arXiv subjects

T. G. Perring

Publications and source records attributed to T. G. Perring.

At least 55 records · Page 3Linked to original sources

arXiv2010

Paramagnetic Spin Correlations in CaFe2As2 Single Crystals

Magnetic correlations in the paramagnetic phase of CaFe2As2 (T_N=172 K) have been examined by means of inelastic neutron scattering from 180 K (~ 1.05 T_N) up to 300 K (~1.8 T_N). Despite the first-order nature of the magnetic ordering, strong but short-range antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasi-elastic scattering centered at the wavevector Q_{AFM} of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (E> 60 meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c-direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt doped superconducting BaFe2As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-range anisotropic spin correlations with overdamped spin dynamics.

cond-mat.supr-con↗

arXiv2010

Dispersive Spin Fluctuations in the near optimally-doped superconductor Ba(Fe1-xCox)2As2 ($x$=0.065)

Inelastic neutron scattering is used to probe the collective spin excitations of the near optimally-doped superconductor Ba(Fe1-xCox)2As2 ($x$=0.065). Previous measurements on the antiferromagnetically ordered parents of this material show a strongly anisotropic spin-wave velocity. Here we measure the magnetic excitations up to 80 meV and show that a similar anisotropy persists for superconducting compositions. The dispersive mode measured here connects directly with the spin resonance previously observed in this compound. When placed on an absolute scale, our measurements show that the local- or wavevector- integrated susceptibility is larger in magnitude than that of the ordered parents over the energy range probed.

cond-mat.supr-con↗

arXiv2009

Confinement of fractional quantum number particles in a condensed matter system

The idea of confinement states that in certain systems constituent particles can be discerned only indirectly being bound by an interaction whose strength increases with increasing particle separation. Though the most famous example is the confinement of quarks to form baryons and mesons in (3+1)-dimensional Quantum Chromodynamics, confinement can also be realized in the systems of condensed matter physics such as spin-ladders which consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S=1/2, are confined even by an infinitesimal interchain coupling. The realizations studied so far cannot illustrate this process due to the large strength of their interchain coupling which leaves no energy window for the spinon excitations of individual chains. Here we present neutron scattering experiments for a weakly-coupled ladder material. At high energies the spectral function approaches that of individual chains; at low energies it is dominated by spin 0,1 excitations of strongly-coupled chains.

cond-mat.str-el↗

arXiv2009

Inelastic neutron scattering studies of the quantum frustrated magnet clinoatacamite, $γ$-Cu2(OD)3Cl, a proposed valence bond solid (VBS)

The frustrated magnet clinoatacamite, $γ$-Cu$_2$(OH)$_3$Cl, is attracting a lot of interest after suggestions that at low temperature it forms an exotic quantum state termed a Valence Bond Solid (VBS) made from dimerised Cu$^{2+}$ ($S=1/2$) spins.\cite{Lee_clinoatacamite} Key to the arguments surrounding this proposal were suggestions that the kagomé planes in the magnetic pyrochlore lattice of clinoatacamite are only weakly coupled, causing the system to behave as a quasi-2-dimensional magnet. This was reasoned from the near 95$^\circ$ angles made at the bridging oxygens that mediate exchange between the Cu ions that link the kagomé planes. Recent work pointed out that this exchange model is inappropriate for $γ$-Cu$_2$(OH)$_3$Cl, where the oxygen is present as a $μ_3$-OH.\cite{Wills_JPC} Further, it used symmetry calculations and neutron powder diffraction to show that the low temperature magnetic structure ($T<6$ K) was canted and involved significant spin ordering on all the Cu$^{2+}$ spins, which is incompatible with the interpretation of simultaneous VBS and Néel ordering. Correspondingly, clinoatacamite is best considered a distorted pyrochlore magnet. In this report we show detailed inelastic neutron scattering spectra and revisit the responses of this frustrated quantum magnet.

cond-mat.str-el↗

arXiv2009

Testing the itinerancy of spin dynamics in superconducting Bi2212

Much of what we know about the electronic states of high-temperature superconductors is due to photoemission and scanning tunneling spectroscopy studies of the compound Bi2212. The demonstration of well-defined quasiparticles in the superconducting state has encouraged many theorists to apply the conventional theory of metals, Fermi-liquid theory, to the cuprates. In particular, the spin excitations observed by neutron scattering at energies below twice the superconducting gap energy are commonly believed to correspond to an excitonic state involving itinerant electrons. Here, we present the first measurements of the magnetic spectral weight of optimally-doped Bi2212 in absolute units. The lack of temperature dependence of the local spin susceptibility across the superconducting transition temperature, T_c, is incompatible with the itinerant calculations. Alternatively, the magnetic excitations could be due to local moments, as the magnetic spectrum is similar to that in LBCO where quasiparticles and local moments coexist.

cond-mat.supr-con↗

arXiv2009

Itinerant magnetic excitations in antiferromagnetic CaFe2As2

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 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.

cond-mat.supr-con↗

arXiv2008

Covalent bonding and magnetism in cuprates

The importance of covalent bonding for the magnetism of 3d metal complexes was first noted by Pauling in 1931. His point became moot, however, with the success of the ionic picture of Van Vleck, where ligands influence magnetic electrons of 3d ions mainly through electrostatic fields. Anderson's theory of spin superexchange later established that covalency is at the heart of cooperative magnetism in insulators, but its energy scale was believed to be small compared to other inter-ionic interactions and therefore it was considered a small perturbation of the ionic picture. This assertion fails dramatically in copper oxides, which came to prominence following the discovery of high critical temperature superconductors (HTSC). Magnetic interactions in cuprates are remarkably strong and are often considered the origin of the unusually high superconducting transition temperature, Tc. Here we report a detailed survey of magnetic excitations in the one-dimensional cuprate Sr2CuO3 using inelastic neutron scattering (INS). We show that although the experimental dynamical spin structure factor is well described by the model S=1/2 nearest-neighbour Heisenberg Hamiltonian typically used for cuprates, the magnetic intensity is modified dramatically by strong hybridization of Cu 3d states with O p states, showing that the ionic picture of localized 3d Heisenberg spin magnetism is grossly inadequate. Our findings provide a natural explanation for the puzzle of the missing INS magnetic intensity in cuprates and have profound implications for understanding current and future experimental data on these materials.

cond-mat.str-el↗

arXiv2008

High energy spin excitations in BaFe2As2

We report neutron scattering measurements of cooperative spin excitations in antiferromagnetically ordered BaFe2As2, the parent phase of an iron pnictide superconductor. The data extend up to ~100meV and show that the spin excitation spectrum is sharp and highly dispersive. By fitting the spectrum to a linear spin-wave model we estimate the magnon bandwidth to be in the region of 0.17eV. The large characteristic spin fluctuation energy suggests that magnetism could play a role in the formation of the superconducting state.

cond-mat.supr-con↗

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↗

arXiv2007

Quantum dynamics and entanglement of spins on a square lattice

Bulk magnetism in solids is fundamentally quantum mechanical in nature. Yet in many situations, including our everyday encounters with magnetic materials, quantum effects are masked, and it often suffices to think of magnetism in terms of the interaction between classical dipole moments. Whereas this intuition generally holds for ferromagnets, even as the size of the magnetic moment is reduced to that of a single electron spin (the quantum limit), it breaks down spectacularly for antiferromagnets, particularly in low dimensions. Considerable theoretical and experimental progress has been made in understanding quantum effects in one-dimensional quantum antiferromagnets, but a complete experimental description of even simple two-dimensional antiferromagnets is lacking. Here we describe a comprehensive set of neutron scattering measurements that reveal a non-spin-wave continuum and strong quantum effects, suggesting entanglement of spins at short distances in the simplest of all two-dimensional quantum antiferromagnets, the square lattice Heisenberg system.

cond-mat.str-el↗

arXiv2007

From Cooperative Paramagnet to Néel Order in Y_2Ru_2O_7

Spin correlations in the pyrochlore antiferromagnet Y_2Ru_2O_7 with Curie-Weiss temperature $Θ_{CW}=-1100$ K and critical temperature T_N=77 K were examined through neutron scattering. For $T_N<T<Θ_{CW}/3$ the data show spin relaxation with a rate $\hbarΓ= 1.17(9)k_BT$. For T<T_N spectral weight moves to higher energies with substantial changes up to $4\times k_BT_N$. For T<<T_N there is a $Δ=11(1)$ meV energy gap and a pronounced spectral maximum at 19.7 meV. Throughout the temperature range examined the wave vector dependence of inelastic scattering exhibits a broad peak for $Qd\approx 3.8$ (d is Ru-Ru spacing) consistent with dipolar spin correlations.

cond-mat.str-el↗

arXiv2007

Zener double exchange from local valence fluctuations in magnetite

Magnetite (Fe$_{3}$O$_{4}$) is a mixed valent system where electronic conductivity occurs on the B-site (octahedral) iron sublattice of the spinel structure. Below $T_{V}=122$ K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B-sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B-site sublattice ($\sim$80 meV) are shifted upwards in energy above $T_{V}$ due to the occurrence of B-B ferromagnetic double exchange in the mixed valent metallic phase. The double exchange interaction affects only spin waves of $Δ_{5}$ symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by electron correlations above $T_{V}$.

cond-mat.str-el↗

arXiv2007

The Persistence of High-Frequency Spin Fluctuations in Overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ ($x$=0.22)

We report a detailed inelastic neutron scattering study of the collective magnetic excitations of overdoped superconducting La1.78Sr0.22CuO4 for the energy range 0-160 meV. Our measurements show that overdoping suppresses the strong response present for optimally doped La2-xSrxCuO4 which is peaked near 50 meV. The remaining response is peaked at incommensurate wavevectors for all energies investigated. We observe a strong high-frequency magnetic response for E >= 80 meV suggesting that significant antiferromagnetic exchange couplings persist well into the overdoped part of the cuprate phase diagram.

cond-mat.supr-con↗

arXiv2007

Spin waves throughout the Brillouin zone and magnetic exchange coupling in ferromagnetic metallic manganites La$_{1-x}$Ca$_{x}$MnO$_3$ ($x=0.25,0.30$)

Using time-of-flight and triple-axis inelastic neutron spectroscopy, we determine spin wave excitations throughout the Brillouin zone for ferromagnetic manganites La$_{1-x}$Ca$_x$MnO$_3$ ($x=0.25,0.3$) in their low temperature metallic states. While spin wave excitations in the long wavelength limit (spin stiffness $D$) have similar values for both compounds, the excitations near the Brillouin zone boundary of La$_{0.7}$Ca$_{0.3}$MnO$_3$ are considerable softened in all symmetry directions compared to that of La$_{0.75}$Ca$_{0.25}$MnO$_3$. A Heisenberg model with the nearest neighbor and the fourth neighbor exchange interactions can describe the overall dispersion curves fairly well. We compare the data with various theoretical models describing the spin excitations of ferromagnetic manganites.

cond-mat.str-el↗

arXiv2007

High-energy magnetic excitations from dynamic stripes in LBCO (x=1/8)

We use inelastic neutron scattering to study the temperature dependence of magnetic excitations (for energies up to 100 meV) in the cuprate La$_{1.875}$Ba$_{0.125}$CuO$_4$. This compound exhibits stripe order below a temperature of ~50 K; previous measurements have shown that the magnetic excitations of the stripe-ordered phase have an hour-glass-like dispersion, with a saddle point at ~50 meV. Here we compare measurements in the disordered phase taken at 65 and 300 K. At energies on the scale of $k_{\rm B}T$, there is substantial momentum-broadening of the signal, and the low-energy incommensurate features can no longer be resolved at 300 K. In contrast, there is remarkably little change in the scattered signal for energies greater than $k_{\rm B}T$. In fact, the momentum-integrated dynamic susceptibility is almost constant with temperature. We suggest that the continuity of higher-energy magnetic excitations is strong evidence for dynamic stripes in the high-temperature, disordered phase. We reconsider the nature of the magnetic dispersion, and we discuss the correspondences between the thermal evolution of magnetic stripe correlations with other electronic properties of the cuprates.

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

Magnetic Excitations and their energy change available to Superconducting Condensation in Optimally Doped YBa$_2$Cu$_3$O$_{6.95}$

Understanding the magnetic excitations in high-transition temperature (high-$T_c$) copper oxides is important because they may mediate the electron pairing for superconductivity. By determining the wavevector ({\bf Q}) and energy ($\hbarω$) dependence of the magnetic excitations, one can calculate the change in the exchange energy available to the superconducting condensation energy. For the high-$T_c$ superconductor YBa$_2$Cu$_3$O$_{6+x}$, the most prominent feature in the magnetic excitations is the resonance. Although the resonance has been suggested to contribute a major part of the superconducting condensation, the accuracy of such an estimation has been in doubt because the resonance is only a small portion of the total magnetic scattering. Here we report an extensive mapping of magnetic excitations for YBa$_2$Cu$_3$O$_{6.95}$ ($T_c\approx 93$ K). Using the absolute intensity measurements of the full spectra, we estimate the change in the magnetic exchange energy between the normal and superconducting states and find it to be about 15 times larger than the superconducting condensation energy. Our results thus indicate that the change in the magnetic exchange energy is large enough to provide the driving force for high-$T_c$ superconductivity in YBa$_2$Cu$_3$O$_{6.95}$.

cond-mat.supr-con↗