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Register a new userDimensional crossover of spin chains in a transverse staggered field: an NMR study
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Heisenberg spin-1/2 chain materials are known to substantially alter their static and dynamic properties when experiencing an effective transverse staggered field originating from the varying local environment of the individual spins. We present a temperature-, angular- and field-dependent 29Si NMR study of the model compound BaCu2Si2O7. The experimental data are interpreted in terms of the divergent low-temperature transverse susceptibility, predicted by theory for spin chains in coexisting longitudinal and transverse staggered fields. Our analysis first employs a finite-temperature Density Matrix Renormalization Group (DMRG) study of the relevant one-dimensional Hamiltonian. Next we compare our numerical with the presently known analytical results. With an analysis based on crystal symmetries we show how the anisotropic contribution to the sample magnetization is experimentally accessible even below the ordering temperature, in spite of its competition with the collinear order parameter of the antiferromagnetic phase. The modification of static and dynamic properties of the system due to the presence of a local transverse staggered field (LTSF) acting on the one-dimensional spin array are argued to cause the unusual spin reorientation transitions observed in BaCu2Si2O7. On the basis of a Ginzburg-Landau type analysis, we discuss aspects of competing spin structures in the presence of magnetic order and the enhanced transverse susceptibility.
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Theoretical and experimental work have not provided a consistent picture of the phase diagram of the nearly ideal Ising ferromagnet LiHoF4 in a transverse magnetic field. Using a newly fabricated capacitive dilatometer, we have investigated the thermal expansion and magnetostriction of LiHoF4 in magnetic fields applied perpendicular to the Ising direction. Critical points for the ferromagnetic phase transition have been determined from both methods in the classical paramagnetic to ferromagnetic regime. Excellent agreement has been found with existing experimental data suggesting that, in this regime, the current theoretical calculations have not entirely captured the physics of this interesting model system.
Hyperuniform states of matter are characterized by anomalous suppression of long-wavelength density fluctuations. While most of interesting cases of disordered hyperuniformity are provided by complex many-body systems like liquids or amorphous solids, classical spin chains with certain long-range interactions have been shown to demonstrate the same phenomenon. It is well-known that the transverse field Ising model shows a quantum phase transition (QPT) at zero temperature. Under the quantum effects of a transverse magnetic field, classical hyperuniform spin chains are expected to lose their hyperuniformity. High-precision simulations of these cases are complicated because of the presence of highly nontrivial long-range interactions. We perform extensive analysis of these systems using density matrix renormalization group to study the possibilities of phase transitions and the mechanism by which they lose hyperuniformity. We discover first-order QPTs in the hyperuniform spin chains. An interesting feature of the phase transitions in these disordered hyperuniform spin chains is that, depending on the parameter values, the presence of transverse magnetic field may remarkably lead to increase in the order of the ground state as measured by the $tau$ order metric, even if hyperuniformity is lost. Therefore, it would be possible to design materials to target specific novel quantum behaviors in the presence of a transverse magnetic field. Our numerical investigations suggest that these spin chains can show no more than two QPTs. We further analyze the long-range interacting spin chains via the Jordan-Wigner mapping, showing that under the pairwise interacting approximation and a mean-field treatment, there can be at most two QPTs. Based on these numerical and theoretical explorations, we conjecture that these spin chains can show a maximum of two QPTs at zero temperature.
Rb-NMR study has been performed on the quasi-one dimensional competing spin chain Rb2Cu2Mo3O12 with ferromagnetic and antiferromagnetic exchange interactions on nearest neighboring and next nearest neighboring spins, respectively. The system changes from a gapped ground state at zero field to the gapless state at H_C simeq 2 T, where the existence of magnetic order below 1 K was demonstrated by a broadening of NMR spectrum, associated with a critical divergence of 1/T_1. In higher temperature region, 1/T_1 showed a power-law type temperature dependence, from which the field dependence of Luttinger parameter K was obtained and compared with theoretical calculations based on the spin nematic Tomonaga Luttinger Liquid (TLL) state.
We consider an alternating Heisenberg spin-$1/2$ antiferromagnetic-ferromagnetic ($AF-F$) chain with the space modulated dominant antiferromagnetic exchange and anisotropic ferromagnetic coupling (tetrameric spin-$1/2$ chain). The zero-temperature effect of a symmetry breaking transverse magnetic field on the model is studied numerically. It is found that the anisotropy effect on the ferromagnetic coupling induces two new gapped phases. We identified their orderings as a kind of the stripe-antiferromagnetic phase. As a result, the magnetic phase diagram of the tetrameric chain shows five gapped quantum phases and the system is characterized by four critical fields which mark quantum phase transitions in the ground state of the system with the changing transverse magnetic field. We have also exploited the well known bipartite entanglement (name as concurrence) and global entanglement tools to verify the occurrence of quantum phase transitions and the corresponding critical points.
By means of a numerical analysis using a non-Abelian symmetry realization of the density matrix renormalization group, we study the behavior of vector chirality correlations in isotropic frustrated chains of spin S=1 and S=1/2, subject to a strong external magnetic field. It is shown that the field induces a phase with spontaneously broken chiral symmetry, in line with earlier theoretical predictions. We present results on the field dependence of the order parameter and the critical exponents.
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