• ### Efficient Bayesian Inference of Atomistic Structure in Complex Functional Materials(1708.09274)

March 12, 2019 cond-mat.mtrl-sci
Tailoring the functional properties of advanced organic/inorganic heterogeonous devices to their intended technological applications requires knowledge and control of the microscopic structure inside the device. Atomistic quantum mechanical simulation methods deliver accurate energies and properties for individual configurations, however, finding the most favourable configurations remains computationally prohibitive. We propose a 'building block'-based Bayesian Optimisation Structure Search (BOSS) approach for addressing extended organic/inorganic interface problems and demonstrate its feasibility in a molecular surface adsorption study. In BOSS, a likelihood-free Bayesian scheme accelerates the identification of material energy landscapes with the number of sampled configurations during active learning, enabling structural inference with high chemical accuracy and featuring large simulation cells. This allowed us to identify several most favourable molecular adsorption configurations for $\mathrm{C}_{60}$ on the (101) surface of $\mathrm{TiO}_2$ anatase and clarify the key molecule-surface interactions governing structural assembly. Inferred structures were in good agreement with detailed experimental images of this surface adsorbate, demonstrating good predictive power of BOSS and opening the route towards large-scale surface adsorption studies of molecular aggregates and films.
• ### Elastic interactions and control of the Mott transition(1701.02318)

Metal to insulator transitions (MITs) driven by strong electronic correlations are common in condensed matter systems, and are associated with some of the most remarkable collective phenomena in solids, including superconductivity and magnetism. Tuning and control of the transition holds the promise of novel, low power, ultrafast electronics, but the relative roles of doping, chemistry, elastic strain and other applied fields has made systematic understanding difficult to obtain. Here we point out that existing data on tuning of the MIT in perovskite transition metal oxides through ionic size effects provides evidence of systematic and large effects on the phase transition due to dynamical fluctuations of the elastic strain, which have been usually neglected. This is illustrated by a simple yet quantitative statistical mechanical calculation in a model that incorporates cooperative lattice distortions coupled to the electronic degrees of freedom. We reproduce the observed dependence of the transition temperature on cation radius in the well-studied manganite and nickelate materials. Since the elastic couplings are generically quite strong, these conclusions will broadly generalize to all MITs that couple to a change in lattice symmetry.
• ### Effects of transmutation elements in tungsten(1706.03252)

March 7, 2019 cond-mat.mtrl-sci
Tungsten (W) is widely considered as the most promising plasma facing material (PFM), which will be used in nuclear fusion devices. Due to the transmutation reaction caused by the fusion neutron irradiation, transmutation elements (such as Re, Os, and Ta) are generated in the W-based PFM during the operation of nuclear fusion devices. In this paper, transmutation elements effects on mechanical properties of the W and the behavior of hydrogen/helium (H/He) atom in the W were investigated by using the first-principles calculation method. The results show that the ductility of the W is enhanced by transmutation elements if dislocation and other defects were ignored, while mechanical properties of the W incompletely depend on transmutation elements content. Compared with the pure W, the formation energy of the H/He in the W is reduced by transmutation elements, but the most favorable sites of the H/He in the W is not changed. Except for a repulsion between Ta and He in the W, the H/He in the W is attracted by transmutation elements. In addition, transmutation elements can change the best diffusion path of the H/He in the W and increase the diffusion rate of the H/He in W. This study provides a theoretical basis for the Tungsten (W) application, and further study on the effects of transmutation elements in the W will be needed.
• ### Electrically Switchable Metadevices via Graphene(1512.08277)

Metamaterials bring sub-wavelength resonating structures together to overcome the limitations of conventional materials. The realization of active metadevices has been an outstanding challenge that requires electrically reconfigurable components operating over a broad spectrum with a wide dynamic range. The existing capability of metamaterials, however, is not sufficient to realize this goal. Here, by integrating passive metamaterials with active graphene devices, we demonstrate a new class of electrically controlled active metadevices working in microwave frequencies. The fabricated active metadevices enable efficient control of both amplitude (> 50 dB) and phase (> 90{\deg}) of electromagnetic waves. In this hybrid system, graphene operates as a tunable Drude metal that controls the radiation of the passive metamaterials. Furthermore, by integrating individually addressable arrays of metadevices, we demonstrate a new class of spatially varying digital metasurfaces where the local dielectric constant can be reconfigured with applied bias voltages. Additionally, we reconfigure resonance frequency of split ring resonators without changing its amplitude by damping one of the two coupled metasurfaces via graphene. Our approach is general enough to implement various metamaterial systems that could yield new applications ranging from electrically switchable cloaking devices to adaptive camouflage systems.
• ### Extension of the General Thermal Field Equation for nanosized emitters(1609.02426)

March 1, 2019 cond-mat.mtrl-sci
During the previous decade, K.L. Jensen et. al. developed a general analytical model that successfully describes electron emission from metals both in the field and thermionic regimes, as well as in the transition region. In that development, the standard image corrected triangular potential barrier was used. This barrier model is valid only for planar surfaces and therefore cannot be used in general for modern nanometric emitters. In a recent publication the authors showed that the standard Fowler-Nordheim theory can be generalized for highly curved emitters if a quadratic term is included to the potential model. In this paper we extend this generalization for high temperatures and include both the thermal and intermediate regimes. This is achieved by applying the general method developed by Jensen to the quadratic barrier model of our previous publication. We obtain results that are in good agreement with fully numerical calculations for radii $R>4nm$, while our calculated current density differs by a factor up to 27 from the one predicted by the Jensen's standard General-Thermal-Field (GTF) equation. Our extended GTF equation has application to modern sharp electron sources, beam simulation models and vacuum breakdown theory.
• ### Study of deteriorating semiopaque turquoise lead-potassium glass beads at different stages of corrosion using micro-FTIR spectroscopy(1705.09394)

Nowadays, a problem of historical beadworks conservation in museum collections is actual more than ever because of fatal corrosion of the 19th century glass beads. Vibrational spectroscopy is a powerful method for investigation of glass, namely, of correlation of the structure-chemical composition. Therefore, Fourier-transform infrared spectroscopy was used for examination of degradation processes in cloudy turquoise glass beads, which in contrast to other color ones deteriorate especially strongly. Micro-X-ray fluorescence spectrometry of samples has shown that lead-potassium glass PbO-K$_2$O-SiO$_2$ with small amount of Cu and Sb was used for manufacture of cloudy turquoise beads. Fourier-transform infrared spectroscopy study of the beads at different stages of glass corrosion was carried out in the range from 200 to 4000 cm$^{-1}$ in the attenuated total reflection mode. In all the spectra, we have observed shifts of two major absorption bands to low-frequency range (~1000 and ~775 cm$^{-1}$) compared to ones typical for amorphous SiO2 (~1100 and 800 cm$^{-1}$, respectively). Such an effect is connected with Pb$^{2+}$ and K$^+$ appending to the glass network. The presence of a weak band at ~1630 cm$^{-1}$ in all the spectra is attributed to the adsorption of H$_2$O. After annealing of the beads, the band disappeared completely in less deteriorated samples and became significantly weaker in more destroyed ones. Based on that we conclude that there is adsorbed molecular water on the beads. However, products of corrosion (e.g., alkali in the form of white crystals or droplets of liquid alkali) were not observed on the glass surface. We have also observed glass depolymerisation in the strongly degraded beads, which is exhibited in domination of the band peaked at ~1000 cm$^{-1}$.
• ### Selecting the suitable dopants: electronic structures of transition metal and rare earth doped thermoelectric sodium cobaltate(1206.5448)

Engineered $Na_{0.75}CoO_2$ is considered a prime candidate to achieve high-efficiency thermoelectric systems to regenerate electricity from waste heat. In this work, three elements with outmost electronic configurations, (1) an open d shell (Ni), (2) a closed d shell (Zn), and (3) a half filled f shell (Eu) with maximum unpaired electrons, were selected to outline the dopants' effects on electronic and crystallographic structures of $Na_{0.75}CoO_2$. Systematic $ab$ $initio$ density functional calculations with $DMOL^3$ package showed that the Ni and Zn were more stable when substituting Co with formation energy $-2.35$ eV, $2.08$ eV when Fermi level equals to the valence band maximum. While Eu is more stable when it substitutes Na having formation energy of $-2.64$ eV. As these results show great harmony with existing experimental data, they provide new insights into the fundamental principle of dopant selection for manipulating the physical properties in the development of high-performance sodium cobaltate based multifunctional materials.
• ### Second order perturbed Heisenberg Hamiltonian of Fe3O4 ultra-thin films(1611.02225)

Feb. 25, 2019 cond-mat.mtrl-sci
Due to the wide range of applications, theoretical models of Fe3O4 films are found to be important. Ultra thin Fe3O4 films with ferrite structure have been theoretically investigated using second order perturbed modified Heisenberg Hamiltonian. Matrices for ultra thin films with two and three spin layers are presented in this manuscript. Total magnetic energy was expressed in terms of spin exchange interaction, magnetic dipole interaction, second order magnetic anisotropy and stress induced magnetic anisotropy. Magnetic properties were observed for films with two spin layers and variant second order magnetic anisotropy. For the film with three spin layers, second order anisotropy constant was fixed to avoid tedious derivations. Magnetic easy axis rotates toward the in plane direction as the number of spin layers is increased from two to three because the stress induced anisotropy energy dominates at higher number of spin layers. According to some other experimental data, the magnetic easy axis of thin films rotates toward the in plane direction as the thickness is increased. For ferrite film with two spin layers, magnetic easy and hard directions can be observed at 0.75 and 1.2 radians, respectively, when the ratio of stress induced anisotropy to the long range dipole interaction strength is 3.9. For ferrite film with three spin layers, magnetic easy and hard directions can be observed at 2.4 and 2.3 radians, respectively, when the ratio of stress induced anisotropy to the long range dipole interaction strength is 4.2.
• ### Extraordinarily large intrinsic magnetodielectric coupling of Tb member within the Haldane spin-chain family, R2BaNiO5(1706.07981)

The Haldane spin-chain compound, Tb2BaNiO5, has been known to order antiferromagnetically below (T_N= ) 63 K. The present magnetic studies on the polycrystals bring out that there is another magnetic transition at a lower temperature (T_2= ) 25 K, with a pronounced magnetic-field induced metamagnetic and metaelectric behavior. Multiferroic features are found below T_2 only, and not at T_N. The most intriguing observation is that the observed change of dielectric constant is intrinsic and largest (e.g., about 18% at 15 K) within this Haldane spin-chain family, R2BaNiO5. Taking into account that this trend (the largest change for Tb case within this family) correlates with a similar trend in T_N (with the values of T_N being about 55, 58, 53 and 32 K for Gd, Dy, Ho and Er cases), we believe that an explanation usually offered for this T_N behavior in rare-earth systems is applicable for this behavior as well . That is, single-ion anisotropy following crystal-field splitting is responsible for this extraordinary magnetodielectric effect in this Tb case. To our knowledge, such an observation was not made in the past literature of multiferroics.
• ### Controlling Morphology-Structure of Gold Tiny Particles, Nanoparticles and Particles at Different Pulse Rates and Pulse Polarity(1605.04408)

June 4, 2019 cond-mat.mtrl-sci
Controlling the shape and structure of metallic colloids is an important topic. Here, different morphology-structures of colloidal gold particles are investigated with different process parameters in a pulse-based electron-photon and solution interface process. Different tiny-shaped particles of gold developed for different packets of supplied nano-energy as per set pulse OFF to ON time. Depending on the set ratios of pulse OFF to ON times and pulse polarity, packets of nano-energy bind transitional state gold atoms resting at electronically-flat solution surface that is controlled in their own shapes. Tiny particles of joined triangular shape in each case developed under set tuned ratio of bipolar pulses OFF to ON time. At unipolar pulse, tiny particles in triangular shape developed directly. When the ratio of bipolar pulse OFF to ON time was large, distorted shapes of the particles developed. Geometrical shapes of particles developed under significant ratios of pulse OFF to ON times. When the ratio of bipolar pulse OFF to ON time was 3, particles developed in low aspect ratio. But under the fraction of this ratio, particles tend to develop in high aspect ratio. For longer pulse ON time, structures of smooth elements get developed in width less than inter-spacing distance and forcing energy of travelling photons along the interface to flatten them further. Morphology and structure of tiny particles, nanoparticles and particles are discussed for different process parameters opening multiple routes for materials research and their counterparts. This is the overall attained orientation of electrons in elongated atoms forming colloidal particles of different size and shape maintaining the certain color of their solution under sunlight.
• ### Development of Gold Tiny Particles and Particles in Different Sizes at Varying Precursor Concentration(1604.07508)

Nov. 29, 2019 cond-mat.mtrl-sci
Coalescence (or Growth) of tiny particles to larger particles has been an exciting and practical research topic in nanotechnology. This study deals with development of gold particles at varying precursor concentration in a custom-built setup. Under the tuned ratio of bipolar pulse OFF to ON time, tiny particles of different sizes and shapes develop depending on the amount of gold precursor. When compact monolayer assembly is formed on solution surface, nano energy in packet shapes bind gold atoms in own shape. Between 0.07 mM to 0.90 mM precursor concentration, tiny particles develop in both triangular and non-triangular shapes. Tiny particles of triangular shape develop in a large number at precursor concentration 0.30 mM and 0.60 mM. Hence, nanoparticles and particles also developed in different geometrical shapes. Such tiny particles pack due to the exerted force in immersing format, where made structures of smooth elements assemble to develop nanoparticles and particles of different shapes. Tiny particles of non-geometrical shapes do not assemble at a common point and their assembling develops a nanoparticle or particle of distorted or spherical shape. This occurs mainly when precursor concentration is 0.05 mM and 1.20 mM. At 50 sccm Argon flow rate, nanoparticles and particles develop in the same shapes as in case of 100 sccm. For different precursor concentrations, different colors of solutions indicate different features of nanoparticles and particles. This study clarifies the necessary concentration of precursor to develop colloids of different sizes.
• ### Modified Bohm's theory for abstruse measurements: application to layer depth profiling by Auger spectroscopy(1610.05100)

Feb. 18, 2019 cond-mat.mtrl-sci
Modified Bohm formalism is applied to solve a problem of abstruse layer depth profiles measured by the Auger electron spectroscopy technique in real physical systems, i.e., the desorbed carbon/passive layer on NiTi substrate and the adsorbed oxygen/surface of NiTi alloy. It is shown that abstruse layer profiles may be converted to real layer structures using the modified Bohm theory, where the quantum potential is due to an Auger electron effect. It is also pointed out that the stationary probability density predicts multilayer structures of abstruse depth profiles caused by carbon desorption and oxygen adsorption processes. The criterion for a kind of break between the physical and unphysical multilayer systems was found. We have concluded that the physics is also characterized by the abstruse measurement and the modified Bohm formalism
• ### Negative reflection of elastic guided waves in chaotic and random scattering media(1710.06150)

Feb. 14, 2019 cond-mat.mtrl-sci
The propagation of waves in complex media can be harnessed either by taming the incident wave-field impinging on the medium or by forcing waves along desired paths through its careful design. These two alternative strategies have given rise to fascinating concepts such as time reversal or negative refraction. Here, we show how these two processes are intimately linked through the negative reflection phenomenon. A negative reflecting mirror converts a wave of positive phase velocity into its negative counterpart and vice versa. In this article, we experimentally demonstrate this phenomenon with elastic waves in a 2D billiard and in a disordered plate by means of laser interferometry. Despite the complexity of such configurations, the negatively reflected wave field focuses back towards the initial source location, thereby mimicking a phase conjugation operation while being a fully passive process. The super-focusing capability of negative reflection is also highlighted in a monochromatic regime. The negative reflection phenomenon is not restricted to guided elastic waves since it can occur in zero-gap systems such as photonic crystals, chiral metamaterials or graphene. Negative reflection can thus become a tool of choice for the control of waves in all fields of wave physics.
• ### Electron emission from conduction band of diamond with negative electron affinity(0910.2189)

Feb. 12, 2019 cond-mat.mtrl-sci
Experimental evidence explaining the extremely low-threshold electron emission from diamond reported in 1996 has been obtained for the first time. Direct observation using combined ultraviolet photoelectron spectroscopy/field emission spectroscopy (UPS/FES) proved that the origin of field-induced electron emission from heavily nitrogen (N)-doped chemical vapour deposited (CVD) diamond was at conduction band minimum (CBM) utilising negative electron affinity (NEA). The significance of the result is that not only does it prove the utilisation of NEA as the dominant factor for the extremely low-threshold electron emission from heavily N-doped CVD diamond, but also strongly implies that such low-threshold emission is possible from other types of diamond, and even other materials having NEA surface. The low-threshold voltage, along with the stable intensity and remarkably narrow energy width, suggests that this type of electron emission can be applied to develop a next generation vacuum nano-electronic devices with long lifetime and high energy resolution.
• ### Ferromagnetism and conductivity in atomically thin SrRuO3(1609.08901)

Atomically thin ferromagnetic and conducting electron systems are highly desired for spintronics, because they can be controlled with both magnetic and electric fields. We present (SrRuO3)1-(SrTiO3)5 superlattices and single-unit-cell-thick SrRuO3 samples that are capped with SrTiO3. We achieve samples of exceptional quality. In these samples, the electron systems comprise only a single RuO2 plane. We observe conductivity down to 50 mK, a ferromagnetic state with a Curie temperature of 25 K, and signals of magnetism persisting up to approximately 100 K.
• ### Probing Many-Body Interactions in Monolayer Transition-Metal Dichalcogenides(1606.07101)

Feb. 2, 2019 cond-mat.mtrl-sci
Many-body interactions in monolayer transition-metal dichalcogenides are strongly affected by their unique band structure. We study these interactions by measuring the energy shift of neutral excitons (bound electron-hole pairs) in gated WSe$_2$ and MoSe$_2$. Surprisingly, while the blueshift of the neutral exciton, $X^0$, in electron-doped samples can be more than 10~meV, the blueshift in hole-doped samples is nearly absent. Taking into account dynamical screening and local-field effects, we present a transparent and analytical model that elucidates the crucial role played by intervalley plasmons in electron-doped conditions. The energy shift of $X^0$ as a function of charge density is computed showing agreement with experiment, where the renormalization of $X^0$ by intervalley plasmons yields a stronger blueshift in MoSe$_2$ than in WSe$_2$ due to differences in their band ordering.
• ### Reliable thermodynamic estimators for screening multicaloric materials(1702.03042)

Reversible, diffusionless, first-order solid-solid phase transitions accompanied by caloric effects are critical for applications in the solid-state cooling and heat-pumping devices. Accelerated discovery of caloric materials requires reliable but faster estimators for predictions and high-throughput screening of system-specific dominant caloric contributions. We assess reliability of the computational methods that provide thermodynamic properties in relevant solid phases at or near a phase transition. We test the methods using the well-studied B2 FeRh alloy as a "fruit fly" in such a materials genome discovery, as it exhibits a metamagnetic transition which generates multicaloric (magneto-, elasto-, and baro-caloric) responses. For lattice entropy contributions, we find that the commonly-used linear-response and small-displacement phonon methods are invalid near instabilities that arise from the anharmonicity of atomic potentials, and we offer a more reliable and precise method for calculating lattice entropy at a fixed temperature. Then, we apply a set of reliable methods and estimators to the metamagnetic transition in FeRh (predicted $346 \pm 12$ K, observed $353 \pm 1$ K) and calculate the associated caloric properties, such as isothermal entropy and isentropic temperature changes.
• ### Multiband quasiparticle interference in the topological insulator Cu_(x)Bi_(2)Te_(3)(1110.4406)

We present angle resolved photoemission experiments and scanning tunneling spectroscopy results on the doped topological insulator Cu0.2Bi2Te3. Quasi-particle interference (QPI) measurements, based on high resolution conductance maps of the local density of states show that there are three distinct energy windows for quasi-particle scattering. Using a model Hamiltonian for this system two new scattering channels are identified: the first between the surface states and the conduction band and the second between conduction band states. We also observe that the real space density modulation has a predominant three-fold symmetry, which rules out a simple, isotropic impurity potential. We obtain agreement between experiment and theory by considering a modified scattering potential that is consistent with having mostly Bi-Te anti-site defects as scatterers.
• ### Band splitting in bilayer stanene electronic structure scrutinized via first principle DFT calculations(1710.05701)

Jan. 23, 2019 cond-mat.mtrl-sci
The recent work on stanene as quantum spin Hall insulators made us investigate bilayer stanene using first principle calculations. With an aim of improving and developing new properties, via modulating the stacking order (and angle) of the bilayers. This stacking of layers has been proven technique for modulating the properties of monolayer materials. Here we design multiple bilayer systems, with different stacking angles and AA and AB configurations. Rather observing an improvement in bandgap due to spin-orbit coupling (SOC), we witness a splitting of the band due to SOC, a characteristic behavior of stacked MoS2 sheets. This splitting of the bands gives rise to different, independent and distinct spin-up and spin-down channels, manifesting a valley dependent spin polarization. Also, as a contrast to stacked MoS2 system we notice in our system the stacking angle and order, does effect electronic states.
• ### Near-infrared intersubband photodetection in GaN/AlN nanowires(1710.00871)

Intersubband optoelectronic devices rely on transitions between quantum-confined electron levels in semiconductor heterostructures, which enables infrared (IR) photodetection in the 1-30 $\mu$m wavelength window with picosecond response times. Incorporating nanowires as active media could enable an independent control over the electrical cross-section of the device and the optical absorption cross-section. Furthermore, the three-dimensional carrier confinement in nanowire heterostructures opens new possibilities to tune the carrier relaxation time. However, the generation of structural defects and the surface sensitivity of GaAs nanowires have so far hindered the fabrication of nanowire intersubband devices. Here, we report the first demonstration of intersubband photodetection in a nanowire, using GaN nanowires containing a GaN/AlN superlattice absorbing at 1.55 $\mu$m. The combination of spectral photocurrent measurements with 8-band k$\cdot$p calculations of the electronic structure supports the interpretation of the result as intersubband photodetection in these extremely short-period superlattices. We observe a linear dependence of the photocurrent with the incident illumination power, which confirms the insensitivity of the intersubband process to surface states and highlights how architectures featuring large surface-to-volume ratios are suitable as intersubband photodetectors. Our analysis of the photocurrent characteristics points out routes for an improvement of the device performance. This first nanowire based intersubband photodetector represents a technological breakthrough that paves the way to a powerful device platform with potential for ultrafast, ultrasensitive photodetectors and highly-efficient quantum cascade emitters with improved thermal stability.
• ### Electric power generation of a liquid self-assembled drop on a semiconductor surface(1609.08361)

Jan. 16, 2019 cond-mat.mtrl-sci
The technological innovation of the direct conversion of solar energy to electric plays an important role in electric power generation. Earlier discussions of band bending in a semiconductor contacting a metal and liquid electrolyte solutions containing redox couples with different electrochemical potentials should not overshadow the fact that under absorption of photons takes place in a solar cell, which can generate free charge for an electrical circuit. Here we propose new band bending of ZnO and Cu2O semiconductors induced by a liquid self-assembled microdrop of a physiological salt solution.
• ### Prediction of pressure-induced stabilization of noble-gas-atom compounds with alkali oxides and alkali sulfides(1409.2227)

Jan. 12, 2019 cond-mat.mtrl-sci
The cubic antifluorite structure comprises a face-centered cubic sublattice of anions with cations on the tetrahedral sites. The voids in the antifluorite structure that are crucial for superionicity in Li2O might also act as atomic traps. Trapping of guest atoms and small molecules within voids of a host structure leads to the formation of what are known as clathrate compounds. Here we investigate the possibility of trapping helium or larger neon guest atoms under pressure within alkali metal oxide and sulfide structures. We find stable helium and neon-bearing compounds at very low pressures. These structures are stabilized by a reduction in volume from incorporation of helium or neon atoms within the antifluorite structure. We predict that NeCs2S could be stable at ambient pressure. Our study suggests a novel class of alkali oxide and sulfide materials incorporating noble gas atoms that might potentially be useful for gas storage.
• ### Effect of anisotropy distribution on local nucleation field in bistable ferromagntic microwires(1703.08066)

June 3, 2019 cond-mat.mtrl-sci
Critical parameters defining the local nucleation field in amorphous ferromagnetic microwires with positive magnetostriction are obtained analytically through scaling procedures. Exact value of the nucleation field is obtained numerically as a function of geometric parameters of anisotropy distribution, which is fully taken into account instead of being averaged out. It is established that the value of nucleation field depends predominantly on the steepness of anisotropy change withing the boundary between axial and radial domains, while the maximal value of anisotropy inside the wire or an overall average is not relevant.
• ### Experimental signatures of emergent quantum electrodynamics in Pr$_2$Hf$_2$O$_7$(1706.03604)

In a quantum spin liquid, the magnetic moments of the constituent electron spins evade classical long-range order to form an exotic state that is quantum entangled and coherent over macroscopic length scales [1-2]. Such phases offer promising perspectives for device applications in quantum information technologies, and their study can reveal fundamentally novel physics in quantum matter. Quantum spin ice is an appealing proposal of one such state, in which the fundamental ground state properties and excitations are described by an emergent U(1) lattice gauge theory [3-7]. This quantum-coherent regime has quasiparticles that are predicted to behave like magnetic and electric monopoles, along with a gauge boson playing the role of an artificial photon. However, this emergent lattice quantum electrodynamics has proved elusive in experiments. Here we report neutron scattering measurements of the rare-earth pyrochlore magnet Pr$_2$Hf$_2$O$_7$ that provide evidence for a quantum spin ice ground state. We find a quasi-elastic structure factor with pinch points - a signature of a classical spin ice - that are partially suppressed, as expected in the quantum-coherent regime of the lattice field theory at finite temperature. Our result allows an estimate for the speed of light associated with magnetic photon excitations. We also reveal a continuum of inelastic spin excitations, which resemble predictions for the fractionalized, topological excitations of a quantum spin ice. Taken together, these two signatures suggest that the low-energy physics of Pr$_2$Hf$_2$O$_7$ can be described by emergent quantum electrodynamics. If confirmed, the observation of a quantum spin ice ground state would constitute a concrete example of a three-dimensional quantum spin liquid - a topical state of matter which has so far mostly been explored in lower dimensionalities.
• ### Ab-initio study of different structures of CaC: Magnetism, Bonding, and Lattice Dynamics(1204.4781)

Dec. 30, 2018 cond-mat.mtrl-sci
On the basis of ab-initio pseudopotential calculations, we study structural, magnetic, dynamical, and mechanical properties of the hypothetical CaC ionic compound in the rock-salt (RS), B2, zinc-blende (ZB), wurtzite (WZ), NiAs (NA), anti-NiAs (NA*), and CrB (B33) structures. It is argued that the ZB, WZ, NA, and RS structures are more ionic while the NA*, B2, and B33 structures are more covalent systems. As a result of that, the nonmagnetic B33-CaC is the energetically preferred system, while the more ionic structures prefer a ferromagnetic ground state with high Fermi level spin polarization. The observed ferromagnetism in the more ionic systems is attributed to the sharp partially filled $p$ states of carbon atom in the system. In the framework of density functional perturbation theory, the phonon spectra of these systems are computed and the observed dynamical instabilities of the NA* and B2 structures are explained in terms of the covalent bonds between carbon atoms. The calculated Helmholtz and Enthalpy free energies indicate the highest stability of the B33 structure in a wide range of temperatures and pressures. Among the ferromagnetic structures, RS-CaC and ZB-CaC are reported, respectively, to be the most and the least metastable systems in various thermodynamics conditions. Several mechanical properties of the dynamically stable structures of CaC are determined from their phonon spectra.