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Register a new userQuantum critical phenomena in a spin-1/2 frustrated square lattice with spatial anisotropy
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We present a model compound with a spin-1/2 spatially anisotropic frustrated square lattice, in which three antiferromagnetic interactions and one ferromagnetic interaction are competing. We observe an unconventional gradual increase in the low-temperature magnetization curve reminiscent of the quantum critical behavior between gapped and gapless phases. In addition, the specific heat and electron spin resonance signals indicate one-dimensional characteristics. These results demonstrate quantum critical behavior associated with one dimensionalization caused by frustrated interactions in the spin-1/2 spatially anisotropic square lattice.
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We present a model compound with a spin-1/2 frustrated square lattice, in which three ferromagnetic (F) interactions and one antiferromagnetic (AF) compet. Considering the effective spin-1 formed by the dominant F dimer, this square lattice can be mapped to a spin-1 spatially anisotropic triangular lattice. The magnetization curve exhibits gapped behavior indicative of a dominant one-dimensional (1D) AF correlation. In the field-induced gapless phase, the specific heat and magnetic susceptibility show a phase transition to an ordered state with 2D characteristics. These results indicate that the spin-1 Haldane state is extended to the 2D system. We demonstrate that the gapped ground state observed in the present spin-1/2 frustrated square lattice originates from the one-dimensionalization caused by frustration.
We present a new model compound with the S = 1/2 frustrated square lattice composed of the charge-transfer salt (o-MePy-V)PF6. Ab initio calculations indicate the formation of an S = 1/2 square lattice, in which six types of nearest-neighbor ferromagnetic- and antiferromagnetic interactions cause frustration. By applying a magnetic field, we observe an unusually gradual increase of magnetization and a subsequent 1/2-plateau-like behavior. A numerical analysis using the tensor network method qualitatively demonstrates such behaviors and suggests a collinear ordered state and a field-enhanced quantum fluctuation. Furthermore, the local magnetization and T1^-1 probed by nuclear magnetic resonance measurements support these findings.
The zero-temperature phase diagram of the spin-$frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{perp}$ model on an $AA$-stacked square-lattice bilayer is studied using the coupled cluster method implemented to very high orders. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor Heisenberg exchange interactions, of strengths $J_{1}>0$ and $J_{2} equiv kappa J_{1}>0$, respectively, are included in each layer. The two layers are coupled via a NN interlayer Heisenberg exchange interaction with a strength $J_{1}^{perp} equiv delta J_{1}$. The magnetic order parameter $M$ (viz., the sublattice magnetization) is calculated directly in the thermodynamic (infinite-lattice) limit for the two cases when both layers have antiferromagnetic ordering of either the N{e}el or the striped kind, and with the layers coupled so that NN spins between them are either parallel (when $delta < 0$) or antiparallel (when $delta > 0$) to one another. Calculations are performed at $n$th order in a well-defined sequence of approximations, which exactly preserve both the Goldstone linked cluster theorem and the Hellmann-Feynman theorem, with $n leq 10$. The sole approximation made is to extrapolate such sequences of $n$th-order results for $M$ to the exact limit, $n to infty$. By thus locating the points where $M$ vanishes, we calculate the full phase boundaries of the two collinear AFM phases in the $kappa$--$delta$ half-plane with $kappa > 0$. In particular, we provide the accurate estimate, ($kappa approx 0.547,delta approx -0.45$), for the position of the quantum triple point (QTP) in the region $delta < 0$. We also show that there is no counterpart of such a QTP in the region $delta > 0$, where the two quasiclassical phase boundaries show instead an ``avoided crossing behavior, such that the entire region that contains the nonclassical paramagnetic phases is singly connected.
Frustration in quantum spin systems promote a variety of novel quantum phases. An important example is the frustrated spin-$1$ model on the square lattice with the nearest-neighbor bilinear ($J_1$) and biquadratic ($K_1$) interactions. We provide strong evidence for a nematic spin liquid phase in a range of $K_1/J_1$ near the SU(3)-symmetric point ($J_1 = K_1$), based on the linear flavor-wave theory and extensive density matrix renormalization group calculation. This phase displays no spin dipolar or quadrupolar order, preserves translational symmetry but spontaneously breaks $C_4$ lattice rotational symmetry, and possesses fluctuations peaked at the wavevector $(pi, 2pi/3)$. The spin excitation gap drops rapidly with system size and appears to be gapless, and the nematic order is attributed to the dominant $(pi, 2pi/3)$ fluctuations. Our results provide a novel mechanism for electronic nematic order and, more generally, open up a new avenue to explore frustration-induced exotic ground states.
The results of high frequency (60-315 GHz) studies of the ESR in CuGeO3 single crystals containing 0.9% of the Mn impurity are reported. The quantitative ESR line shape analysis shows that the low temperature (T<40 K) magnetic susceptibility of Cu2+ chains diverges with the critical exponent a=0.81 and therefore indicates an onset of a quantum critical (QC) regime. The scenario, in which disorder caused by the Mn impurity in the quantum spin chains in CuGeO3 may lead to the co-existence of the QC regime and the spin-Peierls dimerisation, is discussed. For the quantitative description of the temperature dependences of the line width and g-factor a model assuming the crossover from the high temperature semiclassical Nagata and Tazuke limit to the low temperature quantum case described by Oshikawa and Affleck theory is suggested.
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