Statics and dynamics of incommensurate spin order in a geometrically frustrated antiferromagnet CdCr2O4

Using elastic and inelastic neutron scattering we show that a cubic spinel, CdCr2O4, undergoes an elongation along the c axis (c > a = b) at its spin-Peierls-like phase transition at T(N) = 7.8 K. The Néel phase (T < T(N)) has an incommensurate spin structure with a characteristic wave vector Q(M) = (0, delta,1) with delta approximately 0.09 and with spins lying on the ac plane. This is in stark contrast to another well-known Cr-based spinel, ZnCr2O4, that undergoes a c-axis contraction and a commensurate spin order. The magnetic excitation of the incommensurate Néel state has a weak anisotropy gap of 0.6 meV and it consists of at least three bands extending up to 5 meV.     

Nonlocal magnetic field-tuned quantum criticality in cubic CeIn(3-x)Sn(x) (x = 0.25)

We show that antiferromagnetism in lightly (approximately 8%) Sn-doped CeIn3 terminates at a critical field mu0H(c) = 42 +/- 2 T. Electrical transport and thermodynamic measurements reveal the effective mass m* not to diverge, suggesting that cubic CeIn3 is representative of a critical spin-density wave (SDW) scenario, unlike the local quantum critical points reported in anisotropic systems such as CeCu(6-x)Au(x) and YbRh2Si(2-x)Ge(x). The existence of a maximum in m* at a lower field mu0H(x) = 30 +/- 1 T may be interpreted as a field-induced crossover from local moment to SDW behavior as the Néel temperature falls below the Fermi temperature.     

Zero-energy states and fragmentation of spin in the easy-plane antiferromagnet on a honeycomb lattice

The core of the vortex in the Néel order parameter for an easy-plane antiferromagnet on a honeycomb lattice is demonstrated to bind two zero-energy states. Remarkably, a single electron occupying this midgap band has its spin fragmented between the two sublattices: Whereas it yields a vanishing total magnetization, it shows a finite Néel order, orthogonal to the one of the assumed background. The requisite easy-plane anisotropy may be introduced by a magnetic field parallel to the graphene layer, for example. The results are relevant for spin-1/2 fermions on the graphene's or optical honeycomb lattice, in the strongly interacting regime.     

Néel temperature of quasi-low-dimensional Heisenberg antiferromagnets

The Néel temperature T(N) of quasi-one- and quasi-two-dimensional antiferromagnetic Heisenberg models on a cubic lattice is calculated by Monte Carlo simulations as a function of interchain (interlayer) to intrachain (intralayer) coupling J(')/J down to J(')/J approximately = 10(-3). We find that T(N) obeys a modified random-phase approximationlike relation for small J(')/J with an effective universal renormalized coordination number, independent of the size of the spin. Empirical formulas describing T(N) for a wide range of J(') and useful for the analysis of experimental measurements are presented.     

Macroscopic Quantum Coherence in an Antiferromagnetic Spin Chain with Biaxial Anisotropy

Instanton configurations of (1+1)-dimensions in an antiferromagnetic biaxial-anisotropy-spin-chain are obtained explicitly in the strong anisotropy limit, which interpolate between degenerate equilibrium orientations of the Néel vector along easy axis and are seen to be responsible for quantum tunneling. Macroscopic quantum coherence of the domain walls is demonstrated in terms of the instantons.     

Evidence for deconfined quantum criticality in a two-dimensional Heisenberg model with four-spin interactions

Using ground-state projector quantum Monte Carlo simulations in the valence-bond basis, it is demonstrated that nonfrustrating four-spin interactions can destroy the Néel order of the two-dimensional S=1/2 Heisenberg antiferromagnet and drive it into a valence-bond solid (VBS) phase. Results for spin and dimer correlations are consistent with a single continuous transition, and all data exhibit finite-size scaling with a single set of exponents, z=1, nu=0.78+/-0.03, and eta=0.26+/-0.03. The unusually large eta and an emergent U(1) symmetry, detected using VBS order parameter histograms, provide strong evidence for a deconfined quantum critical point.     

Valence bond solid phases in a cubic antiferromagnet

We report on a valence bond projector Monte Carlo simulation of the cubic lattice quantum Heisenberg model with additional higher-order exchange interactions in each unit cell. The model supports two different valence bond solid (VBS) ground states. In one of these states, the dimer pattern is a three-dimensional analogue of the columnar pattern familiar from two dimensions. In the other, the dimers are regularly arranged along the four main diagonals in 1/8 of the unit cells. The phases are separated from one another and from a Néel phase by strongly first-order boundaries. Our results strengthen the case for exotic transitions in two dimensions, where no discontinuities have been detected at the Heisenberg Néel-VBS transition driven by four-spin plaquette interactions.     

Noncollinear spin reorientation transition in S?=?1 ferromagnetic thin films

An in-plane spin reorientation transition in thin ferromagnetic films is discussed in terms of the thermodynamics of inhomogeneous low-dimensional systems based on a Néel sublattices concept while using a spin 1 Heisenberg Hamiltonian. The model allows us to investigate in a straightforward manner the layer-dependent phenomena. In this context, we propose a model of noncollinear magnetization structure based on the appropriate distribution of the anisotropy parameters inside the Fe films on W(110). The spin reorientation transition originates at the Fe/W(110) interface and proceeds via noncollinear magnetization structure toward the surface with increasing film thickness in accordance with the experimental findings. The temperature-driven spin reorientation transition in freestanding Fe films and in Fe/W(110) systems is also discussed in detail.     

Magnetic order in lightly doped La2-xSrxCuO4

We study long wavelength magnetic excitations in lightly doped La2-xSrxCuO4 (x相似文献   

Emergent fluctuation hot spots on the fermi surface of CeIn(3) in strong magnetic fields

de Haas-van Alphen measurements on CeIn3 in pulsed magnetic fields of up to 65 T reveal an increase in the quasiparticle effective mass with the field concentrated at "hot spots" on the Fermi surface as the Néel phase is suppressed. As well as revealing the existence of fluctuations deep within the antiferromagnetic phase, these data suggest that a possible new type of quantum critical point may exist in strong magnetic fields that involves only parts of the Fermi surface.     

Néel order in doped quasi-one-dimensional antiferromagnets

We study the Néel temperature of quasi-one-dimensional S = 1/2 antiferromagnets containing nonmagnetic impurities. We first consider the temperature dependence of the staggered susceptibility of finite chains with open boundary conditions, which shows an interesting difference for even and odd length chains. We then use a mean field theory treatment to incorporate the three-dimensional interchain couplings. The resulting Néel temperature shows a pronounced drop as a function of doping by up to a factor of 5.     

Anomalous curie response of impurities in quantum-critical spin-1/2 Heisenberg antiferromagnets

We consider a magnetic impurity in two different S=1/2 Heisenberg bilayer antiferromagnets at their respective critical interlayer couplings separating Néel and disordered ground states. We calculate the impurity susceptibility using a quantum Monte Carlo method. With intralayer couplings in only one of the layers (Kondo lattice), we observe an anomalous Curie constant C*, as predicted on the basis of field-theoretical work [S. Sachdev, Science 286, 2479 (1999)10.1126/science.286.5449.2479]. The value C* = 0.262 +/- 0.002 is larger than the normal Curie constant C=S(S+1)/3. Our low-temperature results for a symmetric bilayer are consistent with a universal C*.     

Origin of the structural and magnetic anomalies of the layered compound SrFeO2: a density functional investigation

The structural and magnetic anomaly of the layered compound SrFeO2 are examined by first-principles density functional calculations and Monte Carlo simulations. The down-spin Fe 3d electron occupies the d(z(2)) level rather than the degenerate (d(xz), d(yz)) levels, which explains the absence of a Jahn-Teller instability, the easy ab-plane magnetic anisotropy, and the observed three-dimensional (0.5, 0.5, 0.5) antiferromagnetic order. Monte Carlo simulations show that the strong interlayer spin exchange is essential for the high Néel temperature.     

Fermi surface of CeIn3 above the Néel critical field

We report measurements of the de Haas-van Alphen effect in CeIn(3) in magnetic fields extending to approximately 90 T, well above the Néel critical field of mu(0)H(c) approximately 61 T. The unreconstructed Fermi surface a sheet is observed in the high magnetic field polarized paramagnetic limit, but with its effective mass and Fermi surface volume strongly reduced in size compared to that observed in the low magnetic field paramagnetic regime under pressure. The spheroidal topology of this sheet provides an ideal realization of the transformation from a "large Fermi surface" accommodating f electrons to a "small Fermi surface" when the f-electron moments become polarized.     

M?ssbauer spectroscopic study of spin reorientation in Mn-substituted yttrium orthoferrite

The first-order spin-reorientation transition in the Mn-substituted yttrium orthoferrites, YFe(1-x)Mn(x)O(3) (x = 0.1, 0.15 and 0.2), has been investigated using (57)Fe M?ssbauer spectroscopy. Owing to its large anisotropy, substitution of Mn(3+) ions in YFeO(3) induces a spin-reorientation transition from the low-temperature antiferromagnetic state to a high-temperature weak ferromagnetic state. With increasing x, the spin-reorientation transition temperature (T(SR)) increases whereas the Néel temperature (T(N)) decreases. Analysis of the M?ssbauer spectra unambiguously confirms the occurrence of spin reorientation relative to crystal axes. At a given temperature, the mean hyperfine field decreases with the increasing Mn concentration. The variation of canting angle with temperature for YFe(0.85)Mn(0.15)O(3) has been estimated.     

Charge ordering due to magnetic symmetry breaking

It is argued that both transitions observed in 50% doped manganites, at the Néel temperature (T(N)) and the so-called charge ordering temperature (T(CO)), are magnetic. T(N) corresponds to the order-disorder transition, which takes place between ferromagnetic zigzag chains, while the coherent motion of spins within the chains is destroyed only around T(CO). The magnetic structure below T(CO) is highly anisotropic. It is dressed by the lattice distortion and leads to the huge anisotropy of the electronic structure, which explains stability of this state as well as the form of the charge-orbital pattern above T(N). The type of phase transition at T=T(N) is determined by lattice interactions.     

Mott glass in site-diluted S=1 antiferromagnets with single-ion anisotropy

The interplay between site dilution and quantum fluctuations in S=1 Heisenberg antiferromagnets on the square lattice is investigated using quantum Monte Carlo simulations. Quantum fluctuations are tuned by a single-ion anisotropy D. In the clean limit, a sufficiently large D>Dc=5.65(2)J forces each spin into its mS=0 state, and thus destabilizes antiferromagnetic order. In the presence of site dilution, quantum fluctuations are found to destroy Néel order before the percolation threshold of the lattice is reached, if D exceeds a critical value D*=2.3(2)J. This mechanism opens up an extended quantum-disordered Mott-glass phase on the percolated lattice, characterized by a gapless spectrum and vanishing uniform susceptibility.     

Spin correlations and magnetic order in nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta)

We report quantitative neutron scattering measurements of the evolution with doping of the Néel temperature, the antiferromagnetic correlations, and the ordered moment of as-grown, nonsuperconducting Nd(2-x)Ce(x)CuO(4+/-delta) (0相似文献   

Order by distortion and string modes in pyrochlore antiferromagnets

We study the effects of magnetoelastic couplings on pyrochlore antiferromagnets. We employ Landau theory, extending an investigation begun by Yamashita and Ueda for the case of S = 1, and classical analyses to argue that such couplings generate bond order via a spin-Peierls transition. This is followed by, or concurrent with, a transition into one of several possible low-temperature Néel phases, with most simply collinear, but also coplanar or mixed spin patterns. In a collinear Néel phase, a dispersionless stringlike magnon mode dominates the resulting excitation spectrum, providing a distinctive signature of the parent geometrically frustrated state. We comment on the experimental situation.