How a certain ground state of complex physical systems emerges, especially in two-dimensional materials, is a fundamental question in condensed-matter physics. A particularly interesting case is systems belonging to the class of XY Hamiltonian where the magnetic order parameter of conventional nature is unstable in two-dimensional materials leading to a Berezinskii−Kosterlitz−Thouless transition. Here, we report how the XXZ-type antiferromagnetic order of a magnetic van der Waals material, NiPS3, behaves upon reducing the thickness and ultimately becomes unstable in the monolayer limit. Our experimental data are consistent with the findings based on renormalization-group theory that at low temperatures a two-dimensional XXZ system behaves like a two-dimensional XY one, which cannot have a long-range order at finite temperatures. This work provides the experimental examination of the XY magnetism in the atomically thin limit and opens opportunities of exploiting these fundamental theorems of magnetism using magnetic van der Waals materials.

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Magnetic ordering in the two-dimensional limit has been one of the most important issues in condensed matter physics for the past several decades. The recent discovery of new magnetic van der Waals materials heralds a much-needed easy route for the studies of two-dimensional magnetism: the thickness dependence of the magnetic ordering has been examined by using Isingand XXZ-type magnetic van der Waals materials. Here, we investigated the magnetic ordering of MnPS3, a two-dimensional antiferromagnetic material of Heisenberg-type, by Raman spectroscopy from bulk all the way down to bilayer. The phonon modes that involve the vibrations of Mn ions exhibit characteristic changes as temperature gets lowered through the Néel temperature. In bulk MnPS3, the Raman peak at ~155 cm -1 becomes considerably broadened near the Néel temperature and upon further cooling is subsequently red-shifted. The measured peak positions and polarization dependences of the Raman spectra are in excellent agreement with our first-principles calculations. In few-layer MnPS3, the peak at ~155 cm -1 exhibits the characteristic red-shift at low temperatures down to the bilayer, indicating that the magnetic ordering is surprisingly stable at such a thin limit. Our work sheds light on the hitherto unexplored magnetic ordering in the Heisenberg-type antiferromagnetic systems in the atomic-layer limit. ∑ ∑where XY J and I J are spin-exchange energies on the basal plane and along the c-axis, respectively; j S α is the α (α = x, y, or z) component of total spin; and j and δ run through all lattice sites and all nearest-neighbors, respectively. All three fundamental models can be realized with the generic Hamiltonian: 0 XY J = for the Ising model, 0 I J = for the XY model, and XY I J J = for the Heisenberg model. According to the Mermin-Wagner theorem [4], no magnetic ordering is possible at any nonzero temperature in one-or two-dimensional isotropic Heisenberg models. On the other hand, 2D Ising systems can have magnetic ordering at finite temperatures according to Onsager [5].Transition metal phosphorus trisulfides (TMPS3) belong to a class of 2D van der Waals magnetic materials that can be exfoliated to atomically thin layers [6,7]. For transition metal elements like Fe, Ni, and Mn, the materials share the same crystal structures but the magnetic phase at low temperatures vary depending on the magnetic elements: Ising (Fe), XXZ (Ni), and

To date, only a few large-scale studies have measured the effect of dialysis modality on mortality in Asian populations. Here, we sought to compare survival between incident hemodialysis (HD) and peritoneal dialysis (PD) patients using the Korean Health Insurance Review & Assessment Service database. This enabled us to perform a population-based complete survey that included 32,280 incident dialysis patients and followed them for a median of 26.5 months. To reduce biases due to nonrandomization, we first matched 7049 patient pairs with similar propensity scores. Using the log-rank test, we found the mortality rate in PD patients was significantly higher than that in HD patients. Subsequent subgroup analyses indicated that in older patients (55 years and older), with the exception of the subgroup of patients with no comorbidities and the subgroup of patients with malignancy, PD was consistently associated with a higher mortality rate. In younger patients (under 55 years), regardless of the covariates, the survival rate of PD patients was comparable to that of HD patients. Thus, while the overall mortality rate was higher in incident PD patients, mortality rates of some incident PD and HD patients were comparable in Korea.