arXiv subjects

C. D. Frost

Publications and source records attributed to C. D. Frost.

At least 37 records · Page 2Linked to original sources

arXiv2008

The emergence of coherent magnetic excitations in the pseudogap phase of La2-xSrxCuO4

We use inelastic neutron scattering to measure the magnetic excitations in the underdoped superconductor La2-xSrxCuO4 (x=0.085, Tc=22 K) over energy and temperatures ranges 5 < E < 200 meV and 5 < T < 300 K respectively. At high temperature (T = 300 K), we observe strongly damped excitations with a characteristic energy scale of approximately 50 meV. As the temperature is lowered to T = 30 K, and we move into the pseudogap state, the magnetic excitations become highly structured in energy and momentum below about 60 meV. This change appears to be associated with the development of the pseudogap in the electronic excitations.

cond-mat.supr-con↗

arXiv2008

Spin dynamics in the pseudogap state of a high-temperature superconductor

The pseudogap is one of the most pervasive phenomena of high temperature superconductors. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature Tc, or to a hidden order parameter competing with superconductivity. Here we use inelastic neutron scattering from underdoped YBa(2)Cu(3)O(6.6) to show that the dispersion relations of spin excitations in the superconducting and pseudogap states are qualitatively different. Specifically, the extensively studied "hour glass" shape of the magnetic dispersions in the superconducting state is no longer discernible in the pseudogap state and we observe an unusual "vertical" dispersion with pronounced in-plane anisotropy. The differences between superconducting and pseudogap states are thus more profound than generally believed, suggesting a competition between these two states. Whereas the high-energy excitations are common to both states and obey the symmetry of the copper oxide square lattice, the low-energy excitations in the pseudogap state may be indicative of collective fluctuations towards a state with broken orientational symmetry predicted in theoretical work.

cond-mat.supr-con↗

arXiv2008

A study of the quantum classical crossover in the spin dynamics of the 2D S=5/2 antiferromagnet Rb2MnF4: neutron scattering, computer simulations, and analytic theories

We report comprehensive inelastic neutron scattering measurements of the magnetic excitations in the 2D spin-5/2 Heisenberg antiferromagnet Rb2MnF4 as a function of temperature from deep in the Neel ordered phase up to paramagnetic, 0.13 < kBT/4JS < 1.4. Well defined spin-waves are found for wave-vectors larger than the inverse correlation length $η^{-1}$ for temperatures up to near the Curie-Weiss temperature, $Θ_{CW}$. For wave-vectors smaller than $η^{-1}$, relaxational dynamics occurs. The observed renormalization of spin-wave energies, and evolution of excitation line-shapes, with increasing temperature are quantitatively compared with finite-temperature spin-wave theory, and computer simulations for classical spins. Random phase approximation calculations provide a good description of the low-temperature renormalisation of spin-waves. In contrast, lifetime broadening calculated using the first Born approximation shows, at best, modest agreement around the zone boundary at low temperatures. Classical dynamics simulations using an appropriate quantum-classical correspondence were found to provide a good description of the intermediate- and high-temperature regimes over all wave-vector and energy scales, and the crossover from quantum to classical dynamics observed around $Θ_{CW}/S$, where the spin S=5/2. A characterisation of the data over the whole wave-vector/energy/temperature parameter space is given. In this, $T^2$ behaviour is found to dominate the wave-vector and temperature dependence of the line widths over a large parameter range, and no evidence of hydrodynamic behaviour or dynamical scaling behaviour found within the accuracy of the data sets.

cond-mat.stat-mech↗

arXiv2008

Mesoscopic Phase Coherence in a Quantum Spin Fluid

Mesoscopic quantum phase coherence is important because it improves the prospects for handling quantum degrees of freedom in technology. Here we show that the development of such coherence can be monitored using magnetic neutron scattering from a one-dimensional spin chain Y2BaNiO5, a quantum spin fluid where no classical, static magnetic order is present. In the cleanest samples, the quantum coherence length is 20 nm, almost an order of magnitude larger than the classical antiferromagnetic correlation length of 3 nm. We also demonstrate that the coherence length can be modified by static and thermally activated defects in a quantitatively predictable manner.

cond-mat.str-el↗

arXiv2007

Absence of low temperature anomaly in the Debye-Waller factor of solid He-4

The mean square atomic displacement in hcp-phase solid He-4 has been measured in crystals with a molar volume of 21.3 cm^3. It is temperature independent from 1 K to 140 mK, with no evidence for an anomaly in the vicinity of the proposed supersolid transition. The mean square displacement is different for in-plane motions (0.122 +/- 0.001 A^(2)) and out-of-plane motions (0.150 +/- 0.001 A ^(2)).

cond-mat.other↗

arXiv2007

Disappearance of antiferromagnetic spin excitations in over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$

Magnetic excitations in the energy range up to 100 meV are studied for over-doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.25$ and 0.30, using time-of-flight neutron spectroscopy. Comparison of spectra integrated over the width of an antiferromagnetic Brillouin zone demonstrates that the magnetic scattering at intermediate energies, $20 \lesssim ω\lesssim 100$ meV, progressively decreases with over-doping. This strongly suggests that the magnetism is not related to Fermi surface nesting, but rather is associated with a decreasing volume fraction of (probably fluctuating) antiferromagnetic bubbles.

cond-mat.supr-con↗

arXiv2007

Evidence for decay of spin-waves above the pseudogap in underdoped YBa2Cu3O6.35

The magnetic spectrum at high-energies in heavily underdoped YBa$_{2}$Cu$_{3}$O$_{6.35}$ (T$_{c}$=18 K) has been determined throughout the Brillouin zone. At low-energy the scattering forms a cone of spin excitations emanating from the antiferromagnetic (0.5, 0.5) wave vector with an acoustic velocity similar to that of insulating cuprates. At high energy transfers, below the maximum energy of 270 meV at (0.5, 0), we observe zone boundary dispersion much larger and spectral weight loss more extensive than in insulating antiferromagnets. Moreover we report phenomena not found in insulators, an overall lowering of the zone-boundary energies and a large damping of $\sim$ 100 meV of the spin excitations at high-energies. The energy above which the damping occurs coincides approximately with the gap determined from transport measurements. We propose that as the energy is raised the spin excitations encounter an extra channel of decay into particle-hole pairs of a continuum that we associate with the pseudogap.

cond-mat.supr-con↗

arXiv2007

Two energy scales in the spin excitations of the high-T_c superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$

The excitations responsible for producing high-temperature superconductivity in the cuprates have not been identified. Two promising candidates are collective spin excitations and phonons. A recent argument against spin excitations has been their inability to explain structures seen in electronic spectroscopies such as photoemission and tunnelling. Here we use inelastic neutron scattering to demonstrate that collective spin excitations in optimally doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ are more structured than previously thought. The excitations have a two component structure with a low-frequency component strongest around 18 meV and a broader component strongest near 40-70 meV. The second component carries most of the spectral weight and its energy matches structure seen in photoemission and tunnelling spectra in the range 50-90 meV. Our results demonstrate that collective spin excitations can explain features of quasiparticle spectroscopies and are therefore likely to be the strongest coupled excitations.

cond-mat.supr-con↗

arXiv2006

One- and Two-Triplon Spectra of a Cuprate Ladder

We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41, which will serve as a model system for the investigation of doped ladders. A key feature in the analysis was the separation of one-triplon and two-triplon scattering contributions due to an antiphase rung modulation. This enabled extraction of the one-triplon dispersion which has a spin gap of 26.4+/-0.3 meV, and for the first time two-triplon scattering is clearly observed. The exchange constants are determined by fitting the data to be Jleg=186 meV and Jrung=124 meV along the leg and the rung respectively. In addition to the surprisingly large difference between Jleg and Jrung a substantial ring exchange of Jcyc=31 meV was confirmed.

cond-mat.str-el↗

arXiv2006

High Energy Spin Excitations in Electron-Doped Superconducting Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-δ}$ with $T_c=21$ K

We use high-resolution inelastic neutron scattering to study the low-temperature magnetic excitations of electron-doped superconducting Pr$_{0.88}$LaCe$_{0.12}$CuO$_{4-δ}$ ($T_c=21\pm 1$ K) over a wide energy range (4 meV$\le\hbarω\le 330$ meV). The effect of electron-doping is to cause a wave vector ($Q$) broadening in the low-energy ($\hbarω\le 80$ meV) commensurate spin fluctuations at ($π$,$π$) and to suppress the intensity of spin-wave-like excitations at high energies ($\hbarω\ge 100$ meV). This leads to a substantial redistribution in the spectrum of the local dynamical spin susceptibility $χ^{\prime\prime}(ω)$, and reveals a new energy scale similar to that of the lightly hole-doped YB$_2$Cu$_3$O$_{6.353}$ ($T_c=18$ K).

cond-mat.supr-con↗

arXiv2005

Localized excitation in the hybridization gap in YbAl3

The intermediate valence compound YbAl3 exhibits a broad magnetic excitation with characteristic energy E1 ~ 50meV, of order of the Kondo energy (TK ~ 600-700K). In the low temperature (T < Tcoh ~ 40K) Fermi liquid state, however, a new magnetic excitation arises at E2 ~ 33meV, which lies in the hybridization gap that exists in this compound. We show, using inelastic neutron scattering on a single-crystal sample, that while the scattering at energies near E1 has the momentum (Q-) dependence expected for interband scattering across the indirect gap, the scattering near E2 is independent of Q. This suggests that it arises from a spatially-localized excitation in the hybridization gap.

cond-mat.str-el↗

arXiv2005

Two-magnon excitations observed by neutron scattering in the two-dimensional spin-5/2 Heisenberg antiferromagnet Rb2MnF4

The low-temperature magnetic excitations of the two-dimensional spin-5/2 square-lattice Heisenberg antiferromagnet Rb2MnF4 have been probed using pulsed inelastic neutron scattering. In addition to dominant sharp peaks identified with one-magnon excitations, a relatively weak continuum scattering is also observed at higher energies. This is attributed to neutron scattering by pairs of magnons and the observed intensities are consistent with predictions of spin wave theory.

cond-mat.str-el↗

arXiv2005

High-energy Cu spin excitations in PrBa2Cu3O6+x

This paper describes high-energy neutron inelastic scattering measurements of propagating magnetic excitations in PrBa2Cu3O6+x (x approx 0.2 and 0.93). The measurements probe the acoustic and optic modes of the antiferromagnetically-ordered copper--oxygen bilayers in the energy range 50-150 meV. The observed magnon dispersion can be described satisfactorily in this energy range by a spin wave model including intra- and inter-layer nearest-neighbour exchange constants J_parallel and J_perp. We find J_parallel = 127 +/- 10 meV and J_perp = 5.5 +/- 0.9 meV. The value of J_parallel is virtually the same as that found in YBa2Cu3O6.2, but J_perp is a factor of two smaller. To within experimental error the values of J_parallel and J_perp for PrBa2Cu3O6+x do not vary with oxygen doping.

cond-mat.str-el↗

arXiv2005

Quantum criticality and universal scaling of a quantum antiferromagnet

Quantum effects dominate the behaviour of many diverse materials. Of particular current interest are those systems in the vicinity of a quantum critical point (QCP). Their physical properties are predicted to reflect those of the nearby QCP with universal features independent of the microscopic details. The prototypical QCP is the Luttinger liquid (LL) which is of relevance to many quasi-one-dimensional materials. The magnetic material KCuF3 realizes an array of weakly-coupled spin chains (or LLs) and thus lies close to but not exactly at the Luttinger liquid quantum critical point. By using inelastic neutron scattering we have collected a complete data set of the magnetic correlations of KCuF3 as a function of momentum, energy, and temperature. The LL description is found to be valid over an extensive range of these parameters, and departures from this behaviour at high and low energies and temperatures have been identified and explained.

cond-mat.str-el↗

arXiv2004

From incommensurate to dispersive spin-fluctuations: The high-energy inelastic spectrum in superconducting YBa2Cu3O6.5

We have investigated the spin fluctuations at energy transfers up to ~110 meV, well above the resonance energy (33 meV) in the YBa2Cu3O6.5 ortho-II superconductor using neutron time-of-flight and triple-axis techniques. The spectrum at high energies differs from the low-energy incommensurate modulations previously reported where the incommensurate wave vector is largely independent of energy. Well above the resonance the peak of the spin response lies at wave vectors that increase with energy. Within error the excitations at all energies above the resonance are best described by a ring around the (pi, pi) position. The isotropic wave-vector pattern differs from a recently reported square pattern in different but related systems. The spin spectral weight at high-energies is similar to that in the insulator but the characteristic velocity is ~40% lower. We introduce a method of extracting the acoustic and optic weights at all energies from time-of-flight data. We find that the optic spectral weight extends to surprisingly low-energies of ~25 meV, and infer that the bilayer spin correlations weaken with increase in hole doping. When the low-energy optic excitations are taken into account we measure the total integrated weight around (pi, pi), for energies below 120 meV, to agree with that expected from the insulator. As a qualitative guide, we compare spin-wave calculations for an ordered and a disordered stripe model and describe the inadequacy of this and other stripe models for the high-energy fluctuations.

cond-mat.supr-con↗

arXiv2004

Ferromagnetic in-plane spin fluctuations in NaxCoO2 observed by neutron inelastic scattering

We present neutron scattering spectra taken from a single crystal of Na0.75CoO2, the precursor to a novel cobalt-oxide superconductor. The data contain a prominent inelastic signal at low energies (~10 meV), which is localized in wavevector about the origin of two-dimensional reciprocal space. The signal is highly dispersive, and decreases in intensity with increasing temperature. We interpret these observations as direct evidence for the existence of ferromagnetic spin fluctuations within the cobalt-oxygen layers.

cond-mat.str-el↗

arXiv2003

Spinons in the strongly correlated copper oxide chains in SrCuO2

We have investigated the spin dynamics in the strongly correlated chain copper oxide SrCuO$_2$ for energies up to $\gtrsim 0.6$ eV using inelastic neutron scattering. We observe an acoustic band of magnetic excitations which is well described by the "Muller-ansatz" for the two-spinon continuum in the S=1/2 antiferromagnetic Heisenberg spin chain. The lower boundary of the continuum extends up to $\approx 360$ meV, which corresponds to an exchange constant $J = 226(12)$ meV. Our finding that an effective Heisenberg spin Hamlitonian adequately describes the spin sector of this 1D electron system, even though its energy scale is comparable to that of charge excitations, provides compelling experimental evidence for spin-charge separation.

cond-mat.str-el↗

arXiv2003

Magnetic excitations in La$_{1.5}$Sr$_{0.5}$CoO$_4$

We report magnetization and neutron scattering measurements of the half-doped compound La$_{1.5}$Sr$_{0.5}$CoO$_4$, which exhibits a checkerboard pattern of charge ordering below ~800K. In the antiferromagnetically-ordered phase below \~40K the spins are found to be canted in the ab plane. The spin excitation spectrum includes spin-wave excitations with a maximum energy of 16 meV, and diffuse magnetic modes at energies around 30 meV.

cond-mat.str-el↗