Short-range Spin Correlations Research Articles

Finitef-electron bandwidth in a heavy-fermion model

The determinant quantum Monte Carlo (DQMC) method is used to study the effect of nonzero hopping ${t}_{f}$ in the ``localized'' $f$ band of the periodic Anderson model (PAM) in two dimensions. The low-temperature properties are determined in the plane of interband hybridization $V$ and ${t}_{f}$ at fixed ${U}_{f}$ and half filling, including the case when the sign of ${t}_{f}$ is opposite to that of the conduction band ${t}_{d}$. For ${t}_{f}$ and ${t}_{d}$ of the same sign and when ${t}_{f}/{t}_{d}>{(V/4{t}_{d})}^{2}$, the noninteracting system is metallic. We show that a remnant of the band insulator to metal line at ${U}_{f}=0$ persists in the interacting system, manifesting itself as a maximal tendency toward antiferromagnetic correlations at low temperature. In this ``optimal'' ${t}_{f}$ region, short-range (e.g., near-neighbor) and long-range spin correlations develop at similar temperatures and have comparable magnitude. Both observations are in stark contrast to the situation in the widely studied PAM (${t}_{f}=0$) and single-band Hubbard model, where short-range correlations are stronger and develop at higher temperature. The effect that finite ${t}_{f}$ has on Kondo screening is investigated by considering the evolution of the local density of states for selected ${t}_{f}$ as a function of $V$. We use mean-field theory as a tool to discriminate those aspects of the physics that are genuinely many-body in character.

Specific Heat of Stage-2 MnCl2 Graphite Intercalation Compound: Co-Existence of Spin Glass Phase and Incommensurate Short-Range Spin Order

Stage-2 MnCl2 GIC magnetically behaves like a quasi 2D \(XY\) antiferromagnet on the triangular lattice with the spin freezing temperature \(T_{\text{SG}}\) (\(= 1.1\) K). We have undertaken an extensive study on the \(T\) dependence of specific heat in the absence of an external field \(H\) and in the presence of \(H\) along the \(c\)-plane. The magnetic specific heat \(C_{\text{mag}}\) at \(H = 0\) shows no anomaly at \(T_{\text{SG}}\), but exhibits double broad peaks around 5 and 41 K. The anomaly at 41 K is the onset of short range spin correlation. The magnetic specific heat \(C_{\text{mag}}\) at \(H = 0\) is described by Cmag ∝(1/T2)exp (-Δ/T) with Δ= 1.41 ±0.03 K, while \(C_{\text{mag}}\) at \(H = 10\) kOe is proportional to \(T\). The entropy due to the broad peak around 5 K is 1/3 of the total entropy. The residual entropy is 63% of the total entropy because of highly frustrated nature of the system. These results are consistent with the previous DC susceptibility, neutron scattering, and electro...

Frustrated magnetism and resonating valence bond physics in two-dimensional kagome-like magnets

We explore the phase diagram and the low-energy physics of three Heisenberg antiferromagnets which, like the kagome lattice, are networks of corner-sharing triangles but contain two sets of inequivalent short-distance resonance loops. We use a combination of exact diagonalization, analytical strong-coupling theories, and resonating valence bond approaches, and scan through the ratio of the two inequivalent exchange couplings. In one limit, the lattices effectively become bipartite, while at the opposite limit heavily frustrated nets emerge. In between, competing tunneling processes result in short-ranged spin correlations, a manifold of low-lying singlets (which can be understood as localized bound states of magnetic excitations), and the stabilization of valence bond crystals with resonating building blocks.

Finite-temperature properties of strongly correlated fermions in the honeycomb lattice

We study finite-temperature properties of strongly interacting fermions in the honeycomb lattice using numerical linked-cluster expansions and determinantal quantum Monte Carlo simulations. We analyze a number of thermodynamic quantities, including the entropy, the specific heat, uniform and staggered spin susceptibilities, short-range spin correlations, and the double occupancy at and away from half filling. We examine the viability of adiabatic cooling by increasing the interaction strength for homogeneous as well as for trapped systems. For the homogeneous case, this process is found to be more efficient at finite doping than at half filling. That, in turn, leads to an efficient adiabatic cooling in the presence of a trap, which, starting with even relatively high entropies, can drive the system to have a Mott insulating phase with substantial antiferromagnetic correlations.

Crystallographic, electronic, thermal, and magnetic properties of single-crystal SrCo2As2

In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here, structural and thermal expansion, electrical resistivity rho, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility chi, 75As NMR and neutron diffraction measurements of SrCo2As2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient alpha_c is negative from 7 to 300 K, whereas alpha_a is positive over this T range. The rho(T) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy E_F. The band calculations exhibit an extremely sharp peak in the density of states D(E_F) arising from a flat d_{x^2 - y^2} band. A comparison of the Sommerfeld coefficient of the electronic specific heat with chi(T = 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The chi(T) does have a large magnitude, but also exhibits a broad maximum at 115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we propose that metallic SrCo2As2 has a gapless quantum spin-liquid ground state.

Coexistence of magnetic order and spin-glass-like phase in the pyrochlore antiferromagnet Na3Co(CO3)2Cl

We present comprehensive investigations on a pyrochlore antiferromagnet Na${}_{3}$Co(CO${}_{3}$)${}_{2}$Cl. Performed dc magnetization indicates a broad maximum at around 4 K. The field dependence of the peak temperature of this maximum follows the de Almeida--Thouless line, suggesting a spin-glass-like phase transition. The ac susceptibility measurements determine the glassy transition temperature to be 4.5 K and reveal a frequency-independent peak at 17 K. The temperature dependence of the specific heat shows a sharp peak at 1.5 K and a broad hump at around 5 K, which are attributed to a long-range magnetic phase transition and a spin-glass-like freezing process, respectively. The average crystallographic structure of Na${}_{3}$Co(CO${}_{3}$)${}_{2}$Cl has been determined using neutron-powder diffraction. No obvious site disorder has been detected within the experimental resolution. The diffuse neutron scattering with polarization analysis indicates short-range spin correlations characterized by dominating antiferromagnetic coupling between nearest neighbors and weak ferromagnetic coupling between next-nearest neighbors. The long-range magnetic order below 1.5 K is evidenced by the magnetic reflections observed at 50 mK and can be well explained with an all-in--all-out spin configuration. Inelastic neutron scattering of Na${}_{3}$Co(CO${}_{3}$)${}_{2}$Cl exhibits collective magnetic excitations at 3.5 K, suggesting that the spin-glass-like transition temperature ${T}_{\mathrm{g}}$ $=$ 4.5 K does not correspond to a complete spin-glass freezing as expected in canonical spin glasses. The peak observed in magnetic susceptibility at 17 K is attributed to the onset of an intermediate partially ordered phase transition, qualitatively consistent with the theoretical predictions for pyrochlore antiferromagnets with weak ferromagnetic next-nearest-neighbor interactions.

Influence of short-range spin correlations on theμSR polarization functions in the slow dynamic limit: Application to the quantum spin-liquid system Yb2Ti2O7

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Determining the Short-Range Spin Correlations in the Spin-ChainLi2CuO2andCuGeO3Compounds Using Resonant Inelastic X-Ray Scattering

We report a high-resolution resonant inelastic soft x-ray scattering study of the quantum magnetic spin-chain materials Li(2)CuO(2) and CuGeO(3). By tuning the incoming photon energy to the oxygen K edge, a strong excitation around 3.5 eV energy loss is clearly resolved for both materials. Comparing the experimental data to many-body calculations, we identify this excitation as a Zhang-Rice singlet exciton on neighboring CuO(4) plaquettes. We demonstrate that the strong temperature dependence of the inelastic scattering related to this high-energy exciton enables us to probe short-range spin correlations on the 1 meV scale with outstanding sensitivity.

Short-Range Correlations and Cooling of Ultracold Fermions in the Honeycomb Lattice

We use determinantal quantum MonteCarlo simulations and numerical linked-cluster expansions to study thermodynamic properties and short-range spin correlations of fermions in the honeycomb lattice. We find that, at half filling and finite temperatures, nearest-neighbor spin correlations can be stronger in this lattice than in the square lattice, even in regimes where the ground state in the former is a semimetal or a spin liquid. The honeycomb lattice also exhibits a more pronounced anomalous region in the double occupancy that leads to stronger adiabatic cooling than in the square lattice. We discuss the implications of these findings for optical lattice experiments.

Density matrix renormalization group algorithm for Bethe lattices of spin-12or spin-1 sites with Heisenberg antiferromagnetic exchange

An efficient density matrix renormalization group (DMRG) algorithm is presented for the Bethe lattice with connectivity $Z = 3$ and antiferromagnetic exchange between nearest neighbor spins $s= 1/2$ or 1 sites in successive generations $g$. The algorithm is accurate for $s = 1$ sites. The ground states are magnetic with spin $S(g) = 2^g s$, staggered magnetization that persists for large $g > 20$ and short-range spin correlation functions that decrease exponentially. A finite energy gap to $S > S(g)$ leads to a magnetization plateau in the extended lattice. Closely similar DMRG results for $s$ = 1/2 and 1 are interpreted in terms of an analytical three-site model.

Paramagnetic spin correlations in colossal magnetoresistive La0.7Ca0.3MnO3

Neutron spectroscopy measurements reveal dynamic spin correlations throughout the Brillouin zone in the colossal magnetoresistive material La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$ at 265 K ($\ensuremath{\approx}1.03$ ${T}_{C}$). The long-wavelength behavior is consistent with spin diffusion, yet an additional and unexpected component of the scattering is also observed in low-energy constant-$E$ measurements, which takes the form of ridges of strong quasielastic scattering running along ($H$ 0 0) and equivalent directions. Well-defined $Q$-space correlations are observed in constant-$E$ scans at energies up to at least 28 meV, suggesting robust short-range spin correlations in the paramagnetic phase.

Evidence of large nonlinear magnetic effects in the paramagnetic phase of GaFeO3

Magnetic susceptibility and magnetization measurements have been performed on a polycrystalline sample of GaFeO${}_{3}$ and compared with $X$-band electronic spin resonance (ESR) experiments. The persistence of nonlinear effects in the presence of relatively low external fields is observed far above the ferrimagnetic transition. The temperature evolution of the ESR spectra indicates the increase of spin-spin interactions accompanied by significant changes in the internal field as the nonlinear regime becomes more important. This suggests the existence of ferrimagnetic short-range spin correlations induced by the external field over a wide temperature range above the magnetic ordering transition.

Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO3

Terahertz and far-infrared electric and magnetic responses of hexagonal piezomagnetic YMnO${}_{3}$ single crystals are investigated. Antiferromagnetic resonance is observed in the spectra of magnetic permeability ${\ensuremath{\mu}}_{a}$ [H$\phantom{\rule{0.16em}{0ex}}(\ensuremath{\omega})$ oriented within the hexagonal plane] below the N\'eel temperature ${T}_{N}$. This excitation softens from 41 to 32 $\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ upon heating and finally disappears above ${T}_{N}$. An additional weak and heavily-damped excitation is seen in the spectra of complex dielectric permittivity ${\ensuremath{\varepsilon}}_{c}$ within the same frequency range. This excitation contributes to the dielectric spectra in both antiferromagnetic and paramagnetic phases. Its oscillator strength significantly increases upon heating toward room temperature, thus providing evidence of piezomagnetic or higher-order couplings to polar phonons. Other heavily-damped dielectric excitations are detected near 100 $\phantom{\rule{0.16em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ in the paramagnetic phase in both ${\ensuremath{\varepsilon}}_{c}$ and ${\ensuremath{\varepsilon}}_{a}$ spectra, and they exhibit similar temperature behavior. These excitations appearing in the frequency range of magnon branches well below polar phonons could remind electromagnons, however their temperature dependence is quite different. We have used density functional theory for calculating phonon dispersion branches in the whole Brillouin zone. A detailed analysis of these results and of previously published magnon dispersion branches brought us to the conclusion that the observed absorption bands stem from phonon-phonon and phonon-paramagnon differential absorption processes. The latter is enabled by strong short-range in-plane spin correlations in the paramagnetic phase.

Synthesis, structure, and magnetic properties of the layered iron oxychalcogenide Na2Fe2Se2O

A new layered iron-oxychalcogenide Na2Fe2OSe2 has been synthesized and structurally characterized by powder X-ray diffraction. The structure is formed by alternate stacking of the newly discovered [Fe2OSe2] blocks and double layers of Na. Conductivity study shows that Na2Fe2OSe2 is a semiconductor with activation energy of 0.26 eV. Magnetic susceptibility and heat capacity measurements reveal an antiferromagnetic phase transition occurs at TN=73 K. A broad maximum of magnetic susceptibility and a slow decay of the specific heat above TN, arises as a result of two-dimensional short-range spin correlation.

Structural, transport, thermodynamic, and neutron diffraction studies of layeredR2O3Fe2Se2(R=Ce, Pr, Nd, and Sm)

We report the structural, transport, and thermodynamic properties of the semiconducting, layered rare-earth iron oxyselenides $R$${}_{2}$O${}_{3}$Fe${}_{2}$Se${}_{2}$ ($R=\text{Ce, Pr, Nd, and Sm}$), which have crystal structures related to the arsenide superconductors except with in-plane oxygens within the Fe square planes. The activation energies are estimated to be between 0.15 and 0.26 eV for these four compounds. The antiferromagnetic Fe sublattice ordering, which appears as an anomaly in the magnetic susceptibility and a sharp peak in the heat capacity, occurs at 87.2, 88.6, 87.7, and 85.3 K, for the Ce to Sm variants, respectively, and the structural analysis suggests a possible correlation between ${T}_{N\ensuremath{-}\mathrm{Fe}}$ and the Fe-Se distance, rather than the in-plane Fe-O distance. The $R$ sublattice orders antiferromagnetically at 21.8 K for the Pr variant and 5.8 K for the Sm variant; no $R$ sublattice ordering is observed above 2 K for $R=\text{Ce}$ and Nd. The existence of short-range spin correlations above the Fe sublattice ordering temperature is revealed by magnetic entropy analysis, which also suggests that the ordered magnetic states of the $R$ sublattice arise from triply degenerate crystal-electric-field (CEF) states for Pr and doubly degenerate CEF states for Sm. The neutron-scattering measurements performed on Pr${}_{2}$O${}_{3}$Fe${}_{2}$Se${}_{2}$ reveals that the ordered Fe sublattice has the same magnetic structure as the one in La${}_{2}$O${}_{3}$Fe${}_{2}$Se${}_{2}$, with a propagation vector of $\mathbf{k}=(0.5,0,0.5)$ and an ordered magnetic moment of 2.23(3)${\ensuremath{\mu}}_{B}$/Fe at 5 K.

Critical correlations for short-range valence-bond wave functions on the square lattice

We investigate the arguably simplest $SU(2)$-invariant wave functions capable of accounting for spin-liquid behavior, expressed in terms of nearest-neighbor valence-bond states on the square lattice and characterized by different topological invariants. While such wave functions are known to exhibit short-range spin correlations, we perform Monte Carlo simulations and show that four-point correlations decay algebraically with an exponent 1.16(4). This is reminiscent of the classical dimer problem, albeit with a slower decay. Furthermore, these correlators are found to be spatially modulated according to a wave vector related to the topological invariants. We conclude that a recently proposed spin Hamiltonian that stabilizes the here considered wave function(s) as its (degenerate) ground state(s) should exhibit gapped spin and gapless nonmagnetic excitations.

Theory of the lightly doped Mott insulator

A theory for the Hubbard model appropriate in the limit of large $U/t$, small doping away from half-filling and short-ranged antiferromagnetic spin correlations is presented. Despite the absence of any broken symmetry, the Fermi surface takes the form of elliptical hole pockets centered near $(\frac{\ensuremath{\pi}}{2},\frac{\ensuremath{\pi}}{2})$ with a volume proportional to the hole concentration. Short-range antiferromagnetic correlations render the nearest-neighbor hopping almost ineffective so that only second- or third-nearest-neighbor hopping contributes appreciably to the dispersion relation.

Magnetization Step in Spatially Distorted Heisenberg Kagomé Antiferromagnets

Motivated by a recent experiment on volborthite, a typical spin-$1/2$ antiferromagnet with a kagom\'{e} lattice structure, we study the magnetization process of a classical Heisenberg model on a spatially distorted kagom\'{e} lattice using the Monte Carlo (MC) method. We find a distortion-induced magnetization step at low temperatures and low magnetic fields. The magnitude of this step is given by $\Delta m_z=\left|1-\alpha\right|/3\alpha$ at zero temperature, where $\alpha$ denotes the spatial anisotropy in exchange constants. The magnetization step signals a first-order transition at low temperatures, between two phases distinguished by distinct and well-developed short-range spin correlations, one characterized by spin alignment of a local $120^{\circ}$ structure with a $\sqrt{3}\times\sqrt{3}$ period, and the other by a partially spin-flopped structure. We point out the relevance of our results to the unconventional steps observed in volborthite.

Paramagnetic spin correlations inCaFe2As2single crystals

Magnetic correlations in the paramagnetic phase of CaFe2As2 (T_N=172 K) have been examined by means of inelastic neutron scattering from 180 K (~ 1.05 T_N) up to 300 K (~1.8 T_N). Despite the first-order nature of the magnetic ordering, strong but short-range antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasi-elastic scattering centered at the wavevector Q_{AFM} of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (E> 60 meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c-direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt doped superconducting BaFe2As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-range anisotropic spin correlations with overdamped spin dynamics.

Finite Doping Signatures of the Mott Transition in the Two-Dimensional Hubbard Model

Experiments on layered materials call for a study of the influence of short-range spin correlations on the Mott transition. To this end, we solve the cellular dynamical mean-field equations for the Hubbard model on a plaquette with continuous-time quantum Monte Carlo calculations. The normal-state phase diagram as a function of temperature T, interaction strength U, and filling n reveals that upon increasing n towards the insulator, there is a surface of first-order transition between two metals at nonzero doping. For T above the critical end line there is a maximum in scattering rate.