Short-range Spin Correlations Research Articles

Emergence of nontrivial magnetic excitations in a spin-liquid state of kagomé volborthite

When quantum fluctuations destroy underlying long-range ordered states, novel quantum states emerge. Spin-liquid (SL) states of frustrated quantum antiferromagnets, in which highly correlated spins fluctuate down to very low temperatures, are prominent examples of such quantum states. SL states often exhibit exotic physical properties, but the precise nature of the elementary excitations behind such phenomena remains entirely elusive. Here, we use thermal Hall measurements that can capture the unexplored property of the elementary excitations in SL states, and report the observation of anomalous excitations that may unveil the unique features of the SL state. Our principal finding is a negative thermal Hall conductivity [Formula: see text] which the charge-neutral spin excitations in a gapless SL state of the 2D kagomé insulator volborthite Cu3V2O7(OH)2[Formula: see text]2H2O exhibit, in much the same way in which charged electrons show the conventional electric Hall effect. We find that [Formula: see text] is absent in the high-temperature paramagnetic state and develops upon entering the SL state in accordance with the growth of the short-range spin correlations, demonstrating that [Formula: see text] is a key signature of the elementary excitation formed in the SL state. These results suggest the emergence of nontrivial elementary excitations in the gapless SL state which feel the presence of fictitious magnetic flux, whose effective Lorentz force is found to be less than 1/100 of the force experienced by free electrons.

Spin liquid state in the disordered triangular latticeSc2Ga2CuO7revealed by NMR

We present microscopic magnetic properties of a two dimensional triangular lattice Sc2Ga2CuO7, consisting of single and double triangular Cu planes. A Curie-Weiss temperature theta_CW = --44 K and an antiferromagnetic (AFM) exchange interaction J/KB = 35 K between Cu2+ (S = 1/2) spins in the triangular bi-plane are obtained from the analysis of magnetic susceptibility data. The intrinsic magnetic susceptibility extracted from 71Ga NMR shift data displays the presence of AFM short range spin correlations and remains finite down to 50 mK suggesting an non-singlet ground state. The nuclear spin-lattice relaxation rate (1/T1) reveals a slowing down of Cu2+ spin fluctuations with decreasing T down to 100 mK. Magnetic specific heat (Cm) and 1/T1 exhibit a power law behavior at low temperatures implying gapless nature of the spin excitation spectrum. Absence of long range magnetic ordering down to ~ J/700, nonzero spin susceptibility at low T, and power law behavior of Cm and 1/T1 suggest a gapless quantum spin liquid (QSL) state. Our results demonstrate that persistent spin dynamics induced by frustration maintain a quantum-disordered state at T -> 0 in this triangular lattice antiferromagnet. This suggests that the low energy modes are dominated by spinon excitations in the QSL state due to randomness engendered by disorder and frustration.

Exact diagonalization study of the effects of Zn and Ni impurities on the pseudogap of underdoped cupratehigh-Tcsuperconductors

The influence of Zn and Ni impurities on the normal-state pseudogap of underdoped $\text{high-}{T}_{c}$ cuprate superconductors is studied using exact diagonalization of effective $t\ensuremath{-}J\text{-like}$ Hamiltonians describing low energy electronic excitations of the ${\mathrm{CuO}}_{2}$ plane with some of the copper ions replaced with Zn/Ni. The Ni case Hamiltonian has been obtained by a sequence of approximations from a more complete model involving Cu $3d$, Ni $3d$, and O $2p$ orbitals. Our main findings are: (i) The width ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{PG}}$ of the pseudogap occurring in the many body density of states, and manifesting itself also in the $c\text{-axis}$ infrared conductivity, decreases with increasing Zn concentration as a consequence of a suppression of short range spin correlations. (ii) In the case of one hole and one Ni impurity, the hole is---for realistic values of the model parameters---weakly bound to the Ni site. This causes a slight increase of ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{PG}}$ with respect to the pure case. (iii) Based on this result and further results for 1--2 holes and 1--2 Ni impurities, we suggest that in the real Ni substituted ${\mathrm{CuO}}_{2}$ plane ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{PG}}$ is larger than in the pure case due to the binding of the doped holes to the Ni sites and effective underdoping. Our findings clarify the trends observed in the $c\text{-axis}$ infrared conductivity data of Zn and Ni substituted (Sm,Nd)${\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ crystals.

Anomalous Magnetothermopower in a Metallic Frustrated Antiferromagnet.

We report the temperature T and magnetic field H dependence of the thermopower S of an itinerant triangular antiferromagnet PdCrO_{2} in high magnetic fields up to 32T. In the paramagnetic phase, the zero-field thermopower is positive with a value typical of good metals with a high carrier density. In marked contrast to typical metals, however, S decreases rapidly with increasing magnetic field, approaching zero at the maximum field scale for T>70 K. We argue here that this profound change in the thermoelectric response derives from the strong interaction of the 4d correlated electrons of the Pd ions with the short-range spin correlations of the Cr^{3+} spins that persist beyond the Néel ordering temperature due to the combined effects of geometrical frustration and low dimensionality.

Ultra-fast photo-carrier relaxation in Mott insulators with short-range spin correlations.

Ultra-fast spectroscopy can reveal the interplay of charges with low energy degrees of freedom, which underlies the rich physics of correlated materials. As a potential glue for superconductivity, spin fluctuations in Mott insulators are of particular interest. A theoretical description of the coupled spin and charge degrees of freedom is challenging, because magnetic order is often only short-lived and short-ranged. In this work we theoretically investigate how the spin-charge interactions influence the relaxation of a two-dimensional Mott-Hubbard insulator after photo-excitation. We use a nonequilibrium variant of the dynamical cluster approximation, which, in contrast to single-site dynamical mean-field theory, captures the effect of short-range correlations. The relaxation time is found to scale with the strength of the nearest-neighbor spin correlations, and can be 10–20 fs in the cuprates. Increasing the temperature or excitation density decreases the spin correlations and thus implies longer relaxation times. This may help to distinguish the effect of spin-fluctuations on the charge relaxation from the influence of other bosonic modes in the solid.

Magnetic Excitations of Spin Nematic State in Frustrated Ferromagnetic Chain

By exploiting density-matrix renormalization group techniques, we investigate the dynamical spin structure factor of a spin-1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions in an applied magnetic field. In a field-induced spin nematic regime, we find gapless longitudinal and gapped transverse spin excitation spectra, in accordance with quasi-long-ranged longitudinal and short-ranged transverse spin correlations, respectively. The gapless point coincides with the dominant longitudinal spin correlation, whereas the gap position exhibits a characteristic field dependence contradicting the dominant transverse spin correlation.

Improving Room-Temperature Ferromagnetism in Mn-Implanted Si by High-Temperature Annealing

Mn-implanted Si with postannealing treatment at 873, 973, 1073, and 1173 K exhibited ferromagnetism at room temperature. Ferromagnetism was enhanced as the annealing temperature increased. The largest value of saturation magnetization was found in the sample annealed at 1173 K. X-ray absorption near-edge structure spectra showed no formation of substitutional–interstitial Mn–Mn dimmers in all samples. On the other hand, high-resolution transmission electron microscopy and glancing incidence X-ray diffraction revealed a variety of MnSi compounds. We propose that the ferromagnetism has the origin from the MnSi1.7 nanoparticles as well as the MnSi 1:1 phase, which possesses strong short-range spin correlation. The formation of antiferromagnetic Mn5Si3 compound in 873 and 973 K annealed samples substantially reduced the magnetism. This paper provides new insights into the understanding of ferromagnetism in Mn-implanted Si diluted magnetic semiconductors.

Cluster spin-glass ground state in quasi-one-dimensionalKCr3As3

We report structural and physical properties of a new quasi-one-dimensional Cr-based compound, KCr$_{3}$As$_{3}$, which is prepared by potassium deintercalation from the superconductive K$_{2}$Cr$_{3}$As$_{3}$. KCr$_{3}$As$_{3}$ adopts the TlFe$_{3}$Te$_{3}$-type structure with space group $P6_{3}$/$m$ (No. 176). The high-temperature magnetic susceptibility obeys the Curie-Weiss law with an effective magnetic moment of 0.68 $\mu_{\mathrm{B}}$/Cr. Below 56 K the susceptibility deviates from the high-temperature Curie-Weiss behavior, coinciding with the rapid increase in resistivity, which suggests formation of spin clusters. The short-range spin correlations are also supported by the specific-heat data. The title material does not exhibit bulk superconductivity; instead, it shows a cluster spin-glass state below $\sim$ 5 K.

Ground state and low-energy magnetic dynamics in the frustrated magnetCoAl2O4as revealed by local spin probes

We report a combined experimental study of magnetic properties of a single crystal of the frustrated diamond lattice antiferromagnet ${\text{CoAl}}_{2}{\text{O}}_{4}$ with ${\text{Co}}^{2+}$ electron spin resonance, $^{27}\mathrm{Al}$ nuclear magnetic resonance, and muon spin rotation/relaxation techniques. With our local probes, we show that the frustration of spin interactions and the Co/Al site disorder strongly affect the spin dynamics. The experimental results evidence inhomogeneous and slow magnetic fluctuations and the occurrence of short-range electron spin correlations far above a characteristic temperature ${T}^{*}=8$ K at which the spin system turns into in a quasistatic state. Our data indicate that this spin order is likely short range and unconventional with spin fluctuations persistent even at $T\ensuremath{\ll}{T}^{*}$. The results of three spectroscopy techniques highlight a nontrivial role of structural disorder for the magnetism of a frustrated diamond spin lattice at the proximity to the critical point.

Magnetic Orderings in Li2Cu(WO4)2 with Tungstate-Bridged Quasi-1D Spin-1/2 Chains.

By both experimental measurements and theoretical calculations, we investigated the magnetic and electronic properties of Li2Cu(WO4)2 as a tungstate-bridged quasi-one-dimensional (1D) copper spin-(1/2) chain system. Interestingly, magnetic susceptibility χ(T) and specific heat measurements show that the system undergoes a three-dimensional antiferromagnetic (AF)-like ordering at TN ≈ 3.7 K, below a broad χ(T) maximum at ∼8.9 K indicating a low-dimensional short-range AF spin correlation. Bonner-Fisher model fitting of χ(T) leads to an AF intrachain exchange constant of J/kB = 15.8 ± 0.1 K, and mean-field theory estimation gives an interchain coupling constant of J⊥/kB = 1.6 K, which supports the quasi-1D nature of this spin system. Theoretical evaluation of exchange coupling constants within the generalized gradient approximation (GGA) plus on-site Coulomb interaction (U) shows that the dominant AF exchange interaction is of ∼13.9 K along the a-axis with weak interchain coupling, in agreement with the experimental result of a quasi-1D spin-(1/2) chain system. The GGA+U calculations also predict that Li2Cu(WO4)2 is a charge transfer-type AF semiconductor with a direct band gap of 1.5 eV.

Diffuse magnetic neutron scattering in the highly frustrated double perovskiteBa2YRuO6

Here we investigated diffuse magnetic scattering in the highly frustrated double perovskite Ba2YRuO6 using polarized neutrons. Consistent with previous reports, the material shows two apparent transitions at 47 and 36 K to an eventual type I face-centered-cubic magnetic ground state. The (100) magnetic reflection shows different behavior from the five other observed reflections upon heating from 1.8 K, with the former broadening well beyond the resolution limit near 36 K. Closer examination of the latter group reveals a small, but clear, increase in peak widths between 36 and 47 K, indicating that this regime is dominated by short-range spin correlations. Diffuse magnetic scattering persists above 47 K near the position of (100) to at least 200 K, consistent with strong frustration. Reverse Monte Carlo (RMC) modeling of the diffuse scattering from 45 to 200 K finds that the spin-spin correlations between nearest and next-nearest neighbors are antiferromagnetic and ferromagnetic, respectively, at temperatures near the upper ordering temperature, but both become antiferromagnetic and of similar magnitude above 100 K. The significance of this unusual crossover is discussed in light of the super-superexchange interactions between nearest and next-nearest neighbors in this material and the demands of type I order. The dimensionality ofmore » the correlations is addressed by reconstructing the scattering in the (hk0) plane using the RMC spin configurations. This indicates that one-dimensional spin correlations dominate at temperatures close to the first transition. In addition, a comparison between mean-field calculations and (hk0) scattering implies that further neighbor couplings play a significant role in the selection of the ground state. Finally, the results and interpretation are compared with those recently published for monoclinic Sr2YRuO6, and similarities and differences are emphasized.« less

Short-range correlations in the magnetic ground state of Na₄Ir₃O₈.

The magnetic ground state of the J(eff)=1/2 hyperkagome lattice in Na₄Ir₃O₈ is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen state comprised of quasistatic moments develops below a characteristic temperature of T(F)=6 K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasistatic, short-range spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state arises in Na₄Ir₃O₈ driven either by disorder inherent to the creation of the hyperkagome lattice itself or stabilized via quantum fluctuations.

Polaron-mediated spin correlations in metallic and insulatingLa1−xAxMnO3(A=Ca,Sr,orBa)

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.

Two-magnon Raman scattering in LiMnPO4

Abstract Two-magnon Raman scattering is observed in the orthophosphate LiMnPO 4 , carrying quantitative information on the magnetic interactions between local Mn 2+ moments. A simulated annealing fitting procedure of the Raman signal to theoretical curves derived from a magnetic Hamiltonian was carried out, taking exchange and anisotropy constants as free fitting parameters. Previously reported inelastic neutron scattering (INS) data [J. Li et al. , Phys. Rev. B 79 , 144410 (2009)] were also used in the fit. It is shown that the combined application of INS and Raman scattering data in the fit reduces the ambiguity of the determined set of exchange parameters with respect to fitting procedures using INS or Raman data independently. The temperature dependence of the Raman signal does not show a collapse of the two-magnon excitations at the long-range magnetic ordering temperature, T N = 34 K , supporting significant short-range spin correlations above T N .

Approaching the true ground state of frustratedA-site spinels: A combined magnetization and polarized neutron scattering study

We re-investigate the magnetically frustrated, diamond-lattice-antiferromagnet spinels FeAl${}_{2}$O${}_{4}$ and MnAl${}_{2}$O${}_{4}$ using magnetization measurements and diffuse scattering of polarized neutrons. In FeAl${}_{2}$O${}_{4}$, macroscopic measurements evidence a ``cusp'' in zero field-cooled susceptibility around 13 K. Dynamic magnetic susceptibility and memory effect experiments provide results that do not conform with a canonical spin-glass scenario in this material. Through polarized neutron-scattering studies, absence of long-range magnetic order down to 4 K is confirmed in FeAl${}_{2}$O${}_{4}$. By modeling the powder averaged differential magnetic neutron-scattering cross section, we estimate that the spin-spin correlations in this compound extend up to the third nearest-neighbor shell. The estimated value of the Land\'e $g$ factor points towards orbital contributions from Fe${}^{2+}$. This is also supported by a Curie-Weiss analysis of the magnetic susceptibility. MnAl${}_{2}$O${}_{4}$, on the contrary, undergoes a magnetic phase transition into a long-range ordered state below $\ensuremath{\approx}$40 K, which is confirmed by macroscopic measurements and polarized neutron diffraction. However, the polarized neutron studies reveal the existence of prominent spin fluctuations co-existing with long-range antiferromagnetic order. The magnetic diffuse intensity suggests a similar short-range order as in FeAl${}_{2}$O${}_{4}$. Results of the present work support the importance of spin-spin correlations in understanding magnetic response of frustrated magnets like $A$-site spinels which have predominant short-range spin correlations reminiscent of the ``spin-liquid'' state.

Magnetic and ESR studies in a Quasi-two-dimensional Triangular magnet Ba3MnNb2O9

We report the magnetic, specific heat and electron spin resonance (ESR) studies on Ba3MnNb2O9, as one of quasi-two-dimensional triangular–lattice magnet. The magnetic susceptibility and specific heat measurement show that it undergoes an magnetic transition at TN=3.2K. The magnetic capacity (Cmag) follows T2 dependence, and magnetic entropy (Smag) reaching to 14.5JK−1mol−1 at 32K satisfactorily agrees with the theoretical estimations of Mn2+ ion in high-spin configuration with S=5/2. From the ESR spectra, the Landé factor (geff), the linewidth (ΔHpp) and the ESR intensity (I) are derived and analyzed as a function of temperature. As temperature decreases to T⁎~18K, the g-factor shows a minima with geff=1.984(2) before shifts to larger value at lower temperatures, and the ESR intensity starts to be suppressed below this temperature. This anomalous phenomenon is explained in terms of the establishment of short-range AFM spin correlations at much higher temperatures above TN.

Thermodynamics and Magnetic Properties of the Anisotropic 3D Hubbard Model

We study the anisotropic 3D Hubbard model with increased nearest-neighbor tunneling amplitudes along one direction using the dynamical cluster approximation and compare the results to a quantum simulation experiment of ultracold fermions in an optical lattice. We find that the short-range spin correlations are significantly enhanced in the direction with stronger tunneling amplitudes. Our results agree with the experimental observations and show that the experimental temperature is lower than the strong tunneling amplitude. We characterize the system by examining the spin correlations beyond neighboring sites and determine the distribution of density, entropy, and spin correlation in the trapped system. We furthermore investigate the dependence of the critical entropy at the Néel transition on anisotropy.

2D correlation analysis of the magnetic excitations in Raman spectra of HoMnO3

2D correlation analysis is performed on the temperature-dependent Raman spectra of HoMnO3 thin films. As the temperature of the HoMnO3 thin films decrease, the depletion of the spectral weight at 336, 656, and 1304cm−1 occurs at higher temperatures than the increase of the intensity at 508, 766, and 945cm−1 below ∼70K, the Néel temperature. The power spectrum asserts that all the changes in the spectral weight are strongly correlated. Most of the temperature-induced spectral changes of HoMnO3 occur at lower temperature than 70K, while there is slight depletion of the spectral weight at 336, 656, and 1304cm−1 even at higher temperature than 70K. PCA scores and loading vectors plots also support these 2D correlation results. Our 2D correlation analysis supports the existence of the short range spin correlations between Mn sites in HoMnO3 even above the Néel temperature.

Phase Transitions in Li2MnO3 Electrodes at Various States-of-Charge

We studied the structural transformations from the layered-type to spinel-type ordering upon galvanostatic charging (Li+ extraction) of Li2MnO3 electrodes. We have determined by X-ray, electron diffraction techniques and Raman spectroscopy analyses that these transformations partially occurred even at the initial states-of-charge, at potentials of 4.3–4.5V, and in fully charged (4.7V) and discharged (2.0V) states as well. The above transformations are evident also from the analysis of the intrinsic magnetic properties of Li2MnO3 electrodes that indicate short-range spin correlations typical for the Li2Mn2O4 and LiMn2O4 spinel-type structures, depending on the state of charge and the C-rates used for the cycling.

Partial Spin Ordering and Complex Magnetic Structure in BaYFeO4: A Neutron Diffraction and High Temperature Susceptibility Study

The novel iron-based compound, BaYFeO4, crystallizes in the Pnma space group with two distinct Fe(3+) sites, that are alternately corner-shared [FeO5](7-) square pyramids and [FeO6](9-) octahedra, forming into [Fe4O18](24-) rings, which propagate as columns along the b-axis. A recent report shows two discernible antiferromagnetic (AFM) transitions at 36 and 48 K in the susceptibility, yet heat capacity measurements reveal no magnetic phase transitions at these temperatures. An upturn in the magnetic susceptibility measurements up to 400 K suggests the presence of short-range magnetic behavior at higher temperatures. In this Article, variable-temperature neutron powder diffraction and high-temperature magnetic susceptibility measurements were performed to clarify the magnetic behavior. Neutron powder diffraction confirmed that the two magnetic transitions observed at 36 and 48 K are due to long-range magnetic order. Below 48 K, the magnetic structure was determined as a spin-density wave (SDW) with a propagation vector, k = (0, 0, (1)/3), and the moments along the b-axis, whereas the structure becomes an incommensurate cycloid [k = (0, 0, ∼0.35)] below 36 K with the moments within the bc-plane. However, for both cases the ordered moments on Fe(3+) are only of the order ∼3.0 μB, smaller than the expected values near 4.5 μB, indicating that significant components of the Fe moments remain paramagnetic to the lowest temperature studied, 6 K. Moreover, new high-temperature magnetic susceptibility measurements revealed a peak maximum at ∼550 K indicative of short-range spin correlations. It is postulated that most of the magnetic entropy is thus removed at high temperatures which could explain the absence of heat capacity anomalies at the long-range ordering temperatures. Published spin dimer calculations, which appear to suggest a k = (0, 0, 0) magnetic structure, and allow for neither low dimensionality nor geometric frustration, are inadequate to explain the observed complex magnetic structure.