arXiv subjects

Yiming Qiu

Publications and source records attributed to Yiming Qiu.

54 records · Page 3Linked to original sources

arXiv2021

Freezing of a disorder induced quantum spin liquid

$\rm{Sr_2CuTe_{0.5}W_{0.5}O_6}$ is a square-lattice magnet with super-exchange between S=1/2 $\rm{Cu^{2+}}$ spins mediated by randomly distributed Te and W ions. Here, using sub-K temperature and 20 $\rmμ$eV energy resolution neutron scattering experiments we show that this system transits from a gapless disorder-induced quantum spin liquid to a new quantum state below $\rm{T_f}$ = 1.7(1) K, exhibiting a weak frozen moment of /S ~ 0.1 and low energy dynamic susceptibility,$χ''({\hbarω})$ linear in energy which is surprising for such a weak freezing in this highly fluctuating quantum regime.

cond-mat.mtrl-sci↗

arXiv2020

Monopolar and dipolar relaxation in spin ice Ho$_2$Ti$_2$O$_7$

When degenerate states are separated by large energy barriers, the approach to thermal equilibrium can be slow enough that physical properties are defined by the thermalization process rather than the equilibrium. The exploration of thermalization pushes experimental boundaries and provides refreshing insights into atomic scale correlations and processes that impact steady state dynamics and prospects for realizing solid state quantum entanglement. We present a comprehensive study of magnetic relaxation in Ho$_2$Ti$_2$O$_7$ based on frequency-dependent susceptibility measurements and neutron diffraction studies of the real-time atomic-scale response to field quenches. Covering nearly ten decades in time scales, these experiments uncover two distinct relaxation processes that dominate in different temperature regimes. At low temperatures (0.6K<T<1K) magnetic relaxation is associated with monopole motion along the applied field direction through the spin-ice vacuum. The increase of the relaxation time upon cooling indicates reduced monopole conductivity driven by decreasing monopole concentration and mobility as in a semiconductor. At higher temperatures (1K<T<2K) magnetic relaxation is associated with the reorientation of monopolar bound states as the system approaches the single-spin tunneling regime. Spin fractionalization is thus directly exposed in the relaxation dynamics.

cond-mat.str-el↗

arXiv2020

Low energy magnons in the chiral ferrimagnet $\text{Cu}_2\text{OSeO}_3$: a coarse-grained approach

We report a comprehensive neutron scattering study of low energy magnetic excitations in the breathing pyrochlore helimagnetic $\text{Cu}_2\text{OSeO}_3$. Fully documenting the four lowest energy magnetic modes that leave the ferrimagnetic configuration of the "strong tetrahedra" intact ($|\hbarω|<13$ meV), we find gapless quadratic dispersion at the $Γ$ point for energies above 0.2 meV, two doublets separated by 1.6(2) meV at the $R$ point, and a bounded continuum at the $X$ point. Our constrained rigid spin cluster model relates these features to Dzyaloshinskii-Moriya (DM) interactions and the incommensurate helical ground state. Combining conventional spin wave theory with a spin cluster form-factor accurately reproduces the measured equal time structure factor through multiple Brillouin zones. An effective spin Hamiltonian describing the complex anisotropic inter-cluster interactions is obtained.

cond-mat.str-el↗

arXiv2020

Anisotropic effect of a magnetic field on the neutron spin resonance in FeSe

We use inelastic neutron scattering to study the effect of a magnetic field on the neutron spin resonance (Er = 3.6 meV) of superconducting FeSe (Tc = 9 K). While a field aligned along the in-plane direction broadens and suppresses the resonance, a c-axis aligned field does so much more efficiently, consistent with the anisotropic field-induced suppression of the superfluid density from the heat capacity measurements. These results suggest that the resonance in FeSe is associated with the superconducting electrons arising from orbital selective quasi-particle excitations between the hole and electron Fermi surfaces.

cond-mat.str-el↗

arXiv2020

Magnetic structure and spin waves in the frustrated ferro-antiferromagnet Pb$_2$VO(PO$_4$)$_2$

Single crystal neutron diffraction, inelastic neutron scattering and electron spin resonance experiments are used to study the magnetic structure and spin waves in Pb$_2$VO(PO$_4$)$_2$, a prototypical layered $S=1/2$ ferromagnet with frustrating next nearest neighbor antiferromagnetic interactions. The observed excitation spectrum is found to be inconsistent with a simple square lattice model previously proposed for this material. At least four distinct exchange coupling constants are required to reproduce the measured spin wave dispersion. The degree of magnetic frustration is correspondingly revised and found to be substantially smaller than in all previous estimates.

cond-mat.str-el↗

arXiv2019

Sky pixel detection in outdoor imagery using an adaptive algorithm and machine learning

Computer vision techniques enable automated detection of sky pixels in outdoor imagery. In urban climate, sky detection is an important first step in gathering information about urban morphology and sky view factors. However, obtaining accurate results remains challenging and becomes even more complex using imagery captured under a variety of lighting and weather conditions. To address this problem, we present a new sky pixel detection system demonstrated to produce accurate results using a wide range of outdoor imagery types. Images are processed using a selection of mean-shift segmentation, K-means clustering, and Sobel filters to mark sky pixels in the scene. The algorithm for a specific image is chosen by a convolutional neural network, trained with 25,000 images from the Skyfinder data set, reaching 82% accuracy for the top three classes. This selection step allows the sky marking to follow an adaptive process and to use different techniques and parameters to best suit a particular image. An evaluation of fourteen different techniques and parameter sets shows that no single technique can perform with high accuracy across varied Skyfinder and Google Street View data sets. However, by using our adaptive process, large increases in accuracy are observed. The resulting system is shown to perform better than other published techniques.

cs.CV↗

arXiv2019

Anisotropic spin fluctuations in detwinned FeSe

Superconductivity in FeSe emerges from a nematic phase that breaks four-fold rotational symmetry in the iron plane. This phase may arise from orbital ordering, spin fluctuations, or hidden magnetic quadrupolar order. Here we use inelastic neutron scattering on a mosaic of single crystals of FeSe detwinned by mounting on a BaFe2As2 substrate to demonstrate that spin excitations are most intense at the antiferromagnetic wave vectors QAF = (1, 0) at low energies E = 6-11 meV in the normal state. This two-fold (C2) anisotropy is reduced at lower energies 3-5 meV, indicating a gapped four-fold (C4) mode. In the superconducting state, however, the strong nematic anisotropy is again reflected in the spin resonance (E = 3.7 meV) at QAF with incommensurate scattering around 5-6 meV. Our results highlight the extreme electronic anisotropy of the nematic phase of FeSe and are consistent with a highly anisotropic superconducting gap driven by spin fluctuations.

cond-mat.str-el↗

arXiv2018

Quantum critical behavior in Ce(Fe$_{0.76}$Ru$_{0.24}$)$_2$Ge$_2$: the full story

Systems with embedded magnetic ions that exhibit a competition between magnetic order and disorder down to absolute zero can display unusual low temperature behaviors of the resistivity, susceptibility, and specific heat. Moreover, the dynamic response of such a system can display hyperscaling behavior in which the relaxation back to equilibrium when an amount of energy E is given to the system at temperature T only depends on the ratio E/T. Ce(Fe$_{0.755}$Ru$_{0.245}$)$_2$Ge$_2$ is a system that displays these behaviors. We show that these complex behaviors are rooted in a fragmentation of the magnetic lattice upon cooling caused by a distribution of local Kondo screening temperatures, and that the hyperscaling behavior can be attributed to the flipping of the total magnetic moment of magnetic clusters that spontaneously form and order upon cooling. We present our arguments based on the review of two-decades worth of neutron scattering and transport data on this system, augmented with new polarized neutron scattering experiments.

cond-mat.str-el↗

arXiv2017

Continuum of quantum fluctuations in a three-dimensional $S\!=\!1$ Heisenberg magnet

Conventional crystalline magnets are characterized by symmetry breaking and normal modes of excitation called magnons with quantized angular momentum $\hbar$. Neutron scattering correspondingly features extra magnetic Bragg diffraction at low temperatures and dispersive inelastic scattering associated with single magnon creation and annihilation. Exceptions are anticipated in so-called quantum spin liquids as exemplified by the one-dimensional spin-1/2 chain which has no magnetic order and where magnons accordingly fractionalize into spinons with angular momentum $\hbar/2$. This is spectacularly revealed by a continuum of inelastic neutron scattering associated with two-spinon processes and the absence of magnetic Bragg diffraction. Here, we report evidence for these same key features of a quantum spin liquid in the three-dimensional Heisenberg antiferromagnet NaCaNi$_2$F$_7$. Through specific heat and neutron scattering measurements, Monte Carlo simulations, and analytic approximations to the equal time correlations, we show that NaCaNi$_2$F$_7$ is an almost ideal realization of the spin-1 antiferromagnetic Heisenberg model on a pyrochlore lattice with weak connectivity and frustrated interactions. Magnetic Bragg diffraction is absent and 90\% of the spectral weight forms a continuum of magnetic scattering not dissimilar to that of the spin-1/2 chain but with low energy pinch points indicating NaCaNi$_2$F$_7$ is in a Coulomb phase. The residual entropy and diffuse elastic scattering points to an exotic state of matter driven by frustration, quantum fluctuations and weak exchange disorder.

cond-mat.str-el↗

arXiv2017

Direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point

Spontaneous symmetry-breaking quantum phase transitions play an essential role in condensed matter physics. The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu$-$Goldstone modes whereas the massive amplitude mode, analogous to the Higgs boson in particle physics, is prone to decay into a pair of low-energy Nambu$-$Goldstone modes in low dimensions. Especially, observation of a Higgs amplitude mode in two dimensions is an outstanding experimental challenge. Here, using the inelastic neutron scattering and applying the bond-operator theory, we directly and unambiguously identify the Higgs amplitude mode in a two-dimensional S=1/2 quantum antiferromagnet C$_9$H$_{18}$N$_2$CuBr$_4$ near a quantum critical point in two dimensions. Owing to an anisotropic energy gap, it kinematically prevents such decay and the Higgs amplitude mode acquires an infinite lifetime.

cond-mat.str-el↗

arXiv2016

Modulated magnetism and anomalous electronic transport in $\rm Ce_3Cu_4As_4O_2$

The complex magnetism and transport properties of tetragonal Ce$_3$Cu$_4$As$_4$O$_2$ were examined through neutron scattering and physical properties measurements on polycrystalline samples. The lamellar structure consists of alternating layers of $\rm CeCu_4As_4$ with a single square Ce lattice and oxygen-linked Ce bi-layer $\rm Ce_2O_2$. Extending along $\bf c$, a tube-like Fermi surface from DFT calculations points to a quasi-two-dimensional electronic system. Peaks in the specific heat at the Neél temperature $T_{N}=24$ $\rm K$, $T_{2}~=~16 $ $\rm K$ and $T_{3}~=~1.9$ $ \rm K$ indicate three magnetic phase transitions or distinct cross-over phenomena. For $T<T_{N}$ neutron diffraction indicates the development of ferromagnetic ab sheets for both Ce sites, with alternating polarization along $\bf{c}$, a wave vector ${\bf k}_{1}={\bf c}^*$. For $T<T_{2}$, quasi-two-dimensional low-energy spin fluctuations with ${\bf k}_{2}=\frac{1}{2}{\bf a}^*$ and polarized perpendicular to ${\bf k}_{2}$ are suppressed. The data are consistent with quasi-two-dimensional antiferromagnetic order in the $\rm CeCu_4As_4$ planes polarized along the ${\bf k}_{2}$ wave vector. $T_{3}$ marks a spin-flop transition where the ${\bf k}_{1}$ staggered magnetization switches to in-plane polarization. While the narrow 4f bands lie deep below the Fermi surface, there are significant transport anomalies associated with the transitions; in particular a substantial reduction in resistivity for $T<T_{N}$. At $T=100$ $ \rm mK$ the ${\bf k}_1$ modulated staggered moment is $0.85~μ_B$, which matches the $0.8~μ_B$ saturation magnetization achieved for H $~=~7$ $ \rm T$ at $T~=~2$ $ \rm K$. From low T Lorentzian fits the correlation length is in excess of 75 Å. We argue the unusual sequence of magnetic transitions results from competing interactions and anisotropies for the two Ce sites.

cond-mat.str-el↗

arXiv2014

Polaron-mediated spin correlations in metallic and insulating La$_{1-x}A_{x}$MnO$_{3}$ ($A$=Ca, Sr, or Ba)

Neutron spectroscopy measurements reveal short-range spin correlations near and above the ferromagnetic-paramagnetic phase transition in manganite materials of the form La$_{1-x}A_{x}$MnO$_{3}$, including samples with an insulating ground state as well as colossal magnetoresistive samples with a metallic ground state. Quasielastic magnetic scattering is revealed that forms clear ridges running along the [100]-type directions in momentum space. A simple model consisting of a conduction electron hopping between spin polarized Mn ions that becomes self-trapped after a few hops captures the essential physics of this magnetic component of the scattering. We associate this scattering component with the magnetic part of diffuse polarons, as we observe a temperature dependence similar to that of the diffuse structural scattering arising from individual polarons.

cond-mat.str-el↗

arXiv2012

Magnetic Order and Fluctuations in the Presence of Quenched Disorder in the Kagome Staircase System (Co(1-x)Mg(x))3V2O8

Co3V2O8 is an orthorhombic magnet in which S=3/2 magnetic moments reside on two crystallographically inequivalent Co2+ sites, which decorate a stacked, buckled version of the two dimensional kagome lattice, the stacked kagome staircase. The magnetic interactions between the Co2+ moments in this structure lead to a complex magnetic phase diagram at low temperature, wherein it exhibits a series of five transitions below 11 K that ultimately culminate in a simple ferromagnetic ground state below T~6.2 K. Here we report magnetization measurements on single and polycrystalline samples of (Co(1-x)Mg(x))3V2O8 for x<0.23, as well as elastic and inelastic neutron scattering measurements on single crystals of magnetically dilute (Co(1-x)Mg(x))3V2O8 for x=0.029 and x=0.194, in which non-magnetic Mg2+ ions substitute for magnetic Co2+. We find that a dilution of 2.9% leads to a suppression of the ferromagnetic transition temperature by ~15% while a dilution level of 19.4% is sufficient to destroy ferromagnetic long-range order in this material down to a temperature of at least 1.5 K. The magnetic excitation spectrum is characterized by two spin-wave branches in the ordered phase for (Co(1-x)Mg(x))3V2O8 (x=0.029), similar to that of the pure x=0 material, and by broad diffuse scattering at temperatures below 10 K in (Co(1-x)Mg(x))3V2O8 (x=0.194). Such a strong dependence of the transition temperatures to long range order in the presence of quenched non-magnetic impurities is consistent with two-dimensional physics driving the transitions. We further provide a simple percolation model that semi-quantitatively explains the inability of this system to establish long-range magnetic order at the unusually-low dilution levels which we observe in our experiments.

cond-mat.str-el↗

arXiv2010

Dzyaloshinsky-Moriya interaction and long life time of the spin state in the Cu$_3$ triangular spin cluster by inelastic neutron scattering measurements

Inelastic neutron scattering (INS) experiments have been performed on the Cu$_3$ triangular molecular nanomagnet using powder samples. In the medium resolution INS spectrum measured, there are two peaks at $\hbarω=0.5$ and 0.6 meV. Comparing the observed $Q$ dependences of these peaks with calculations, these two INS peaks originate from the Cu$_3$ cluster. From the observed peak position, width, and intensity, we have determined the optimum parameters of the spin Hamiltonian consisted of the antiferromagnetic exchange and the Dzyaloshinsky-Moriya interactions, which can also reproduce the magnetic susceptibility measurement. In addition, we have directly observed that the ground state quartet is split into two doublets with the energy separation of 0.103 meV using high-resolution neutron spectroscopy, which exactly corresponds to that expected from the optimum parameters obtained from the medium resolution xperiment. The temperature dependences of the integrated intensities of the 0.5 and 0.6 meV peaks are well reproduced by the Boltzmann distribution of the energy levels of the model Hamiltonian below 10 K. Furthermore, the inelastic peaks were visible even at very high temperatures as 50 K. This indicates extraordinary weak coupling between phonons (or any other perturbations) and spin states in the Cu$_3$ cluster, compared to the other known molecular nanomagnets.

cond-mat.other↗

arXiv2009

Incommensurate itinerant antiferromagnetic excitations and spin resonance in the FeTe$_{0.6}$Se$_{0.4}$ superconductor

We report on inelastic neutron scattering measurements that find incommensurate itinerant like magnetic excitations in the normal state of superconducting FeTe$_{0.6}$Se$_{0.4}$ (\Tc=14K) at wave-vector $\mathbf{Q}_{inc}=(1/2\pmε,1/2\mpε)$ with $ε$=0.09(1). In the superconducting state only the lower energy part of the spectrum shows significant changes by the formation of a gap and a magnetic resonance that follows the dispersion of the normal state excitations. We use a four band model to describe the Fermi surface topology of iron-based superconductors with the extended $s(\pm)$ symmetry and find that it qualitatively captures the salient features of these data.

cond-mat.supr-con↗

arXiv2009

Spin Waves and Switching: The Dynamics of Exchange - Biased Co Core - CoO Shell Nanoparticles

The utility of nanoscaled ferromagnetic particles requires both stabilized moments and maximized switching speeds. During reversal, the spatial modulation of the nanoparticle magnetization evolves in time, and the energy differences between each new configuration are accomodated by the absorption or emission spin waves with different wavelengths and energy profiles. The switching speed is limited by how quickly this spin wave energy is dissipated. We present here the first observation of dispersing spin waves in a nanoscaled system, using neutron scattering to detect spin waves in the CoO shells of exchange biased Co core- CoO shell nanoparticles. Their dispersion is little affected by finite size effects, but the spectral weight shifts to energies and wave vectors which increase with decreasing system size. Core-shell coupling leads to a substantial enhancement of the CoO spin wave population above its conventional thermal level, suggesting a new mechanism for dissipating core switching energy.

cond-mat.mes-hall↗

arXiv2009

Spin Gap and Resonance at the Nesting Wavevector in Superconducting FeSe0.4Te0.6

Neutron scattering is used to probe magnetic excitations in FeSe_{0.4}Te_{0.6} (T_c=14 K). Low energy spin fluctuations are found with a characteristic wave vector $(0.5,0.5,L)$ that corresponds to Fermi surface nesting and differs from Q_m=(δ,0,0.5) for magnetic ordering in Fe_{1+y}Te. A spin resonance with \hbarΩ_0=6.5 meV \approx 5.3 k_BT_c and \hbarΓ=1.25 meV develops in the superconducting state from a normal state continuum. We show that the resonance is consistent with a bound state associated with s+/- superconductivity and imperfect quasi-2D Fermi surface nesting.

cond-mat.supr-con↗

arXiv2006

The ground state of a quantum critical system

The competition between the tendency of magnetic moments to order at low temperatures, and the tendency of conduction electrons to shield these moments, can result in a phase transition that takes place at zero Kelvin, the quantum critical point (QCP). So far, the ground state of these types of systems has remained unresolved. We present neutron scattering experiments that show that the ground state of a sample representative of a class of QCP-systems is determined by the residual interactions between the conduction electrons, resulting in a state with incommensurate intermediate-range order. However, long-range order is thwarted by quantum fluctuations that locally destroy magnetic moments, leaving the system with too few moments to achieve long-range order.

cond-mat.str-el↗