arXiv subjects

R. J. Birgeneau

Publications and source records attributed to R. J. Birgeneau.

At least 109 records · Page 6Linked to original sources

arXiv2002

Resonant inelastic x-ray scattering study of charge excitations in La2CuO4

We report a resonant inelastic x-ray scattering study of the dispersion relations of charge transfer excitations in insulating La$_2$CuO$_4$. These data reveal two peaks, both of which show two-dimensional characteristics. The lowest energy excitation has a gap energy of $\sim 2.2$ eV at the zone center, and a dispersion of $\sim 1$ eV. The spectral weight of this mode becomes dramatically smaller around ($π$, $π$). The second peak shows a smaller dispersion ($\sim 0.5$ eV) with a zone-center energy of $\sim 3.9$ eV. We argue that these are both highly dispersive exciton modes damped by the presence of the electron-hole continuum.

cond-mat.str-el↗

arXiv2002

Enhancement of long-range magnetic order by magnetic field in superconducting La2CuO(4+y)

We report a detailed study, using neutron scattering, transport and magnetization measurements, of the interplay between superconducting (SC) and spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below the same critical temperature. However, the SDW order grows with applied magnetic field, whereas SC order is suppressed. Most importantly, the field dependence of the SDW Bragg peak intensity has a cusp at zero field, as predicted by a recent theory of competing SDW and SC order. This leads us to conclude that there is a repulsive coupling between the two order parameters. The question of whether the two kinds of order coexist or microscopically phase separate is discussed.

cond-mat.supr-con↗

arXiv2002

Ferroelectric Ordering in the Relaxor Pb(Mg1/3Nb2/3)O3 as Evidenced by Low-Temperature Phonon Anomalies

Neutron scattering measurements of the lowest-energy TO phonons in the relaxor Pb(Mg1/3Nb2/3)O3 (PMN) are reported for 10<=T<=750 K. The soft mode, which is overdamped by the polar nanoregions below the Burns temperature T_d = 620 K, surprisingly recovers below 220 K. The square of the soft mode energy hw0^2 increases linearly with decreasing temperature, and is consistent with the behavior of a ferroelectric soft mode. At 10 K, hw0 reaches 11 meV, the same value observed in ferroelectric Pb(Zn1/3Nb2/3)O3 at low-T. An unusual broadening of the TA phonon starts at T_d and disappears at 220 K, coincident with the recovery of the TO mode. These dynamics suggest that a well-developed ferroelectric state is established below 220 K.

cond-mat.mtrl-sci↗

arXiv2002

Muon spin relaxation studies of incommensurate magnetism and superconductivity in stage-4 La$_{2}$CuO$_{4.11}$ and La$_{1.88}$Sr$_{0.12}$CuO$_{4}$

This paper reports muon spin relaxation (MuSR) measurements of two single crystals of the title high-Tc cuprate systems where static incommensurate magnetism and superconductivity coexist. By zero-field MuSR measurements and subsequent analyses with simulations, we show that (1) the maximum ordered Cu moment size (0.36 Bohr magneton) and local spin structure are identical to those in prototypical stripe spin systems with the 1/8 hole concentration; (2) the static magnetism is confined to less than a half of the volume of the sample, and (3) regions with static magnetism form nano-scale islands with the size comparable to the in-plane superconducting coherence length. By transverse-field MuSR measurements, we show that Tc of these systems is related to the superfluid density, in the same way as observed in cuprate systems without static magnetism. We discuss a heuristic model involving percolation of these nanoscale islands with static magnetism as a possible picture to reconcile heterogeneity found by the present MuSR study and long-range spin correlations found by neutron scattering.

cond-mat.supr-con↗

arXiv2002

Electronic phase separation in lightly-doped La$_{2-x}$Sr$_x$CuO$_4$

The hole concentration dependence of the magnetic correlations is studied in very lightly-doped La$_{2-x}$Sr$_x$CuO$_4$ ($x<$0.02), which shows coexistence of a three-dimensional antiferromagnetic (AF) long-range ordered phase and a spin-glass phase at low temperatures. It is found that the spin-glass phase in the coexistence region also shows a diagonal spin modulation as in the pure spin-glass phase in La$_{2-x}$Sr$_x$CuO$_4$ (0.02$\le x\le$0.055). Below $x\sim$0.02 the diagonal stripe structure is the same as that in $x\sim$0.02 with a volume fraction almost proportional to hole concentration, suggesting that electronic phase separation of the doped holes occurs so that some regions with hole concentration $h_c\sim$0.02 and the rest with $h_c\sim$0 are formed. This result also indicates that the transition between the AF and spin-glass phase around $x$=0.02 is first order.

cond-mat.str-el↗

arXiv2001

Hydrogen-bonded Silica Gels Dispersed in a Smectic Liquid Crystal: A Random Field XY System

The effect on the nematic to smectic-A transition in octylcyanobiphenyl (8CB) due to dispersions of hydrogen-bonded silica (aerosil) particles is characterized with high-resolution x-ray scattering. The particles form weak gels in 8CB creating a quenched disorder that replaces the transition with the growth of short range smectic correlations. The correlations include thermal critical fluctuations that dominate at high temperatures and a second contribution that quantitatively matches the static fluctuations of a random field system and becomes important at low temperatures.

cond-mat.soft↗

arXiv2001

Diagonal static spin correlation in the low temperature orthorhombic Pccn phase of La1.55Nd0.4Sr0.05CuO4

Elastic neutron scattering measurements have been performed on Nd, Sr co-doped La1.55Nd0.4Sr0.05CuO4, which exhibits a structural phase transition at Ts ~ 60K from the low temperature orthorhombic Bmab phase (labelled LTO1) to the low temperature orthorhombic Pccn phase (labelled LTO2). At low temperatures, well below Ts, elastic magnetic peaks are observed at the ``diagonal'' incommensurate (IC) positions (0, 1+/-0.055, 0), with modulation direction only along the orthorhombic b-axis just as in Nd-free La1.95Sr0.05CuO4. In the present study, the one-dimensionality of the IC modulation, which is naturally explained by a stripe model, is clearly demonstrated with our "single-domain" crystal. The temperature dependence of the IC peak intensity suggests a substantial contribution from the Nd3+ spins below ~3K. Consistent with this, the L dependence of the magnetic scattering is accurately accounted for by a model in which the contribution of the Nd3+ spins is explicitly included.

cond-mat.supr-con↗

arXiv2001

Spin Fluctuations in the Underdoped High-Tc Cuprate La1.93Sr0.07CuO4

We performed magnetic inelastic neutron-scattering experiments on La(1.93)Sr(0.07)CuO(4) over a wide range of E and T;2 < E < 44 meV and 1.5 < T < 300 K. The dynamic susceptibility X"(q,E) of this underdoped high-Tc superconductor (Tc = 17 K) is characterized by broad, incommensurate peaks. Here, the incommensurate wavevector is approximately 0.07 reciprocal lattice units at low T and E. The superconducting phase does not possess an observable gap in the spin excitation spectrum down to at least 2 meV. Scaling behavior is demonstrated for the q-integrate energy spectrum X"(E) with respect to (E/kT). This scaling establishes a connnection between the magnetic excitations of the compositions on either side of the insulator-superconductor boundary in the La(2-x)Sr(x)CuO(4) phase diagram. We note a possible cross-over from an incommensurate to a commensurate response for E > 20 meV or T > 300 K.

cond-mat.supr-con↗

arXiv2001

Hole concentration dependence of the magnetic moment in superconducting and insulating La2-xSrxCuO4

Recent neutron scattering measurements on the La2-xSrxCuO4 system have revealed a drastic change of the incommensurate static spin correlations from diagonal in the insulating region to parallel in the superconducting region. We report the doping dependence of the ordered magnetic moment for the hole concentration region 0.03 <= x <= 0.12, focusing on the relationship between the static magnetism and the superconductivity. The elastic magnetic crosssection decreases monotonically with increasing x for 0.03 <= x <= 0.07. We find that the ordered magnetic moment μvaries from ~0.18 (x=0.03) to \~0.06 μ_B/Cu (x=0.07). No significant anomaly is observed at the insulator-superconductor boundary (x ~ 0.055). The elastic magnetic crosssection is enhanced in the vicinity of x=0.12 whose resolution limited width peaks are observed in neutron scattering measurements and where the apparent magntic and superconducting transitions coincide.

cond-mat.supr-con↗

arXiv2001

Critical spin dynamics of the 2D quantum Heisenberg antiferromagnets: Sr_2CuO_2Cl_2 and Sr_2Cu_3O_4Cl_2

We report a neutron scattering study of the long-wavelength dynamic spin correlations in the model two-dimensional $S=1/2$ square lattice Heisenberg antiferromagnets Sr$_2$CuO$_2$Cl$_2$ and Sr$_2$Cu$_3$O$_4$Cl$_2$. The characteristic energy scale, $ω_0 (T/J)$, is determined by measuring the quasielastic peak width in the paramagnetic phase over a wide range of temperature ($0.2 \alt T/J \alt 0.7$). The obtained values for $ω_0 (T/J)$ agree {\it quantitatively} between the two compounds and also with values deduced from quantum Monte Carlo simulations. The combined data show scaling behavior, $ω\sim ξ^{-z}$, over the entire temperature range with $z=1.0(1)$, in agreement with dynamic scaling theory.

cond-mat.str-el↗

arXiv2001

Critical dynamics of a spin-5/2 2D isotropic antiferromagnet

We report a neutron scattering study of the dynamic spin correlations in Rb$_2$MnF$_4$, a two-dimensional spin-5/2 antiferromagnet. By tuning an external magnetic field to the value for the spin-flop line, we reduce the effective spin anisotropy to essentially zero, thereby obtaining a nearly ideal two-dimensional isotropic antiferromagnet. From the shape of the quasielastic peak as a function of temperature, we demonstrate dynamic scaling for this system and find a value for the dynamical exponent $z$. We compare these results to theoretical predictions for the dynamic behavior of the two-dimensional Heisenberg model, in which deviations from $z=1$ provide a measure of the corrections to scaling.

cond-mat↗

arXiv2000

Neutron Scattering study of Sr_2Cu_3O_4Cl_2

We report a neutron scattering study on the tetragonal compound Sr_2Cu_3O_4Cl_2, which has two-dimensional (2D) interpenetrating Cu_I and Cu_{II} subsystems, each forming a S=1/2 square lattice quantum Heisenberg antiferromagnet (SLQHA). The mean-field ground state is degenerate, since the inter-subsystem interactions are geometrically frustrated. Magnetic neutron scattering experiments show that quantum fluctuations lift the degeneracy and cause a 2D Ising ordering of the Cu_{II} subsystem. Due to quantum fluctuations a dramatic increase of the Cu_I out-of-plane spin-wave gap is also observed. The temperature dependence and the dispersion of the spin-wave energy are quantitatively explained by spin-wave calculations which include quantum fluctuations explicitly. The values for the nearest-neighbor superexchange interactions between the Cu_I and Cu_{II} ions and between the Cu_{II} ions are determined experimentally to be J_{I-II} = -10(2)meV and J_{II}= 10.5(5)meV, respectively. Due to its small exchange interaction, J_{II}, the 2D dispersion of the Cu_{II} SLQHA can be measured over the whole Brillouin zone with thermal neutrons, and a novel dispersion at the zone boundary, predicted by theory, is confirmed. The instantaneous magnetic correlation length of the Cu_{II} SLQHA is obtained up to a very high temperature, T/J_{II}\approx 0.75. This result is compared with several theoretical predictions as well as recent experiments on the S=1/2 SLQHA.

cond-mat.str-el↗

arXiv2000

Quantum Fluctuations in the Frustrated Antiferromagnet Sr_2Cu_3O_4Cl_2

Sr_2Cu_3O_4Cl_2 is an antiferromagnet consisting of weakly coupled CuO planes which comprise two weakly interacting antiferromagnetic subsystems, I and II, which order at respective temperatures T_I \approx 390K and T_{II} \approx 40K. Except asymptotically near the ordering temperature, these systems are good representations of the two-dimensional quantum spin 1/2 Heisenberg model. For T< T_{II} there are four low-energy modes at zero wave vector, three of whose energies are dominated by quantum fluctuations. For T_{II} < T < T_I there are two low energy modes. The mode with lower energy is dominated by quantum fluctuations. Our calculations of the energies of these modes (including dispersion for wave vectors perpendicular to the CuO planes) agree extremely well with the experimental results of inelastic neutron scattering (in the accompanying paper) and for modes in the sub meV range observed by electron spin resonance. The parameters needed to describe quantum fluctuations are either calculated here or are taken from the literature. These results show that we have a reasonable qualitative understanding of the band structure of the lamellar cuprates needed to calculate the anisotropic exchange constants used here.

cond-mat.str-el↗

arXiv2000

Magnetic properties of the S=1/2 quasi-one-dimensional antiferromagnet CaCu2O3

We report single crystal growth and magnetic susceptibility and neutron diffraction studies of the S=1/2 quasi-1D antiferromagnet CaCu2O3. The structure of this material is similar to that of the prototype two-leg spin-ladder compound SrCu2O3. However, the Cu-O-Cu bond angle in the ladder rungs in CaCu2O3 is equal to 123 deg, and therefore the magnetic interaction along the rungs is expected to be much weaker in this material. At high temperatures, the magnetic susceptibility of CaCu2O3 can be decomposed into a contribution from 1D antiferromagnetic chains of finite-size chain segments together with a weak Curie contribution. The intrachain magnetic exchange constant, determined from the magnetic susceptibility measurements, is 2000 K. CaCu2O3 undergoes a Neel transition at T_N=25 K with ordering wavevector of (0.429(5), 0.5, 0.5). The magnetic structure is incommensurate in the direction of the frustrated interchain interaction. Weak commensurate (0.5, 0.5, 0.5) magnetic peaks are also observed below T_N. Application of a magnetic field induces a metamagnetic transition at which the incommensurability of the magnetic structure is substantially reduced. The material possesses only short-range magnetic order above the transition field.

cond-mat↗

arXiv2000

Direct Observation of the Quantum Energy Gap in S = 1/2 Tetragonal Cuprate Antiferromagnets

Using an electron spin resonance spectrometer covering a wide range of frequency and magnetic field, we have measured the low energy excitations of the S=1/2 tetragonal antiferromagnets, Sr_{2}CuO_{2}Cl_{2} and Sr_{2}Cu_{3}O_{4}Cl_{2}. Our observation of in-plane energy gaps of order 0.1 meV at zero external magnetic field are consistent with a spin wave calculation, which includes several kinds of quantum fluctuations that remove frustration. Results agree with other experiments and with exchange anisotropy parameters determined from a five band Hubbard model.

cond-mat↗

arXiv2000

Monte Carlo Study of the S=1/2 and S=1 Heisenberg Antiferromagnet on a Spatially Anisotropic Square Lattice

We present a quantum Monte Carlo study of a Heisenberg antiferromagnet on a spatially anisotropic square lattice, where the coupling strength in the x-direction ($J_x$) is different from that in the y-direction ($J_y$). By varying the anisotropy $α$ from 0 to 1, we interpolate between the one-dimensional chain and the two-dimensional isotropic square lattice. Both $S=1/2$ and S=1 systems are considered separately in order to facilitate comparison. The temperature dependence of the uniform susceptibility and the spin-spin correlation length are computed down to very low temperatures for various values of $α$. For S=1, the existence of a quantum critical point at $α^{S=1}_c=0.040(5)$ as well as the scaling of the spin gap is confirmed. Universal quantities predicted from the ${\cal O}(3)$ nonlinear $σ$ model agree with our results at $α=0.04$ without any adjustable parameters. On the other hand, the $S=1/2$ results are consistent with $α^{S=1/2}_c=0$, as discussed by a number of previous theoretical studies. Experimental implications for $S=1/2$ compounds such as Sr$_2$CuO$_3$ are also discussed.

cond-mat.str-el↗

arXiv2000

Static and dynamic spin correlations in the spin-glass phase of slightly-doped La_{2-x}Sr_xCuO_4

Neutron scattering experiments reveal that a diagonal spin modulation, which is a one-dimensional modulation rotated away by 45$^\circ$ from that in the superconducting phase, occurs universally across the insulating spin-glass phase in La$_{2-x}$Sr$_x$CuO$_4$ (0.02$\le x\le$0.055). This establishes an intimate relation between the magnetism and the transport properties in the high-temperature copper oxide superconductors. Furthermore, it is found that the charge density per unit length estimated using a charge stripe model is almost constant throughout the phase diagram, even when the modulation rotates away by 45$^\circ$ at the superconducting boundary. However, at the lowest values for $x$ the density changes approaching 1 hole/Cu as in La$_{2-x}$Sr$_x$NiO$_4$. Magnetic excitation spectra suggest that magnetic correlations change from incommensurate to commensurate at $ω\sim$7 meV and $T\sim$70 K, indicating a characteristic energy for the incommensurate structure of 6-7 meV.

cond-mat.supr-con↗

arXiv2000

X-ray scattering study of two length scales in the critical fluctuations of CuGeO3

The critical fluctuations of CuGeO$_3$ have been measured by synchrotron x-ray scattering, and two length scales are clearly observed. The ratio between the two length scales is found to be significantly different along the $a$ axis, with the $a$ axis along the surface normal direction. We believe that such a directional preference is a clear sign that surface random strains, especially those caused by dislocations, are the origin of the long length scale fluctuations.

cond-mat.str-el↗