arXiv subjects

Suman Karmakar

Publications and source records attributed to Suman Karmakar.

3 recordsLinked to original sources

arXiv2024

Symmetry, Superposition and Fragmentation in Classical Spin Liquids: A General Framework and Applications to Square Kagome Magnets

Classical magnets exhibit exotic ground state properties such as spin liquids and fractionalization, promising a manifestation of superposition and projective symmetry construction in classical theory. While system-specific spin-ice or soft-spin models exist, a formal theory for general classical magnets remains elusive. Here, we introduce a generic symmetry group construction built from a vector field in a plaquette of classical spins, demonstrating how classical spins superpose in irreducible representations (irreps) of the symmetry group. The corresponding probability amplitudes serve as order parameters and local spins as fragmented excitations. The formalism offers a many-body vector field representation of diverse ground states, including spin liquids and fragmented phases described as degenerate ensembles of irreps. We apply the theory to a frustrated square Kagome lattice, where spin-ice or soft spin rules are inapt, to describe spin liquids and fragmented phases, all validated through irreps ensembles and unbiased Monte Carlo simulation. Our generic theory sheds light on previously unknown aspects of spin-liquid phases and fragmentation and broadens their applications to other branches of field theory.

cond-mat.str-el↗

arXiv2024

Crystal growth, magnetic, and magnetocaloric properties of J_eff = 1/2 quantum antiferromagnet CeCl_3

We report growth of high-quality single crystals of CeCl3 using a modified Bridgman Stockbarger method in an infrared image furnace. The grown crystals are characterized using single-crystal/powder X-ray diffraction, Laue X-ray diffraction, Raman spectroscopy, magnetization, and heat capacity probes. CeCl3 crystallizes in a hexagonal structure with a weak trigonal distortion. The Raman spectrum at 300 K showcases five, clearly resolvable, phonon modes at 106.8, 181.2, 189, 213, and 219.7 wavenumbers. The magnetic susceptibility show a large anisotropy with a broad peak in the perpendicular orientation, which is explained using the crystal field theory. The crystal field in CeCl3 splits the J = 5/2 manifold of Ce3+ into three Kramers doublets, resulting in a well-isolated ground state. In the specific heat, no magnetic ordering is detected above 2 K. However, in non-zero fields the low-temperature specific heat changes dramatically, showcasing a peak at 2.5 K under a moderate field of 30 kOe. The weak Ce-Ce exchange, large Ce moment in the crystal field ground state, and significant anisotropy are ingredients for realizing a high magnetocaloric effect. A maximum entropy change of 23 J/Kg/K is observed near 2.5 K in fields ranging from 50 kOe to 60 kOe. These values are comparable to some of the best Gd-based magnetocaloric materials, signifying the potential of CeCl3 as a magnetic coolant.

cond-mat.str-el↗

arXiv2023

Investigating the cause of crossover from charge/spin stripe insulator to correlated metallic phase in layered T' nickelates -- R$_4$Ni$_3$O$_8$

The $T^{\prime}$ infinite layered nickelates have recently garnered significant attention owing to the discovery of superconductivity in hole-doped RNiO$_2$ (R $=$ La, Pr, or Nd), which is the $n = \infty$ member of the series R$_{n+1}$Ni$_n$O$_{2n+2}$. Here, we investigate the $n = 3$ member, namely R$_4$Ni$_3$O$_8$ (R $=$ La, Pr, or Nd) of this family. The compound La$_4$Ni$_3$O$_8$ exhibits simultaneous charge/spin-stripe ordering at T$_N^\ast$ $=$ 105 K, which is also concomitant with the onset of metal-to-insulator (MIT) transition upon lowering the temperature below T$_N^\ast$. We investigate the conspicuous absence of this transition in the Pr and Nd analogues of La$_4$Ni$_3$O$_8$. To achieve this purpose, we synthesized solid-solutions of the form (La, Pr)$_4$Ni$_3$O$_8$ and (La, Nd)$_4$Ni$_3$O$_8$ and examined the behavior of T$_N^\ast$ as a function of the average R-site ionic radius ($r_{\overline{R}}$). We show that after an initial quasilinear decrease with decreasing $r_{\overline{R}}$, T$_N^\ast$ suddenly vanishes in the narrow range 1.134 $Å$ $\leq$ $r_{\overline{R}}$ $\leq$ 1.143 $Å$. In the same range, we observed the emergence of a new transition below T$^\ast$, whose onset temperature increases as $r_{\overline{R}}$ further decreases. We, therefore, argue that the sudden vanishing of charge/spin-stripe/MIT ordering upon decreasing $r_{\overline{R}}$ is due to the appearance of a new competing phase. The point $r_{\overline{R}}$ $\approx$ $r_c$, where T$_N^\ast$ vanishes and T$^\ast$ appears -- a quantum critical point -- should be investigated further. In this regard, Pr$_4$Ni$_3$O$_8$ and Pr-rich samples should be useful due to the weak magnetization response associated with the Pr-sublattice, as shown here.

cond-mat.str-el↗