Quantum Magnets

Impurities in low-dimensional systems

Low-dimensional quantum magnets (LDQMs) are spin systems with a spin quantum number S = 1/2 or 1, and fewer nearest neighbors (nn) than in the conventional three-dimensional magnets (Fe, Ni, NiO, etc). For example, the number of nearest neighbors in a chain are 2, and 3 in a honeycomb lattice. Due to low-dimensionality (fewer neighbors) and small spin state (1/2 or 1), the magnetic ground state of LDQMs show pronounced quantum effects. These effects persist up high temperature showing their influence on the finite temperature properties. Thus, intriguing magnetic properties, including, fractional spin 1/2 excitations, fractionalization of spin, charge and orbital degrees of freedom, Tomonaga-Luttinger liquid state, and strong electron correlation induced charge localization, to name a few, are seen in these systems. The typical lattices that display low-dimensional behavior include chains, ladders, and 2D Vander Waals layered materials. The chains are strictly one-dimensional; planer magnets, on the other hand, are two-dimensional and ladders are intermediate between one- and two-dimensions. In the layered materials several interesting lattice geometries can be studied: triangular, Kagome, honeycomb, etc., for realizing quantum spin liquids where the spins do not show long-range ordering despite strong spin-spin correlations. While on one hand no long-range order exists but at the same time these strongly correlated spins exhibit a fair degree of quantum entanglement which has led to a significant interest in these materials.

Since magnetic excitations in these systems are "quantum confined" along the magnetic low-dimensional structure, new and unexpected properties often emerge when the quasi-particles associated with these excitation (for example, spin 1/2 spinon) encounters a defect or impurity potential in its path. In our ongoing research, we primarily focus on these new degrees of freedom that emerge when a controlled level of disorder, in the form of quantum impurities of defects, is introduced in these systems.

Below, we have taken the example of the spin chain compound SrCuO2. The first row of this figure explains the incongruent melting behavior, which is the manner of melting in these materials. Briefly, the solid decomposes into a solid-liquid mixture whose average composition remains as before but the solid and liquid will have highly different, temperature dependent, compositions. An image of the Floating-zone in the growth of SrCuO2.

The second row shows the mirror-like cleaved crystal facets of SrCuO2. The cleaved surface runs from the bottom till the top of the several cm long grown crystal boule.

The third row shows the inelastic neutron scattering data where two contrasting behaviors can be seen: In the Ni-doped chains the spinon excitation spectrum is gapped; in the Co-doped chains the spectrum remains gapless. In both cases the concentration of impurities is order of 1 in 100 in the chain.

Qunatum Magnetism in Nickaltes

Recently there has been an enormous uproar in the condensed matter community regarding the discovery of superconductivity in thin films of hole doped layered nickelates i.e., the R_1-xSr_xNiO₂ (R = La, Pr and Nd) family. Layered nickelates have the potential for exotic physics as they have an electronic and crystal structure analogous to HTSC cuprates. In this vein, the mixed valent Ruddlesden Popper nickelates – R_n+1Ni_nO_3n+1 (R = La, Pr and Nd) provide an ideal test bed to explore this conjecture. We have carried out detailed low temperature structural analysis of the R₄Ni₃O₁₀ (R = La, Pr and Nd) compounds and could successfully capture the metal-to-metal transition (MMT) in low temperature synchrotron PXRD, resistivity, specific heat, magnetic susceptibility, thermopower, thermal conductivity and zero field capacitive dilatometry experiments. The thermal expansion measurements were done in collaboration with Prof. Rudiger Klingeler from Kirchhoff Institute of Physics, Heidelberg University, Germany. We show that all three compounds crystallize with a monoclinic symmetry (space group P21/a; Z = 4), and undergo a MMT near T_MMT = 135 K (La), 156 K (Pr) and 160 K (Nd). Upon cooling below MMT, the lattice parameters in each case show a distinct anomaly at T_MMT, however, without any lowering of the lattice symmetry. Unambiguous signatures of MMT are also observed in magnetic and thermal measurements, suggesting that there is a strong coupling between the electronic, magnetic and structural degrees of freedom in these nickelates.