3 have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH3 up to 220 GPa. Experiments observed the Fm3̅m phase of DyH3 to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH3 recovered under ambient conditions. Our calculations predicted a series of phases for DyH3 at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P63/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH3, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH3 is systematically compared with other REH3 compounds at high pressures. The phase transition pressure in REH3 shows an inverse relation with the ionic radius of RE atoms.

Revealing the Intrinsic Electronic Structure of 3D Half-Heusler Thermoelectric Materials by Angle-Resolved Photoemission Spectroscopy.

Accurate determination of the intrinsic electronic structure of thermoelectric materials is a prerequisite for utilizing an electronic band engineering strategy to improve their thermoelectric performance. Herein, with high-resolution angle-resolved photoemission spectroscopy (ARPES), the intrinsic electronic structure of the 3D half-Heusler thermoelectric material ZrNiSn is revealed. An unexpectedly large intrinsic bandgap is directly observed by ARPES and is further confirmed by electrical and optical measurements and first-principles calculations. Moreover, a large anisotropic conduction band with an anisotropic factor of 6 is identified by ARPES and attributed to be one of the most important reasons leading to the high thermoelectric performance of ZrNiSn. These successful findings rely on the grown high-quality single crystals, which have fewer Ni interstitial defects and negligible in-gap states on the electronic structure. This work demonstrates a realistic paradigm to investigate the electronic structure of 3D solid materials by using ARPES and provides new insights into the intrinsic electronic structure of the half-Heusler system benefiting further optimization of thermoelectric performance.

Synthesis of clathrate cerium superhydride CeH9 at 80-100 GPa with atomic hydrogen sublattice.

Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 at 80-100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH9. CeH9 crystallizes in a P63/mmc clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.

Pressure-induced phase transitions and superconductivity in a quasi-1-dimensional topological crystalline insulator α-Bi4Br4.

Great progress has been achieved in the research field of topological states of matter during the past decade. Recently, a quasi-1-dimensional bismuth bromide, Bi4Br4, has been predicted to be a rotational symmetry-protected topological crystalline insulator; it would also exhibit more exotic topological properties under pressure. Here, we report a thorough study of phase transitions and superconductivity in a quasihydrostatically pressurized α-Bi4Br4 crystal by performing detailed measurements of electrical resistance, alternating current magnetic susceptibility, and in situ high-pressure single-crystal X-ray diffraction together with first principles calculations. We find a pressure-induced insulator-metal transition between ∼3.0 and 3.8 GPa where valence and conduction bands cross the Fermi level to form a set of small pockets of holes and electrons. With further increase of pressure, 2 superconductive transitions emerge. One shows a sharp resistance drop to 0 near 6.8 K at 3.8 GPa; the transition temperature gradually lowers with increasing pressure and completely vanishes above 12.0 GPa. Another transition sets in around 9.0 K at 5.5 GPa and persists up to the highest pressure of 45.0 GPa studied in this work. Intriguingly, we find that the first superconducting phase might coexist with a nontrivial rotational symmetry-protected topology in the pressure range of ∼3.8 to 4.3 GPa; the second one is associated with a structural phase transition from monoclinic C2/m to triclinic P-1 symmetry.

Exceptional oxygen evolution reactivities on CaCoO3 and SrCoO3.

We investigated the roles of covalent bonding, separation of surface oxygen, and electrolyte pH on the oxygen evolution reaction (OER) on transition metal oxides by comparing catalytic onset potentials and activities of CaCoO3 and SrCoO3. Both cubic, metallic perovskites have similar CoIV intermediate spin states and onset potentials, but a substantially smaller lattice parameter and shorter surface oxygen separation make CaCoO3 a more stable catalyst with increased OER activity. The onset potentials are similar, occurring where H+ is removed from surface -OH-, but two competing surface reactions determine the catalytic activity. In one, the surface -O- is attacked by electrolyte OH- to form the surface -OOH-; in the other, two -O- form a surface peroxide ion and an oxygen vacancy with electrolyte OH- attacking the oxygen vacancy. The second pathway can be faster if the surface oxygen separation is smaller.

Coupling of Spinons with Defects and Phonons in the Spin Chain Compound Ca_{2}CuO_{3}.

Extrinsic spinon scattering by defects and phonons instead of intrinsic spinon-spinon coupling is responsible for resistive magnetic heat transport in one-dimensional (1D) quantum magnets. Here we report an investigation of the elusive extrinsic effect in the 1D Heisenberg S=1/2 spin chain compound Ca_{2}CuO_{3}, where the defect concentration is determined from the measured specific heat and first-principles calculations are used to separate the lattice component of the measured thermal conductivity to isolate a large magnetic contribution (κ_{m}). The obtained temperature-dependent spinon-defect and spinon-phonon mean free paths can enable a quantitative understanding of both κ_{m} and the spinon-induced spin Seebeck effect.

Thermal Conductivity Enhancement in MoS_{2} under Extreme Strain.

Because of their weak interlayer bonding, van der Waals (vdW) solids are very sensitive to external stimuli such as strain. Experimental studies of strain tuning of thermal properties in vdW solids have not yet been reported. Under ∼9% cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed an increase of cross-plane thermal conductivity in bulk MoS_{2} from 3.5 to about 25 W m^{-1} K^{-1}, measured with a picosecond transient thermoreflectance technique. First-principles calculations and coherent phonon spectroscopy experiments reveal that this drastic change arises from the strain-enhanced interlayer interaction, heavily modified phonon dispersions, and decrease in phonon lifetimes due to the unbundling effect along the cross-plane direction. The contribution from the change of electronic thermal conductivity is negligible. Our results suggest possible parallel tuning of structural, thermal, and electrical properties of vdW solids with strain in multiphysics devices.

Pressure-induced phase transitions and superconductivity in a black phosphorus single crystal.

We report a thorough study of the transport properties of the normal and superconducting states of black phosphorus (BP) under magnetic field and high pressure with a large-volume apparatus that provides hydrostatic pressure to induce transitions from the layered A17 phase to the layered A7 phase and to the cubic phase of BP. Quantum oscillations can be observed at P ≥ 1 GPa in both resistivity and Hall voltage, and their evolutions with pressure in the A17 phase imply a continuous enlargement of Fermi surface. A significantly large magnetoresistance (MR) at low temperatures is observed in the A7 phase that becomes superconducting below a superconducting transition temperature Tc ∼ 6-13 K. Tc increases continuously with pressure on crossing the A7 to the cubic phase boundary. The strong MR effect can be fit by a modified Kohler's rule. A correlation between Tc and fitting parameters suggests that phonon-mediated interactions play dominant roles in driving the Cooper pairing, which is further supported by our density functional theory (DFT) calculations. The change of effective carrier mobility in the A17 phase under pressure derived from the MR effect is consistent with that obtained from the temperature dependence of the quantum oscillations. In situ single-crystal diffraction under high pressure indicates a total structural reconstruction instead of simple stretching of the A17 phase layers in the A17-to-A7-phase transition. This finding helps us to interpret transport properties on crossing the phase transition under high pressure.

Unusual high thermal conductivity in boron arsenide bulk crystals.

Conventional theory predicts that ultrahigh lattice thermal conductivity can only occur in crystals composed of strongly bonded light elements, and that it is limited by anharmonic three-phonon processes. We report experimental evidence that departs from these long-held criteria. We measured a local room-temperature thermal conductivity exceeding 1000 watts per meter-kelvin and an average bulk value reaching 900 watts per meter-kelvin in bulk boron arsenide (BAs) crystals, where boron and arsenic are light and heavy elements, respectively. The high values are consistent with a proposal for phonon-band engineering and can only be explained by higher-order phonon processes. These findings yield insight into the physics of heat conduction in solids and show BAs to be the only known semiconductor with ultrahigh thermal conductivity.

Large positive linear magnetoresistance in the two-dimensional t 2g electron gas at the EuO/SrTiO3 interface.

The development of novel nano-oxide spintronic devices would benefit greatly from interfacing with emergent phenomena at oxide interfaces. In this paper, we integrate highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy results in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70 K of EuO, this spin-polarized two-dimensional t 2g electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance (MR). Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the t 2g nature of the carriers. First principles calculations strongly suggest that Zeeman splitting, caused by proximity magnetism and oxygen vacancies in SrTiO3, is responsible for the MR. This system offers an as-yet-unexplored route to pursue proximity-induced effects in the oxide two-dimensional t 2g electron gas.

New Mechanism for Ferroelectricity in the Perovskite Ca2-xMnxTi2O6 Synthesized by Spark Plasma Sintering.

Perovskite oxides hosting ferroelectricity are particularly important materials for modern technologies. The ferroelectric transition in the well-known oxides BaTiO3 and PbTiO3 is realized by softening of a vibration mode in the cubic perovskite structure. For most perovskite oxides, octahedral-site tilting systems are developed to accommodate the bonding mismatch due to a geometric tolerance factor t = (A-O)/[√2(B-O)] < 1. In the absence of cations having lone-pair electrons, e.g., Bi3+ and Pb2+, all simple and complex A-site and B-site ordered perovskite oxides with a t < 1 show a variety of tilting systems, and none of them become ferroelectric. The ferroelectric CaMnTi2O6 oxide is, up to now, the only one that breaks this rule. It exhibits a columnar A-site ordering with a pronounced octahedral-site tilting and yet becomes ferroelectric at Tc ≈ 650 K. Most importantly, the ferroelectricity at T < Tc is caused by an order-disorder transition instead of a displacive transition; this character may be useful to overcome the critical thickness problem experienced in all proper ferroelectrics. Application of this new ferroelectric material can greatly simplify the structure of microelectronic devices. However, CaMnTi2O6 is a high-pressure phase obtained at 7 GPa and 1200 °C, which limits its application. Here we report a new method to synthesize a gram-level sample of ferroelectric Ca2-xMnxTi2O6, having the same crystal structure as CaMnTi2O6 and a similarly high Curie temperature. The new finding paves the way for the mass production of this important ferroelectric oxide. We have used neutron powder diffraction to identify the origin of the peculiar ferroelectric transition in this double perovskite and to reveal the interplay between magnetic ordering and the ferroelectric displacement at low temperatures.

High-Pressure Synthesis, Crystal Structure, and Magnetic and Transport Properties of a Six-Layered SrRhO3.

A SrRhO3 polytype with six-layered (6M) structure was synthesized under high pressure and high temperature. The crystal structure was obtained by refining X-ray powder diffraction with the monoclinic space group C2/c with lattice parameters a = 5.5650(1) Å, b = 9.5967(2) Å, c = 14.0224(4) Å, and ß = 92.846(2)°. It is isostructural with SrIrO3 synthesized under ambient pressure and consists of dimers of the face-shared Rh(2)O6 octahedra connected by their vertices to the corner-shared Rh(1)O6 octahedra along the c axis with a stacking of SrO3 layers in the sequence of CCHCCH, where C and H denote cubic and hexagonal closed packing, respectively. With increasing pressure, the 6M SrRhO3 transforms to an orthorhombic perovskite (Pv) phase, having a = 5.5673(1) Å, b = 5.5399(2) Å, c = 7.8550(2) Å in the space group Pbnm. A pressure-temperature phase diagram shows that the 6M-Pv phase boundary moves to lower temperatures with increasing pressure. Both the 6M and the Pv phases of SrRhO3 were characterized by magnetic susceptibility, resistivity, and thermopower; they are all metals with an enhanced and temperature-dependent magnetic susceptibility; no long-range magnetic order has been found. The polytype structures are normally found in ABO3 oxides with the geometric tolerance factor t > 1. SrRhO3 represents another example (in addition to SrIrO3) where the polytype 6M structure can be stabilized with a t < 1.

A soft X-ray spectroscopic perspective of electron localization and transport in tungsten doped bismuth vanadate single crystals.

Doped BiVO4 is a promising photoelectrochemical water splitting anode, whose activity is hampered by poor charge transport. Here we use a set of X-ray spectroscopic methods to probe the origin and nature of localized electron states in W:BiVO4. Furthermore, using the polarized nature of the X-rays, we probe variations in the electronic structure along the crystal axes. In this manner, we reveal aspects of the electronic structure related to electron localization and observations consistent with conductivity anisotropy between the ab-plane and c-axis. We verify that tungsten substitutes as W6+ for V5+ in BiVO4. This is shown to result in the presence of inter-band gap states related to electrons at V4+ sites of e symmetry. The energetic position of the states in the band gap suggest that they are highly localized and may act as recombination centres. Polarization dependent X-ray absorption spectra reveal anisotropy in the electronic structure between the ab-plane and c-axis. Results show the superior hybridization between V 3d and O 2p states, higher V wavefunction overlap and broader conduction bands in the ab-plane than in the c-axis. These insights into the electronic structure are discussed in the context of existing experimental and theoretical reports regarding charge transport in BiVO4.

Magnons and Phonons Optically Driven out of Local Equilibrium in a Magnetic Insulator.

The coupling and possible nonequilibrium between magnons and other energy carriers have been used to explain several recently discovered thermally driven spin transport and energy conversion phenomena. Here, we report experiments in which local nonequilibrium between magnons and phonons in a single crystalline bulk magnetic insulator, Y_{3}Fe_{5}O_{12}, has been created optically within a focused laser spot and probed directly via micro-Brillouin light scattering. Through analyzing the deviation in the magnon number density from the local equilibrium value, we obtain the diffusion length of thermal magnons. By explicitly establishing and observing local nonequilibrium between magnons and phonons, our studies represent an important step toward a quantitative understanding of various spin-heat coupling phenomena.