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Electronic correlations lead to heavy quasiparticles in three-dimensional metals, and their collapse can destabilize magnetic moments. It is an open question whether there is an analogous instability in one-dimensional (1D) systems, unanswered due to the lack of metallic spin chains. We report neutron scattering measurements and Density Matrix Renormalization Group calculations establishing spinons in the correlated metal Ti4MnBi2, confirming it is 1D. Ti4MnBi2 is inherently frustrated, forming near a quantum critical point separating two T = 0 phases of the J1-J2 XXZ model. The lack of magnetic order above 0.3 K results from these quantum critical fluctuations, potentially compounded by Kondo moment compensation. Ti4MnBi2 provides the first experimental evidence that 1D magnetism, previously the exclusive domain of insulators, persists in metallic systems with moderate correlations., Comment: 13 pages, 4 figures

We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr$_2$RuO$_4$ by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculations. We observed a spin resonance with energy of $\hbar\omega_\text{res}=0.56$~meV centered at a characteristic wavevector $\mathbf{Q}_\text{res}=(0.3, 0.3, 0.5)$. The resonance energy corresponds well to the superconducting gap $2\Delta=0.56$~meV estimated by the tunneling spectroscopy. The spin resonance shows the $L$ modulation with a maximum at around $L = 0.5$. The $L$ modulated intensity of the spin resonance and our RPA calculations indicate that the superconducting gaps regarding the quasi-one-dimensional $\alpha$ and $\beta$ sheets at the Fermi surfaces have the horizontal line nodes. These results may set a strong constraint on the pairing symmetry of Sr$_2$RuO$_4$. We also discuss the implications on possible superconducting order parameters., Comment: arXiv admin note: substantial text overlap with arXiv:1904.01234

The dimerized quantum magnet BaCuSi$_2$O$_6$ was proposed as an example of "dimensional reduction" arising near the magnetic-field-induced quantum critical point (QCP) due to perfect geometrical frustration of its inter-bilayer interactions. We demonstrate by high-resolution neutron spectroscopy experiments that the effective intra-bilayer interactions are ferromagnetic, thereby excluding frustration. We explain the apparent dimensional reduction by establishing the presence of three magnetically inequivalent bilayers, with ratios 3:2:1, whose differing interaction parameters create an extra field-temperature scaling regime near the QCP with a non-trivial but non-universal exponent. We demonstrate by detailed quantum Monte Carlo simulations that the magnetic interaction parameters we deduce can account for all the measured properties of BaCuSi$_2$O$_6$, opening the way to a quantitative understanding of non-universal scaling in any modulated layered system., Comment: 14 pages, 9 figures

Spin crossover is expected to enrich unusual physical states in various types of condensed matter. Through inelastic neutron scattering, we study the spin-state excitations in the canonical and advanced platform, LaCoO$_3$, and reveal that the spatial correlation robustly maintains the seven-Co-site size below 300 K and the internal Co-$d$ electrons are spatially delocalized. By combining theoretical calculations, this dynamical short-range order is identified as a new collective unit for describing spin-state with dual spin-state nature beyond the conventional one-Co-site classification., Comment: 11 pages, 7 figures, 1 table

Spin-state transition, also known as spin crossover, plays a key role in diverse systems, including minerals and biological materials. In theory, the boundary range between the low- and high-spin states is expected to enrich the transition and give rise to unusual physical states. However, no compound that realizes a nearly degenerate critical range as the ground state without requiring special external conditions has yet been experimentally identified. This study reports that, by comprehensive measurements of macroscopic physical properties, X-ray diffractometry, and neutron spectroscopy, the Sc substitution in LaCoO$_3$ destabilizes its nonmagnetic low-spin state and generates an anomalous paramagnetic state accompanied by the enhancement of transport gap and magneto-lattice-expansion as well as the contraction of Co--O distance with the increase of electron site-transfer. These phenomena are not well described by the mixture of conventional low- and high-spin states, but by their quantum superposition occurring on the verge of a spin-state transition. The present study enables us to significantly accelerate the design of new advanced materials without requiring special equipment based on the concept of quantum spin-state criticality., Comment: Keywords: excitonic insulator, bi-exciton

We investigate the imaginary part of the wave vector dependent dynamic spin susceptibility in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ as a function of temperature using neutron scattering. At T=5 K, two-dimensional incommensurate (IC) magnetic fluctuations are clearly observed around $\mathbf{Q}_\text{c}=(0.3,0.3,L)$ up to approximately 60 meV energy transfer. We find that the IC excitations disperse to ridges around the $(\pi,\pi)$ point. Below 50 K, the energy and temperature dependent excitations are well described by the phenomenological response function for a Fermi liquid system with a characteristic energy of 4.0(1) meV. Although the wave vector dependence of the IC magnetic fluctuations in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ is similar to that in the Fermi liquid state of the parent compound, Sr$_2$RuO$_4$, the magnetic fluctuations are clearly suppressed by the Ti-doping., Comment: 5 pages, 4 figures

By performing time-of-flight neutron scattering measurements on a large amount of single crystals of Sr$_2$RuO$_4$, we studied detailed structure of the imaginary part of the dynamic spin susceptibility over a wide range of phase space. In the normal state at T=5 K, strong incommensurate (IC) peaks were clearly observed at around $\mathbf{Q}_\text{c}=(0.3,0.3)$ up to at least $\hbar\omega=80$ meV. In addition, our data also show strong magnetic fluctuations that exist on the ridges connecting the IC peaks around the $(\pi,\pi)$ point rather than around the $\Gamma$ point. Our results are consistent with the semi-mean-field random phase approximation calculation for a two dimensional Fermi liquid with a characteristic energy of 5.0 meV. Furthermore, the IC fluctuations were observed even at room temperature.

We have studied the magnetic excitations in impurity doped La$_{1.85}$Sr$_{0.15}$Cu$_{1-y}$A$_{y}$O$_{4}$ (A=Ni or Zn) by neutron scattering. The dispersion for Zn:$y=0.017$ is similar to that for the impurity free sample: incommensurate peaks with the incommensurability $\delta=0.12\pm0.01$ (rlu) do not change their positions up to 21 meV. On the other hand, for Ni:$y=0.029$, two incommensurate peaks observed at low energies suddenly change into a broad commensurate peak at $E_\mathrm{cross}=15$ meV. Compared to the impurity free sample with a similar Sr-concentration $x=0.16$, [B. Vignolle {\it et al.} Nature Physics {\bf 3} (2007) 163], $E_\mathrm{cross}$ for Ni:$y=0.029$ is decreased by nearly the same factor for the reduction in $T_{c}$. This is very similar to the shift of the resonance energy ($E_\mathrm{res}$) in Ni-doped YBa$_{2}$Cu$_{3}$O$_{7}$.[Y. Sidis {\it et al.}: Phys. Rev. Lett. {\bf 84} (2000) 5900]. These common impurity effects on the shift of $E_\mathrm{cross}$ and $E_\mathrm{res}$ suggest the same magnetic origin for the resonance peak in YBa$_{2}$Cu$_{3}$O$_{\delta}$ and that for a crossing point of upward and downward dispersions in the La$_{2-x}$Sr$_{x}$CuO$_{4}$. We propose that the sudden change in the dispersion is better described by a crossover from incommensurate spin fluctuations to a gapped spin wave rather than a hourglass-like dispersion., Comment: 11 pages, 5 figures

By using bulk magnetization, electron spin resonance (ESR), heat capacity, and neutron scattering techniques, we characterize the thermodynamic and quantum phase diagrams of Ba$_3$Cr$_2$O$_8$. Our ESR measurements indicate that the low field paramagnetic ground state is a mixed state of the singlet and the S$_z$ = 0 triplet for $H \perp c$. This suggests the presence of an intra-dimer Dzyaloshinsky-Moriya (DM) interaction with a DM vector perpendicular to the c-axis.

The magnetic structure of BaFe2As2 was completely determined from polycrystalline neutron diffraction measurements soon after the ThCr2Si2-type FeAs-based superconductors were discovered. Both the moment direction and the in-plane antiferromagnetic wavevector are along the longer a-axis of the orthorhombic unit cell. There is only one combined magnetostructural transition at about 140 K. However, a later single-crystal neutron diffraction work reported contradicting results. Here we show neutron diffraction results from a clean single crystal sample, grown by a self-flux method, that support the original polycrystalline work., Comment: 10 pages, 2 figures

Using single crystal inelastic neutron scattering with and without application of an external magnetic field and powder neutron diffraction, we have characterized magnetic interactions in Ba$_3$Cr$_2$O$_8$. Even without field, we found that there exist three singlet-to-triplet excitation modes in $(h,h,l)$ scattering plane. Our complete analysis shows that the three modes are due to spatially anisotropic interdimer interactions that are induced by local distortions of the tetrahedron of oxygens surrounding the Jahn-Teller active Cr$^{5+} (3d^1)$. The strong intradimer coupling of $J_0 = 2.38(2)$ meV and weak interdimer interactions ($|J_{\rm inter}| \leq 0.52(2)$ meV) makes Ba$_3$Cr$_2$O$_8$ a good model system for weakly-coupled $s = 1/2$ quantum spin dimers.

The newly discovered superconductor La(O,F)FeAs (Tc = 26 K) was investigated using the neutron scattering technique. No spin-density-wave (SDW) order was observed in the normal state nor in the superconducting state, both with and without an applied magnetic field of 9 T, consistent with the proposal that SDW and superconductivity are competing in the laminar materials. While our inelastic measurements offer no constraints on the spin dynamic response from d-wave pairing, an upper limit for the magnetic resonance peak predicted from an extended s-wave pairing mechanism is provided. Our measurements also support the energy scale of the calculated phonon spectrum which is used in electron-phonon coupling theory, and fails to produce the high observed Tc., Comment: 4 pages, 4 figures

Low energy spin excitations were investigated in the static stripe phase of La_{2-x}Sr_xCuO_4 using elastic and inelastic neutron scattering on single crystals. For x = 1/8 in which long-range static stripe order exists, an energy gap of E_g = 4 meV exists in the excitation spectrum in addition to strong quasi-elastic, incommensurate spin fluctuations associated with the static stripes. When x increases, the spectral weight of the spin fluctuations shifts from the quasi-elastic continuum to the excitation spectrum above E_g. The dynamic correlation length as a function of energy and the temperature evolution of the energy spectrum suggest a phase separation of two distinct magnetic phases in real space., Comment: 5pages, 4figures

We performed inelastic neutron experiments on underdoped La_2-xSr_xCuO_4(x=0.10, T_c=28.6K) using a time-of-flight neutron scattering technique. Four incommensurate peaks on the two-dimensional reciprocal plane disperse inwards toward an antiferromagnetic zone center as the energy increases. These peaks merge into a single peak at an energy E_cross around w=40+-3meV. Beyond E_cross, the peak starts to broaden and ``hourglass-like'' excitations are observed. The E_cross in the underdoped sample is smaller than that reported for the optimally doped La_1.84Sr_0.16CuO_4. The reduction of the E_cross is explained by the doping-independent slope of the downward dispersion below the E_cross combined with the smaller incommensurability in the underdoped sample. In the energy spectrum of chi"(w), we observed a similar "peak-dip-hump" structure in the energy region of 10~45meV to that reported for the optimally doped sample. We discuss the relation between the hourglass-shaped dispersion and the peak-dip-hump energy spectrum., Comment: 4pages, 3figures. submitted to PRB Rapid Communications. submitted to PRB Rapid Communications

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) on single-layered cuprate superconductor Bi2Sr2CuO6 to elucidate the origin of shadow band. We found that the shadow band is exactly the c(2x2) replica of the main band irrespective of the carrier concentration and its intensity is invariable with respect to temperature, doping, and substitution constituents of block layers. This result rules out the possibility of antiferromagnetic correlation and supports the structural origin of shadow band. ARPES experiments on optimally doped La1.85Sr0.15CuO4 also clarified the existence of the c(2x2) shadow band, demonstrating that the shadow band is not a unique feature of Bi-based cuprates. We conclude that the shadow band is related to the orthorhombic distortion at the crystal surface., Comment: 6 pages, 4figures

Impurity effects of Zn and Ni on the low-energy spin excitations were systematically studied in optimally doped La1.85Sr0.15Cu1-yAyO4 (A=Zn, Ni) by neutron scattering. Impurity-free La1.85Sr0.15CuO4 shows a spin gap of 4meV below Tc in the antiferromagnetic(AF) incommensurate spin excitation. In Zn:y=0.004, the spin excitation shows a spin gap of 3meV below Tc. In Zn:y=0.008 and Zn:y=0.011, however, the magnetic signals at 3meV decrease below Tc and increase again at lower temperature, indicating an in-gap state. In Zn:y=0.017, the low-energy spin state remains unchanged with decreasing temperature, and elastic magnetic peaks appear below 20K then exponentially increase. As for Ni:y=0.009 and Ni:y=0.018, the low-energy excitations below 3meV and 2meV disappear below Tc. The temperature dependence at 3meV, however, shows no upturn in constrast with Zn:y=0.008 and Zn:y=0.011, indicating the absence of in-gap state. In Ni:y=0.029, the magnetic signals were observed also at 0meV. Thus the spin gap closes with increasing Ni. Furthermore, as omega increases, the magnetic peak width broadens and the peak position, i.e. incommensurability, shifts toward the magnetic zone center (pi pi). We interpret the impurity effects as follows: Zn locally makes a non-superconducting island exhibiting the in-gap state in the superconducting sea with the spin gap. Zn reduces the superconducting volume fraction, thus suppressing Tc. On the other hand, Ni primarily affects the superconducting sea, and the spin excitations become more dispersive and broaden with increasing energy, which is recognized as a consequence of the reduction of energy scale of spin excitations. We believe that the reduction of energy scale is relevant to the suppression of Tc., Comment: 13pages, 14figures; submitted to Phys. Rev. B

Low-energy spin excitations of La1.85Sr0.15Cu1-yZnyO4 were studied by neutron scattering. In y=0.004, the incommensurate magnetic peaks show a well defined ``spin gap'' below Tc. The magnetic signals at omega=3 meV decrease below Tc=27 K for y=0.008, also suggesting the gap opening. At lower temperatures, however, the signal increases again, implying a novel in-gap spin state. In y=0.017, the spin gap vanishes and elastic magnetic peaks appear. These results clarify that doped Zn impurities induce the novel in-gap state, which becomes larger and more static with increasing Zn., Comment: 4 pages, 4 figures