Excitations In Single Crystals Research Articles

High-energy spin excitations in the quantum spin liquid candidate Zn-substituted barlowite probed by resonant inelastic x-ray scattering

A quantum spin liquid is a novel ground state that can support long-range entanglement between magnetic moments, resulting in exotic spin excitations involving fractionalized $S=\frac{1}{2}$ spinons. Here, we measure the excitations in single crystals of the spin liquid candidate Zn-barlowite using resonant inelastic x-ray scattering. By analyzing the incident polarization and temperature dependences, we deduce a clear magnetic scattering contribution forming a broad continuum that surprisingly extends up to $\ensuremath{\sim}200$ meV ($\ensuremath{\sim}14J$, where $J$ is the magnetic exchange). The excitation spectrum indicates that significant contributions may arise from multiple pairs of spinons and/or antispinons at high energies. Furthermore, the observation of similar scattering in the hexagonal barlowite parent material above its ordering temperature suggests the presence of similar high-energy excitations, irrespective of the low-energy physics.

Optical Raman measurements of low frequency magnons under high pressure.

The application of giga-Pascal scale pressures has been widely used as a tool to systematically tune the properties of materials in order to access such general questions as the driving mechanisms underlying phase transitions. While there is a large and growing set of experimental tools successfully applied to high-pressure environments, the compatibility between diamond anvil cells and optical probes offers further potential for examining lattice, magnetic, and electronic states, along with their excitations. Here, we describe the construction of a highly efficient optical Raman spectrometer that enables measurements of magnetic excitations in single crystals down to energies of 9 cm-1 (1.1 meV or 13 K) at cryogenic temperatures and under pressures of tens of GPa.

Dynamic Spin-Lattice Coupling and Nematic Fluctuations in NaFeAs

We use inelastic neutron scattering to study acoustic phonons and spin excitations in single crystals of NaFeAs, a parent compound of iron pnictide superconductors. NaFeAs exhibits a tetragonal-to-orthorhombic structural transition at $T_s\approx 58$ K and a collinear antiferromagnetic (AF) order at $T_N\approx 45$ K. While longitudinal and out-of-plane transverse acoustic phonons behave as expected, the in-plane transverse acoustic phonons reveal considerable softening on cooling to $T_s$, and then harden on approaching $T_N$ before saturating below $T_N$. In addition, we find that spin-spin correlation lengths of low-energy magnetic excitations within the FeAs layer and along the $c$-axis increase dramatically below $T_s$, and show weak anomaly across $T_N$. These results suggest that the electronic nematic phase present in the paramagnetic tetragonal phase is closely associated with dynamic spin-lattice coupling, possibly arising from the one-phonon-two-magnon mechanism.

Lattice dynamics and the nature of structural transitions in organolead halide perovskites

Organolead halide perovskites are a family of hybrid organic-inorganic compounds whose remarkable optoelectronic properties have been under intensive scrutiny in recent years. Here we use inelastic X-ray scattering to study low-energy lattice excitations in single crystals of methylammonium lead iodide and bromide perovskites. Our findings confirm the displacive nature of the cubic-to-tetragonal phase transition, which is further shown, using neutron and x-ray diffraction, to be close to a tricritical point. Lastly, we detect quasistatic symmetry-breaking nanodomains persisting well into the high-temperature cubic phase, possibly stabilized by local defects. These findings reveal key structural properties of these materials, and also bear important implications for carrier dynamics across an extended temperature range relevant for photovoltaic applications.

Orbital Selective Spin Excitations and their Impact on Superconductivity of LiFe_{1-x}Co_{x}As.

We use neutron scattering to study spin excitations in single crystals of LiFe_{0.88}Co_{0.12}As, which is located near the boundary of the superconducting phase of LiFe_{1-x}Co_{x}As and exhibits non-Fermi-liquid behavior indicative of a quantum critical point. By comparing spin excitations of LiFe_{0.88}Co_{0.12}As with a combined density functional theory and dynamical mean field theory calculation, we conclude that wave-vector correlated low energy spin excitations are mostly from the d_{xy} orbitals, while high-energy spin excitations arise from the d_{yz} and d_{xz} orbitals. Unlike most iron pnictides, the strong orbital selective spin excitations in the LiFeAs family cannot be described by an anisotropic Heisenberg Hamiltonian. While the evolution of low-energy spin excitations of LiFe_{1-x}Co_{x}As is consistent with the electron-hole Fermi surface nesting conditions for the d_{xy} orbital, the reduced superconductivity in LiFe_{0.88}Co_{0.12}As suggests that Fermi surface nesting conditions for the d_{yz} and d_{xz} orbitals are also important for superconductivity in iron pnictides.

Temperature dependence of the resonance and low-energy spin excitations in superconducting FeTe0.6Se0.4

We use inelastic neutron scattering to study the temperature dependence of the low-energy spin excitations in single crystals of superconducting FeTe$_{0.6}$Se$_{0.4}$ ($T_c=14$ K). In the low-temperature superconducting state, the imaginary part of the dynamic susceptibility at the electron and hole Fermi surfaces nesting wave vector $Q=(0.5,0.5)$, $\chi^{\prime\prime}(Q,\omega)$, has a small spin gap, a two-dimensional neutron spin resonance above the spin gap, and increases linearly with increasing $\hbar\omega$ for energies above the resonance. While the intensity of the resonance decreases like an order parameter with increasing temperature and disappears at temperature slightly above $T_c$, the energy of the mode is weakly temperature dependent and vanishes concurrently above $T_c$. This suggests that in spite of its similarities with the resonance in electron-doped superconducting BaFe$_{2-x}$(Co,Ni)$_x$As$_2$, the mode in FeTe$_{0.6}$Se$_{0.4}$ is not directly associated with the superconducting electronic gap.

Antiferromagnetic spin excitations in single crystals of nonsuperconducting Li1−xFeAs

We use neutron scattering to determine spin excitations in single crystals of nonsuperconducting Li1-xFeAs throughout the Brillouin zone. Although angle resolved photoemission experiments and local density approximation calculations suggest poor Fermi surface nesting conditions for antiferromagnetic(AF) order, spin excitations in Li1-xFeAs occur at the AF wave vectors Q = (1, 0) at low energies, but move to wave vectors Q = (\pm 0.5, \pm0.5) near the zone boundary with a total magnetic bandwidth comparable to that of BaFe2As2. These results reveal that AF spin excitations still dominate the low-energy physics of these materials and suggest both itinerancy and strong electron-electron correlations are essential to understand the measured magnetic excitations.

Itinerant Magnetic Excitations in AntiferromagneticCaFe2As2

Neutron scattering measurements of the magnetic excitations in single crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and well-defined spin waves up to an energy of approximately 100 meV. Magnetic excitations above 100 meV and up to the maximum energy of 200 meV are however broader in energy and momentum than the experimental resolution. While the low energy modes can be fit to a Heisenberg model, the total spectrum cannot be described as arising from excitations of a local moment system. Ab initio calculations of the dynamic magnetic susceptibility suggest that the high energy behavior is dominated by the damping of spin waves by particle-hole excitations.

Spin dynamics of the low-dimensional magnet (CH 3) 2NH 2CuCl 3

Dimethylammonium copper (II) chloride (also known as DMACuCl 3 or MCCL) is a low–dimensional S = 1 2 quantum spin system proposed to be an alternating ferro-antiferromagnetic chain with similar magnitude ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions. Subsequently, it was shown that the existing bulk measurements could be adequately modeled by considering DMACuCl 3 as independent AFM and FM dimer spin pairs. We present here new inelastic neutron scattering measurements of the spin excitations in single crystals of DMACuCl 3. These results show significant quasi-one-dimensional coupling, however the magnetic excitations do not propagate along the expected direction. We observe a band of excitations with a gap of Δ=0.95 MeV and a bandwidth of 0.82 MeV.

Inelastic neutron scattering study on low-energy excitations of the heavy-fermion superconductor PrOs 4Sb 12

Low-energy magnetic excitations in single crystals of the heavy-fermion superconductor PrOs 4Sb 12 have been studied by inelastic neutron scattering. The clear softening of excitations at a wave vector Q = ( 1 , 0 , 0 ) , which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and the intensity analyses of excitations directly evidence that the nonmagnetic quadrupolar fluctuations are dominant in this system. Furthermore, the narrowing of the linewidths of the excitations in the superconducting phase indicates the close connection between the superconductivity and the excitations. The preliminary data under magnetic fields are also presented.

Scientific Review: Maps and Magnetism in Single Crystals

The MAPS spectrometer at ISIS is a direct geometry instrument that has been designed to measure coherent excitations in single crystals. Knowledge of the momentum, Q, and energy, _ε, dependent spectrum of excitations in a single crystal permits the most exacting tests of models for the interactions in a material. For example, the imaginary part of the generalized magnetic susceptibility, _χ'' (Q, ε), contains the full information of the spin-spin correlation functions in a magnetic material from which the strength, range, and symmetry of the magnetic interactions can be determined. This quantity is directly measured by inelastic neutron scattering.

Magnetic excitations in single crystals of Cu 1− xNi xGeO 3

We have studied magnetic excitations in two single crystals of CuGeO 3 doped with Ni 2+, using inelastic neutron scattering at wave vectors close to the antiferromagnetic zone centre, Q=(0,1, 1 2 ) . Pure CuGeO 3 is a one-dimensional compound with a spin-Peierls (SP) gap of ≈1.95 meV. When Ni 2+ is substituted for Cu 2+ in CuGeO 3, the 1D chains are broken into finite segments, suppressing the SP phase and inducing a low-temperature transition to coexistence with antiferromagnetic order. We show that for the 1.7% Ni-doped crystal the SP gap is renormalised to ≈1.7 meV, while approximate doubling of the dopant concentration to 3.2% results in an almost complete collapse of this excitation. Instead, measurements on the 3.2% Ni-doped crystal revealed a magnetic excitation that could be clearly resolved from the elastic magnetic peak. This excitation followed the dispersion expected for the anisotropy gap in an antiferromagnet and comparison with an equivalent model of the SP gap in CuGeO 3 has yielded modified exchange coefficients. However, measurements could be made only for wave vectors close to (0, 1, 1 2 ) as the intensity declined rapidly with movement away from the zone centre.

Magnetic excitations in praseodymium alloys

We have studied the effect of alloying on the magnetic excitations in Pr with the PRISMA spectrometer at ISIS. The dispersion of the excitations in single crystals of Pr 0.9Tm 0.1 and Pr 0.9Ce 0.1 along the a *- and c *-directions has been compared with that in pure Pr. We find that alloying Pr with Tm changes the excitation spectrum very significantly, due to strong interactions with the Tm impurity mode, whereas alloying with Ce produces more subtle changes.

A comparison of time-of-flight (TOF) and three axis spectrometer (TAS) techniques for the study of excitations in single crystals

Abstract Arguments on advantages and disadvantages of TOF against TAS are quite old and the situation for single crystals seemed clear for many years. But the advent of spallation sources restimula...

Resonant Raman scattering from superconducting gap excitations in single crystals of (BEDT-TTF)2I3

A study of low-energetic resonant Raman scattering of αt- and βH-(BEDT-TTF)2I3 superconductors was performed. The softening and weakening of the low frequency optical phonons at about 30 cm−1 in the superconducting state of both αt- and βH-phases were observed below Tc.

Raman scattering from spin fluctuations in Pr2-xCexCuO4-y, Nd2-xCexCuO4-y, and Sm2-xCexCuO4-y.

Antiferromagnetic excitations in single crystals of Pr2−xCexCuO4−y, Nd2xCexCuO4−y and Sm2−xCexCuO4−y were investigated as a function of Ce concentration using Raman scattering at room temperature. The introduction of electrons by Ce doping significantly broadens the two-magnon peak in the B1g mode, indicating that zone-boundary magnons become damped with increasing Ce concentration. The existence of broad spin-pair-excitation spectra in the B1g and B2g modes at x=0.15 suggests that short-range spin fluctuations persist in these electron-doped superconductors. The result for the spin-pair-excitation peak in Pr2−xCexCuO4−y up to x=0.2 demonstrates the absence of softening of the short-wavelength magnons.

Temperature-dependent lifetime of spin excitations in RBa2Cu3O6 (R=Eu, Y).

We have measured the light scattering by spin-pair excitations in single crystals of R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ (R=Y,Eu) for several polarization directions in the temperature range of 10 to 700 K. A strong broadening of the ${\mathit{B}}_{1\mathit{g}}$ and an enhancement of the ${\mathit{A}}_{1\mathit{g}}$ component is observed with increasing temperature. For all temperatures agreement between calculations and experimental light-scattering data has been obtained with J=810 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and a zone-edge spin-excitation damping \ensuremath{\Gamma} which changes from 184\ifmmode\pm\else\textpm\fi{}50 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (T=10 K) to 913\ifmmode\pm\else\textpm\fi{}100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (T=600 K). We conjecture that spin-lattice interaction significantly decreases the spin-excitation lifetime.

Low-frequency relaxation modes and structural disorder in KTa1-xNbxO3.

A light-scattering study of the low-frequency excitations in single crystals of cubic and tetragonal KTa{sub 1{minus}{ital x}}Nb{sub {ital x}}O{sub 3} ({ital x}=0.26 and 0.28) has been carried out by employing a technique which uses an Iodine filter to remove elastically scattered light. Low-frequency Raman and Fabry-Perot components related to structural disorder in the mixed-crystal tantalate-niobate system were observed. The spectral shape, symmetry properties, and thermal behavior of these components are consistent with an eight-site order-disorder model of the sequence of structural phase transitions observed in ferrodistortive perovskites.

Anisotropy in the near-edge absorption fine structure of YBa2Cu

We have observed orientational anisotropy of the near-edge absorption fine structure for the Cu 2p and O 1s excitations in single crystals of YBa/sub 2/Cu/sub 3/O/sub 7-//sub delta/ using electron-energy-loss spectroscopy. The O 1s absorption results suggest that O-derived holes near the Fermi level have p/sub x//sub ,//sub y/ symmetry. The Cu 2p edge is weakly anisotropic and consistent with some holes having d/sub z//sub =/ symmetry.

Recent research on radio-luminescence

During the last few years the development of sensitive photoelectron multipliers and of single crystals has made possible a quantitative study of luminescence in solids excited by electrons or by heavy particles. Previous empirical relations found between luminescence, electron potential and beam current are shown to be due to the complex structure of the powder-form screens used. Much simpler behaviour is indicated when the luminescence emission due to excitation of single crystals by single particles is investigated. In many inorganic crystals the luminescence is found to be proportional to the energy absorbed. Deviations from this linearity, particularly in organic crystals, provide indications of the nature of energy transport mechanisms in crystals. Studies of phosphors deposited as optically clear films on glass provide similar evidence and enable range energy relations to be studied for the exciting electrons. Recent theoretical researches have provided ideas on the nature of non-radiative processes in particle excited phosphors. These processes are of practical interest since they account for at least 80% of the energy absorbed by phosphor screens.