arXiv subjects

C. M. Brown

Publications and source records attributed to C. M. Brown.

17 recordsLinked to original sources

arXiv2024

Tuning the spontaneous exchange bias effect in La1.5Sr0.5CoMnO6 with sintering temperature

Here, we present a study of the influence of microstructure on the magnetic properties of polycrystalline samples of the La1.5Sr0.5CoMnO6 double perovskite, with primary attention to the spontaneous exchange bias effect, a fascinating recently discovered phenomena for which some materials exhibit unidirectional magnetic anisotropy after being cooled in zero magnetic fields. By sintering La1.5Sr0.5CoMnO6 at different temperatures, we obtained samples with distinct average grain sizes, ranging from 1.54 to 6.65 mu_m. A detailed investigation of the material's structural, morphologic, electronic, and magnetic properties using X-ray powder diffraction, powder neutron diffraction, X-ray absorption near edge structure spectroscopy, scanning electron microscopy, and AC and DC magnetometry has revealed a systematic enhancement of the exchange bias effect with increasing the average grain size. This evolution is discussed in terms of changes in the material's porosity and grain morphology and its influence on the exchange couplings at the magnetic interfaces.

cond-mat.mtrl-sci↗

arXiv2024

Layer buckling and absence of superconductivity in LaNiO2

The recent observation of unconventional superconductivity in thin films of LaNiO2 (critical temperature, Tc~10 K) and in bulk single crystals of La3Ni2O7 (327) under pressure (Tc~80 K), has brought to light a long sought-after class of superconducting nickelates. Through structural measurements in the 327-system, it was shown that the absence of superconductivity is related to bending of the O-Ni-O bonds. Similarly, the bond bending may be linked to the absence of superconductivity in bulk LaNiO2. Neutron diffraction was used on bulk non-superconducting La(1-x)Sr(x)NiO(2) samples to show that the layers are naturally buckled, creating a Ni-O-Ni bond angle of 177 at 2 K and ambient pressure. The buckling angle increases to 170 on warming to room temperature. Furthermore, a broad paramagnetic continuum is observed that decreases in intensity on cooling from room temperature signaling a possible transition to a coherent state. However, no antiferromagnetic (AFM) peaks are detected, although enhancement of ferromagnetic (FM) correlations cannot be excluded.

cond-mat.supr-con↗

arXiv2021

Néel Ordering in the Distorted Honeycomb Pyrosilicate: C-Er$_{2}$Si$_{2}$O$_{7}$

The rare-earth pyrosilicate family of compounds (RE$_{2}$Si$_{2}$O$_{7}$) hosts a variety of polymorphs, some with honeycomb-like geometries of the rare-earth sublattices, and the magnetism has yet to be deeply explored in many of the cases. Here we report on the ground state properties of C-Er$_{2}$Si$_{2}$O$_{7}$. C-Er$_{2}$Si$_{2}$O$_{7}$ crystallizes in the C2/m space group and the Er$^{3+}$ atoms form a distorted honeycomb lattice in the $a$-$b$ plane. We have utilized specific heat, DC susceptibility, and neutron diffraction measurements to characterize C-Er$_{2}$Si$_{2}$O$_{7}$. Our specific heat and DC susceptibility measurements show signatures of antiferromagnetic ordering at 2.3 K. Neutron powder diffraction confirms this transition temperature and the relative intensities of the magnetic Bragg peaks are consistent with a collinear Néel state in the magnetic space group C2'/m, with ordered moment of 6.61 $μ_{B}$ canted 13$^{\circ}$ away from the $c$-axis toward the $a$-axis. These results are discussed in relation to the isostructural quantum dimer magnet compound Yb$_{2}$Si$_{2}$O$_{7}$.

cond-mat.str-el↗

arXiv2019

Common acoustic phonon lifetimes in inorganic and hybrid lead halide perovskites

The acoustic phonons in the organic-inorganic lead halide perovskites have been reported to have anomalously short lifetimes over a large part of the Brillouin zone. The resulting shortened mean free paths of the phonons have been implicated as the origin of the low thermal conductivity. We apply neutron spectroscopy to show that the same acoustic phonon energy linewidth broadening (corresponding to shortened lifetimes) occurs in the fully inorganic CsPbBr$_{3}$ by comparing the results on the organic-inorganic CH$_{3}$NH$_{3}$PbCl$_{3}$. We investigate the critical dynamics near the three zone boundaries of the cubic $Pm\overline{3}m$ Brillouin zone of CsPbBr$_{3}$ and find energy and momentum broadened dynamics at momentum points where the Cs-site ($A$-site) motions contribute to the cross section. Neutron diffraction is used to confirm that both the Cs and Br sites have unusually large thermal displacements with an anisotropy that mirrors the low temperature structural distortions. The presence of an organic molecule is not necessary to disrupt the low-energy acoustic phonons at momentum transfers located away from the zone center in the lead halide perovskites and such damping may be driven by the large displacements or possibly disorder on the $A$ site.

cond-mat.mtrl-sci↗

arXiv2016

Investigation of the physical properties of the Ce2MAl7Ge4 (M = Co, Ir, Ni, Pd) heavy fermion compounds

We report the synthesis, crystal structure and characterization by means of single crystal x-ray diffraction, neutron powder diffraction, magnetic, thermal and transport measurements of the new heavy fermion compounds Ce$_{2}$MAl$_{7}$Ge$_{4}$ (M = Co, Ir, Ni, Pd). These compounds crystallize in a noncentrosymmetic tetragonal space group P\={4}2$_{1}$m, consisting of layers of square nets of Ce atoms separated by Ge-Al and M-Al-Ge blocks. Ce$_{2}$CoAl$_{7}$Ge$_{4}$, Ce$_{2}$IrAl$_{7}$Ge$_{4}$ and Ce$_{2}$NiAl$_{7}$Ge$_{4}$ order magnetically behavior below $T_{M}=$ 1.8, 1.6, and 0.8 K, respectively. There is no evidence of magnetic ordering in Ce$_{2}$PdAl$_{7}$Ge$_{4}$ down to 0.4 K. The small amount of entropy released in the magnetic state of Ce$_{2}$MAl$_{7}$Ge$_{4}$ (M = Co, Ir, Ni) and the reduced specific heat jump at $T_M$ suggest a strong Kondo interaction in these materials. Ce$_{2}$PdAl$_{7}$Ge$_{4}$ shows non-Fermi liquid behavior, possibly due to the presence of a nearby quantum critical point.

cond-mat.str-el↗

arXiv2015

Rotational Dynamics of Organic Cations in CH3NH3PbI3 Perovskite

Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown impressive power conversion efficiencies of above 20%. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH3NH3+ cations and their impact on relevant processes such as charge recombination and exciton dissociation are still poorly understood. Here, using elastic and quasi-elastic neutron scattering techniques and group theoretical analysis, we studied rotational modes of the CH3NH3+ cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327K) and tetragonal (165K < T < 327K) phases, the CH3NH3+ ions exhibit four-fold rotational symmetry of the C-N axis (C4) along with three-fold rotation around the C-N axis (C3), while in orthorhombic phase (T < 165K) only C3 rotation is present. Around room temperature, the characteristic relaxation times for the C4 rotation is found to be ps while for the C3 rotation ps. The -dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C4 rotational mode and the temperature dependent dielectric permittivity. Our findings on the rotational dynamics of CH3NH3+ and the associated dipole have important implications on understanding the low exciton binding energy and slow charge recombination rate in CH3NH3PbI3 which are directly relevant for the high solar cell performance.

cond-mat.mtrl-sci↗

arXiv2014

Evolution of Magnetism in the $Na_{3-d}Na_{1-x}Mg_xIr_2O_6$ Series of Honeycomb Iridates

The structural and magnetic properties of a new series of iridium-based honeycomb lattices with the formula Na3-delta(Na1-xMgx)Ir2O6 (x = 0 to 1) are reported. As x and delta are increased, the honeycomb lattice contracts and the strength of the antiferromagnetic interactions decreases systematically due to a reduction in Ir-O-Ir bond angles. Samples with imperfect stoichiometry exhibit disordered magnetic freezing at temperatures Tf between 3.4 K and 5 K. This glassy magnetism likely arises due to the presence of non-magnetic Ir3+, which are distributed randomly throughout the lattice, with a possible additional contribution from stacking faults. Together, these results demonstrate that chemical defects and non-stoichiometry have a significant effect on the magnetism of compounds in the A2IrO3 materials family.

cond-mat.str-el↗

arXiv2011

Magnetic Neutron Scattering in Solid Oxygen and its Applications to Ultracold Neutron Production

Using neutron scattering, we investigate the static and dynamic structure functions $S(Q,ω)$ of 3 distinct solid phases (using powder average techniques), and characterize the corresponding modes of energy excitation, focusing primarily on the dynamics of spin interaction. With the $S(Q,ω)$ maps, we extract the temperature dependent production rate and upscattering cross section, that are relevant for understanding experimental data on ultracold neutron production in solid oxygen at the saturated vapor pressure.

cond-mat.mtrl-sci↗

arXiv2009

Neutron scattering evidence for isolated spin-1/2 ladders in (C$_5$D$_{12}$N)$_2$CuBr$_4$

Inelastic neutron scattering was used to determine the spin Hamiltonian for the singlet ground state system of fully deuterated BPCB, (C$_{5}$D$_{12}$N)$_{2}$CuBr$_{4}$. A 2-leg spin-1/2 ladder model, with $J_\bot = (1.09 \pm 0.01)$ meV and $J_\| = (0.296 \pm 0.005)$ meV, accurately describes the data. The experimental limit on the effective inter-ladder exchange constant is $|J_{\rm int}^{\rm eff}|\lesssim 0.006$ meV, and the limit on total diagonal, intra-ladder exchange is $|J_F+J_{F'}|\leq 0.1$ meV. Including the effects of copper to bromide covalent spin transfer on the magnetic form-factor, the experimental ratios of intra-ladder bond energies are consistent with the predictions of continuous unitary transformation.

cond-mat.str-el↗

arXiv2009

One-dimensional magnetic fluctuations in the spin-2 triangular lattice α-NaMnO2

The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T ~ 45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Delta ~ 7.5 meV) has been analysed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k_B ~ 73 K), along the monoclinic b-axis and a sizeable easy-axis magnetic anisotropy (|D|/k_B ~ 3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.

cond-mat.str-el↗

arXiv2008

The crystalline electric field as a probe for long range antiferromagnetic order and superconductivity in CeFeAsO$_{1-x}$F$_x$

We use inelastic neutron scattering to study the crystalline electric field (CEF) excitations of Ce$^{3+}$ in CeFeAsO$_{1-x}$F$_{x}$($x=0,0.16$). For nonsuperconducting CeFeAsO, the Ce CEF levels have three magnetic doublets in the paramagnetic state, but these doublets split into six singlets when Fe ions order antiferromagnetically. For superconducting CeFeAsO$_{0.84}$F$_{0.16}$ ($T_c=41$ K), where the static AF order is suppressed, the Ce CEF levels have three magnetic doublets at $\hbarω=0,18.7,58.4$ meV at all temperatures. Careful measurements of the intrinsic linewidth $Γ$ and the peak position of the 18.7 meV mode reveal clear anomaly at $T_c$, consistent with a strong enhancement of local magnetic susceptibility $χ^{\prime\prime}(\hbarω)$ below $T_c$. These results suggest that CEF excitations in the rare-earth oxypnictides can be used as a probe of spin dynamics in the nearby FeAs planes.

cond-mat.supr-con↗

arXiv2007

EUV emission lines and diagnostics observed with Hinode/EIS

Quiet Sun and active region spectra from the Hinode/EIS instrument are presented, and the strongest lines from different temperature regions discussed. A list of emission lines recommended to be included in EIS observation studies is presented based on analysis of blending and diagnostic potential using the CHIANTI atomic database. In addition we identify the most useful density diagnostics from the ions covered by EIS.

astro-ph↗

arXiv2006

Static and dynamic Jahn-Teller effect in the alkali metal fulleride salts A4C60 (A = K, Rb, Cs)

We report the temperature dependent mid- and near-infrared spectra of K4C60, Rb4C60 and Cs4C60. The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C604- which brings the fulleride anion from D2h to either a D3d or a D5d distortion. In contrast to Cs4C60, low temperature neutron diffraction measurements did not reveal a structural phase transition in either K4C60 and Rb4C60. This proves that the molecular transition is driven by the molecular Jahn-Teller effect, which overrides the distorting potential field of the surrounding cations at high temperature. In K4C60 and Rb4C60 we suggest a transition from a static to a dynamic Jahn-Teller state without changing the average structure. We studied the librations of these two fullerides by temperature dependent inelastic neutron scattering and conclude that both pseudorotation and jump reorientation are present in the dynamic Jahn-Teller state.

cond-mat.mtrl-sci↗

arXiv2003

Structure and Dynamics of Superconducting NaxCoO(2) Hydrate and Its Unhydrated Analog

Neutron scattering has been used to investigate the crystal structure and lattice dynamics of superconducting Na0.3CoO2 1.4(H/D)2O, and the parent Na0.3CoO2 material. The structure of Na0.3CoO2 consists of alternate layers of CoO2 and Na and is the same as the structure at higher Na concentrations. For the superconductor, the water forms two additional layers between the Na and CoO2, increasing the c-axis lattice parameter of the hexagonal P63/mmc space group from 11.16 A to 19.5 A. The Na ions are found to occupy a different configuration from the parent compound, while the water forms a structure that replicates the structure of ice. Both types of sites are only partially occupied. The CoO2 layer in these structures is robust, on the other hand, and we find a strong inverse correlation between the CoO2 layer thickness and the superconducting transition temperature (TC increases with decreasing thickness). The phonon density-of-states for Na0.3CoO2 exhibits distinct acoustic and optic bands, with a high-energy cutoff of ~100 meV. The lattice dynamical scattering for the superconductor is dominated by the hydrogen modes, with librational and bending modes that are quite similar to ice, supporting the structural model that the water intercalates and forms ice-like layers in the superconductor.

cond-mat.supr-con↗

arXiv2003

Probing hole-induced ferromagnetic exchange in magnetic semiconductors by inelastic neutron scattering

The effect of hole doping on the exchange coupling of the nearest neighbor (NN) Mn pairs in Zn$_{1-x}$Mn$_x$Te is probed by inelastic neutron scattering. The difference in the NN exchange energy $ΔJ_1$ in the presence and in the absence of the holes is determined. The obtained value of $ΔJ_1$ is in good agreement with the predictions of the Zener/RKKY model, even on the insulator side of the metal-insulator transition.

cond-mat.mtrl-sci↗

arXiv2001

High-Resolution Spectroscopy of G191-B2B in the Extreme Ultraviolet

We report a high-resolution (R=3000-4000) spectroscopic observation of the DA white dwarf G191-B2B in the extreme ultraviolet band 220-245 A. A low- density ionised He component is clearly present along the line-of-sight, which if completely interstellar implies a He ionisation fraction considerably higher than is typical of the local interstellar medium. However, some of this material may be associated with circumstellar gas, which has been detected by analysis of the C IV absorption line doublet in an HST STIS spectrum. A stellar atmosphere model assuming a uniform element distribution yields a best fit to the data which includes a significant abundance of photospheric He. The 99-percent confidence contour for the fit parameters excludes solutions in which photospheric He is absent, but this result needs to be tested using models allowing abundance gradients.

astro-ph↗

arXiv2001

Giant anharmonicity and non-linear electron-phonon coupling in MgB$_{2}$; A combined first-principles calculations and neutron scattering study

We report first-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB$_{2}$. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. The numerical results reveal that the in-plane boron phonons (with E$_{2g}$ symmetry) near the zone-center are very anharmonic, and are strongly coupled to the partially occupied planar B $σ$ bands near the Fermi level. This giant anharmonicity and non-linear electron-phonon coupling is key to explaining the observed high T$_{c}$ and boron isotope effect in MgB$_{2}$

cond-mat.mtrl-sci↗