Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userFirst-order Neel-cVBS transition in a model square lattice $S=1$ antiferromagnet
178
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study the Neel to four-fold columnar valence bond solid (cVBS) quantum phase transition in a sign free $S=1$ square lattice model. This is the same kind of transition that for $S=1/2$ has been argued to realize the prototypical deconfined critical point. Extensive numerical simulations of the square lattice $S=1/2$ Neel-VBS transition have found consistency with the DCP scenario with no direct evidence for first order behavior. In contrast to the $S=1/2$ case, in our quantum Monte Carlo simulations for the $S=1$ model, we present unambiguous evidence for a direct conventional first-order quantum phase transition. Classic signs for a first order transition demonstrating co-existence including double peaked histograms and switching behavior are observed. The sharp contrast from the $S=1/2$ case is remarkable, and is a striking demonstration of the role of the size of the quantum spin in the phase diagram of two dimensional lattice models.
rate research
Read More
The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor $J_1$ and next-nearest-neighbor $J_2$ interactions ($J_2/J_1 approx 0.5$). The double perovskites Sr$_2$CuTeO$_6$ and Sr$_2$CuWO$_6$ are isostructural spin-1/2 square-lattice antiferromagnets with Neel ($J_1$ dominates) and columnar ($J_2$ dominates) magnetic order, respectively. Here we characterize the full isostructural solid solution series Sr$_2$Cu(Te$_{1-x}$W$_x$)O$_6$ ($0 leq x leq 1$) tunable from Neel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the $x$ = 0.5 phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of $x approx$ 0.1-0.6. This coincides with significant $T$-linear terms in the low-temperature specific heat. However, density functional theory calculations predict most of the materials are not in the highly frustrated $J_2/J_1 approx 0.5$ region square-lattice Heisenberg model. Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in $x$ = 0.5.
Static and dynamic properties of the quasi-two-dimensional antiferromagnet K$_2$V$_3$O$_8$ have been investigated by $^{51}$V-NMR experiments on nonmagnetic V$^{5+}$ sites. Above the structural transition temperature $T_{rm{S}}$ = 115 K, NMR spectra are fully compatible with the $P4bm$ space group symmetry. The formation of superstructure below $T_{rm{S}}$ causes splitting of the NMR lines, which get broadened at lower temperatures so that individual peaks are not well resolved. Evolution of NMR spectra with magnetic field along $c$-axis below the magnetic transition temperature $T_{rm{N}} sim 4$ K is qualitatively consistent with a simple N{e}el order and a spin flop transition. However, broad feature of the spectra does not rule out possible incommensurate spin structure. The spin-lattice relaxation rate $1/T_1$ below $T_{rm{N}}$ shows huge enhancement for a certain range of magnetic field, which is independent of temperature and attributed to cross relaxation due to anomalously large nuclear spin-spin coupling between V$^{5+}$ and magnetic V$^{4+}$ sites. The results indicate strong gapless spin fluctuations, which could arise from incommesurate orders or complex spin textures.
We have explored the magnetic excitation spectrum of the S=1/2 square lattice Heisenberg antiferromagnet, K2V3O8 using both triple-axis and time-of-flight inelastic neutron scattering. The long-wavelength spin waves are consistent with the previously determined Hamiltonian for this material. A small energy gap of 72+/-9 micro-eV is observed at the antiferromagnetic zone center and the near-neighbor exchange constant is determined to be 1.08+/-0.03 meV. A finite ferromagnetic interplanar coupling is observed along the crystallographic c-axis with a magnitude of Jc=-0.0036+/-0.006 meV. However, upon approaching the zone boundary, the observed excitation spectrum deviates significantly from the expectation of linear spin wave theory resulting in split modes at the (pi/2,pi/2) zone boundary point. The effects of magnon-phonon interaction, orbital degrees of freedom, multimagnon scattering, and dilution/site randomness are considered in the context of the mode splitting. Unfortunately, no fully satisfactory explanation of this phenomenon is found and further theoretical and experimental work is needed.
We successfully synthesize single crystals of the verdazyl radical $alpha$-2,3,5-Cl$_3$-V. $Ab$ $initio$ molecular orbital calculations indicate that the two dominant antiferromagnetic interactions, $J_{rm{1}}$ and $J_{rm{2}}$ ($alpha =J_{rm{2}}/J_{rm{1}}simeq 0.56$), form an $S$ = 1/2 distorted square lattice. We explain the magnetic properties based on the $S$ = 1/2 square lattice Heisenberg antiferromagnet using the quantum Monte Carlo method, and examine the effects of the lattice distortion and the interplane interaction contribution. In the low-temperature regions below 6.4 K, we observe anisotropic magnetic behavior accompanied by a phase transition to a magnetically ordered state. The electron spin resonance signals exhibit anisotropic behavior in the temperature dependence of the resonance field and the linewidth. We explain the frequency dependence of the resonance fields in the ordered phase using a mean-field approximation with out-of-plane easy-axis anisotropy, which causes a spin-flop phase transition at approximately 0.4 T for the field perpendicular to the plane. Furthermore, the anisotropic dipole field provides supporting information regarding the presence of the easy-axis anisotropy. These results demonstrate that the lattice distortion, anisotropy, and interplane interaction of this model are sufficiently small that they do not affect the intrinsic behavior of the $S$ = 1 / 2 square lattice Heisenberg antiferromagnet.
Recently, several putative quantum spin liquid (QSL) states were discovered in ${tilde S} = 1/2$ rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL state towards long-range magnetic order. Here, we introduce the Ce-based TLAF KCeS$_2$ and show via low-temperature specific heat and $mu$SR investigations that it yields magnetic order below $T_{mathrm N} = 0.38$ K despite the same delafossite structure. We identify a well separated ${tilde S} = 1/2$ ground state for KCeS$_2$ from inelastic neutron scattering and embedded-cluster quantum chemical calculations. Magnetization and electron spin resonance measurements on single crystals indicate a strong easy-plane $g$~factor anisotropy, in agreement with the ab initio calculations. Finally, our specific-heat studies reveal an in-plane anisotropy of the magnetic field-temperature phase diagram which may indicate anisotropic magnetic interactions in KCeS$_2$.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
