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Neutron scattering from high-quality YBa2Cu3O6.33 (YBCO6.33) single crystals with a Tc of 8.4 K shows no evidence of a coexistence of superconductivity with long-range antiferromagnetic order at this very low, near-critical doping of p~0.055. However, we find short-range three dimensional spin correlations that develop at temperatures much higher than Tc. Their intensity increases smoothly on cooling and shows no anomaly that might signify a Neel transition. The system remains subcritical with spins correlated over only one and a half unit cells normal to the planes. At low energies the short-range spin response is static on the microvolt scale. The excitations out of this ground state give rise to an overdamped spectrum with a relaxation rate of 3 meV. The transition to the superconducting state below Tc has no effect on the spin correlations. The elastic interplanar spin response extends over a length that grows weakly but fails to diverge as doping is moved towards the superconducting critical point. Any antiferromagnetic critical point likely lies outside the superconducting dome. The observations suggest that conversion from Neel long-range order to a spin glass texture is a prerequisite to formation of paired superconducting charges. We show that while pc =0.052 is a critical doping for superconducting pairing, it is not for spin order.

We report magnetoresistance data in magnetic semiconductor multilayers, which exhibit a clear step-wise behavior as a function of external field. We attribute this highly non-trivial step-wise behavior to next-nearest-neighbor interlayer exchange coupling. Our microscopic calculation suggests that this next-nearest-neighbor coupling can be as large as 24% of the nearest-neighbor coupling. It is argued that such unusually long-range interaction is made possible by the quasi-one-dimensional nature of the system and by the long Fermi wavelength characteristic of magnetic semiconductors.

Using inelastic x-ray scattering we have investigated lattice vibrations in a geometric frustrated system CdCr2O4 that upon cooling undergoes a spin-Peierls phase transition at TN = 7.8 K from a cubic and paramagnetic to a tetragonal and Neel state. Phonon modes measured around Brillouin zone boundaries show energy shifts when the transition occurs. Our analysis shows that the shifting can be understood as the ordinary effects of the lowering of the crystal symmetry.

We studied magnetic excitations in a low-temperature ferroelectric phase of the multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify low-energy magnon modes and establish a correspondence between the magnon peaks observed by INS and electromagnon peaks observed in optical absorption [1]. Furthermore, we explain the microscopic mechanism, which results in the lowest-energy electromagnon peak, by comparing the inelastic neutron spectral weight with the polarization in the commensurate ferroelectric phase., Comment: 4 pages, 4 figures

Neutron scattering is used to investigate spin correlations in ultra pure single crystals of the S=1 triangular lattice NiGa2S4. Despite a Curie-Weiss temperature of Theta_CW=-80(2) K, static (tau > 1 ns) short range (xi_ab=26(3) \AA) incommensurate order prevails for T>1.5 K. The incommensurate modulation Q_0=(0.155(3),0.155(3),0), Theta_CW, and the spin wave velocity (c=4400 m/s) can be accounted for by antiferromagnetic third-nearest neighbor interactions J_3=2.8(6) meV and ferromagnetic nearest neighbor coupling J_1=-0.35(9) J_3. Inter-plane correlations are limited to nearest neighbors and weakened by an in-plane field. These observations show that the short range ordered glassy phase that has been observed in a number of highly degenerate systems, can persist near the clean limit., Comment: 4 pages, 4 figures, to be published in Physical Review Letters

Using neutron scattering, we investigate the effect of a magnetic field on the static and dynamic spin response in heavily underdoped superconducting YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO$_{6+x}$) with x=0.33 (T$_{c}$=8 K) and 0.35 (T$_{c}$=18 K). In contrast to the heavily doped and superconducting monolayer cuprates, the elastic central peak characterizing static spin correlations does not respond observably to a magnetic field which suppresses superconductivity. Instead, we find a magnetic field induced resonant enhancement of the spin fluctuations. The energy scale of the enhanced fluctuations matches the Zeeman energy within both the normal and vortex phases while the momentum dependence is the same as the zero field bilayer response. The magnitude of the enhancement is very similar in both phases with a fractional intensity change of $(I/I_{0}-1) \sim 0.1$. We suggest that the enhancement is not directly correlated with superconductivity but is the result of almost free spins located near hole rich regions., Comment: (8 pages, 5 figures, submitted to Physical Review B)

We use neutron reflectometry to investigate the interlayer exchange coupling between Ga$_{0.97}$Mn$_{0.03}$As ferromagnetic semiconductor layers separated by non-magnetic Be-doped GaAs spacers. Polarized neutron reflectivity measured below the Curie temperature of Ga$_{0.97}$Mn$_{0.03}$As reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers are antiferromagnetic (AFM). When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the spacers are undoped, suggesting that the observed AFM coupling is mediated by charge carriers introduced via Be doping. The behavior of magnetization of the multilayers measured by DC magnetometry is consistent with the neutron reflectometry results., Comment: 4 pages, 4 figures

By combining neutron four-circle diffraction and polarized neutron diffraction techniques we have determined the complex spin structures of a multiferroic, YMn2O5, that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magneto-electric coupling, our results are consistent with both the spin-current model that requires a magnetic transverse (cycloidal) spiral structure in order to induce a spontaneous electric polarization and the magneto-restriction model. Our results also explain why the electric polarization changes at the commensurate-to-incommensurate phase transition.

Temperature dependencies of gap energies and magnon lifetimes are measured in the quasi-1-dimensional S=1/2 gapped quantum magnets IPA-CuCl3 and Sul-Cu2Cl4 using inelastic neutron scattering. The results are compared to those found in literature for S=1 Haldane spin chain materials and to theoretical calculations for the O(3)- and O(N)- quantum non-linear sigma-models. It is found that when the T=0 energy gap Delta is used as the temperature scale, all experimental and theoretical curves are identical to within system-dependent but temperature-independent scaling factors of the order of unity. This quasi-universality extends over a surprising broad T range, at least up to kappa T ~ 1.5 Delta., Comment: 4 pages, 4 figures, accepted to PRL

Inelastic neutron scattering is used to investigate magnetic excitations in the quasi-one-dimensional quantum spin-liquid system Cu2Cl4 D8C4SO2. Contrary to previously conjectured models that relied on bond-alternating nearest neighbor interactions in the spin chains, the dominant interactions are actually next-nearest-neighbor in-chain antiferromagnetic couplings. The appropriate Heisenberg Hamiltonian is equivalent to that of a S = 1/2 4-leg spin-tube with almost perfect one dimensionality and no bond alternation. A partial geometric frustration of rung interactions induces a small incommensurability of short-range spin correlations.

Recently, two consecutive phase transitions were observed, upon cooling, in an antiferromagnetic spinel GeNi$_2$O$_4$ at $T_{N1}=12.1$ K and $T_{N2}=11.4$ K, respectively \cite{matsuno, crawford}. Using unpolarized and polarized elastic neutron scattering we show that the two transitions are due to the existence of frustrated minority spins in this compound. Upon cooling, at $T_{N1}$ the spins on the $<111>$ \kagome planes order ferromagnetically in the plane and antiferromagnetically between the planes (phase I), leaving the spins on the $<111>$ triangular planes that separate the \kagome planes frustrated and disordered. At the lower $T_{N2}$, the triangular spins also order in the $<111>$ plane (phase II). We also present a scenario involving exchange interactions that qualitatively explains the origin of the two purely magnetic phase transitions.

Inelastic and elastic neutron scattering is used to study spin correlations in the quasi-one dimensional quantum antiferromagnet IPA-CuCl3 in strong applied magnetic fields. A condensation of magnons and commensurate transverse long-range ordering is observe at a critical filed $H_c=9.5$ T. The field dependencies of the energies and polarizations of all magnon branches are investigated both below and above the transition point. Their dispersion is measured across the entire 1D Brillouin zone in magnetic fields up to 14 T. The critical wave vector of magnon spectrum truncation [Masuda et al., Phys. Rev. Lett. 96, 047210 (2006)] is found to shift from h_c 0.35 at H>H_c. A drastic reduction of magnon bandwidths in the ordered phase [Garlea et al., Phys. Rev. Lett. 98, 167202 (2007)] is observed and studied in detail. New features of the spectrum, presumably related to this bandwidth collapse, are observed just above the transition field.

High resolution inelastic neutron scattering is used to study excitations in the organometallic magnet DMACuCl$_3$. The correct magnetic Hamiltonian describing this material has been debated for many years. Combined with high field bulk magnetization and susceptibility studies, the new results imply that DMACuCl$_3$ is a realization of the $S=1/2$ alternating antiferromagnetic-ferromagnetic (AFM-FM) chain. Coupled-cluster calculations are used to derive exchange parameters, showing that the AFM and FM interactions have nearly the same strength. Analysis of the scattering intensities shows clear evidence for inter-dimer spin correlations, in contrast to existing results for conventional alternating chains. The results are discussed in the context of recent ideas concerning quantum entanglement., Comment: 5 pages, 4 figures included in text. Submitted to APS Journals

Insulating La1.95Sr0.05CuO4 shares with superconducting cuprates the same Fincher-Burke-like spin excitations, which usually are observed in itinerant antiferromagnets. The local spectral function satisfies omega/T scaling above ~16 K for this incommensurate insulating cuprate. Together with previous results in commensurate insulating and incommensurate superconducting cuprates, these results further support the general scaling prediction for square-lattice quantum spin S=1/2 systems. The width of incommensurate peaks in La1.95Sr0.05CuO4 scales to a similar finite value as at optimal doping, strongly suggesting that they are similarly distant from a quantum critical point. They might both be limited to a finite correlation length by the partial spin-glass freezing., Comment: 5 pages, 4 figures

We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagome lattice antiferromagnet Zn Cu_3 (OH)_6 Cl_2. The susceptibility indicates a Curie-Weiss temperature of ~ -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law with exponent less than or equal to 1. These results suggest that an unusual spin-liquid state with essentially gapless excitations is realized in this kagome lattice system., Comment: 4 pages, 3 figures; v2: Updates to authors list and references; v3: Updated version; v4: Published version

The weakly coupled quasi-one-dimensional spin ladder compound (CH$_3$)$_2$CHNH$_3$CuCl$_3$ is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in a reference 3D quantum magnet. However, for the upper two massive magnon branches the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point., Comment: 4 pages, 4 figures

The compound Ce3Co4Sn13 is an extremely heavy cubic heavy fermion system with a low temperature electronic specific heat of order ~4 J/mol-K2. If the compound is nonmagnetic, it would be one of the heaviest nonmagnetic Ce-based heavy fermions reported to date and therefore would be expected to lie extremely close to a quantum critical point. However, a broad peak of unknown origin is observed at 0.8 K in the specific heat and magnetic susceptibility, suggesting the possibility of antiferromagnetic order. We present neutron diffraction data from polycrystalline samples which do not show any sign of magnetic scattering below 0.8 K. In addition, we present inelastic neutron scattering data from a single crystal sample which is consistent with the 1.2 K energy scale for Kondo spin fluctuations determined from specific heat measurements., Comment: 4 pages, 2 figures, submitted to J. Mag. Mag. Mater. for ICM 2006

Magnetic excitations in the quasi-one-dimensional antiferromagnet IPA-CuCl3 are studied by cold neutron inelastic scattering. Strongly dispersive gap excitations are observed. Contrary to previously proposed models, the system is best described as an asymmetric quantum spin ladder. The observed spectrum is interpreted in terms of ``composite'' Haldane spin chains. The key difference from actual S=1 chains is a sharp cutoff of the single-magnon spectrum at a certain critical wave vector., Comment: 4 pages 4 figures

Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr$_2$O$_4$, undergoes an elongation along the c-axis ($c > a = b$) at its spin-Peierls-like phase transition at $T_N$ = 7.8 K. The N\'{e}el phase ($T < T_N$) has an incommesurate spin structure with a characteristic wave vector \textbf{Q}$_M$ = (0,$\delta$,1) with $\delta \sim$ 0.09 and with spins lying on the $ac$-plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr$_2$O$_4$, that undergoes a c-axis contraction and a commensurate spin order. The magnetic excitations of the incommensurate N\'{e}el state has a weak anisotropy gap of 0.6 meV and it consists of at least three bands extending up to 5 meV., Comment: PRL in press

Dimethylammonium copper (II) chloride (also known as DMACuCl3 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 DMACuCl3 as independent AFM and FM dimer spin pairs. We present here new inelastic neutron scattering measurements of the spin-excitations in single crystals of DMACuCl3. 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 0.95 meV and a bandwidth of 0.82 meV., Comment: 3 pages, 2 figures included in text, submitted to proceedings of International Conference on Neutron Scattering, December 2005

The spin gap material IPA-CuCl3 has been extensively studied as a ferromagnetic-antiferromagnetic bondalternating S = 1/2 chain. This description of the system was derived from structural considerations and bulk measurements. New inelastic neutron scattering experiments reveal a totally different picture: IPA-CuCl3 consists of weakly coupled spin ladders with antiferromagnetic legs and ferromagnetic rungs. The ladders run perpendicular to the originally supposed bondalternating chain direction. The ferromagnetic rungs make this system equivalent to a Haldane S = 1 antiferromagnet. With a gap energy of 1.17(1) meV, a zone-boundary energy of 4.1(1) meV, and almost no magnetic anisotropy, IPA-CuCl3 may the best Haldane-gap material yet, in terms of suitability for neutron scattering studies in high magnetic fields., Comment: 2 pages, 2 figures, submitted to proceedings of LT24, Orlando, FL, August 2005

We have mapped the neutron scattering spin spectrum at low-energies in YBa2Cu3O6.353 (Tc=18 K) where the doping ~0.06 is near the critical value (pc=0.055) for superconductivity. No coexistence with long range ordered antiferromagnetism is found. The spins fluctuate on two energy scales, one a damped spin response with a ~2 meV relaxation rate and the other a central mode with a relaxation rate that slows to less than 0.08 meV below Tc. The spectrum mirrors that of a soft mode driving a central mode. Extremely short correlation lengths, 42+-5 Angstrom in-plane and 8+-2 Angstrom along the c direction, and isotropic spin orientations for the central mode indicate that the correlations are subcritical with respect to any second order transition to Neel order. The dynamics follows a model where damped spin fluctuations are coupled to the slow fluctuations of regions with correlations shortened by the hole doping., Comment: 5 pages 4 figures. One figure revised and some text revision. Accepted PRB Rapids February 14, 2006

Inelastic neutron scattering experiments on the S=1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under magnetic fields below and above a critical field Hc at which the energy gap closes. Normal field dependece of Zeeman splitting of the excited triplet modes below Hc has been observed, but the highest mode is unusually small and smears out with increasing field. This can be explained by an interaction with a low-lying two magnon continuum at q=pi that is present in dimerized chains but absent in uniform ones. Above Hc, we find only one excited mode, in stark contrast with three massive excitations previously observed in the structurally similar Haldane-gap material NDMAP [A. Zheludev et al., Phys. Rev. B 68, 134438 (2003)]., Comment: 4 pages, 4 figures

We use inelastic neutron scattering to study magnetic excitations of Sc$_{1-x}$U$_{x}$Pd$_3$ for U concentrations ($x=0.25$, 0.35) near the spin glass quantum critical point (QCP). The excitations are spatially incoherent, broad in energy ($E=\hbar\omega$), and follow $\omega/T$ scaling at all wave vectors investigated. Since similar $\omega/T$ scaling has been observed for UCu$_{5-x}$Pd$_x$ and CeCu$_{6-x}$Au$_{x}$ near the antiferromagnetic (AF) QCP, we argue that the observed non Fermi liquid (NFL) behavior in these $f$-electron materials arises from the critical phenomena near a T=0 K phase transition, irrespective of the nature of the transition., Comment: Submitted to PRL

Follow-up neutron measurements, performed on a sample much larger than the one used in the original study, show that in the energy range 0.5-45 meV the magnetic excitations in BaCu2SiGeO7 are indistinguishable from those in conventional (disorder-free) quantum S=1/2 chains. Scrutinizing the previous data, we found that the analysis was affected by a poorly identified structured background and an additional technical mistake in the data reduction., Comment: This is a complete withdrawal of the original paper, also published as in Phys. Rev. Lett 93, 077206 (2004). One page, one figure

Inelastic neutron scattering data from a twinned single-crystal of YBa2Cu3O6.95 are presented that show a distinct a-b plane anisotropy in the oxygen vibrations. The Cu-O bond-stretching type phonons are simultaneously observed along the a and b directions due to a 4 meV splitting arising from the orthorhombicity. The present results show the bond-stretching branch along b (parallel to the chain) has a continuous dispersion, while the branch along a is discontinuous, suggesting a possibility of short-range cell-doubling along a. Furthermore, the LO mode along a is split in en-ergy from its TO partner at non-zero q-vectors, while the b mode is not. These results imply strong anisotropy in the electronic screening and a one-dimensional character in underlying charge fluctuations., Comment: 4 pages, 4 figures