arXiv subjects

Kavita Mehlawat

Publications and source records attributed to Kavita Mehlawat.

11 recordsLinked to original sources

arXiv2026

Dominant Kitaev Interaction and Field-induced Quantum Disordered Phase in the Cobaltate Na$_2$Co$_2$TeO$_6$

The identification of quantum spin liquid phases in Kitaev candidate materials remains a major experimental challenge. Since most Kitaev candidates develop antiferromagnetic (AFM) order at low temperatures, currently there are great interest on the field-induced magnetic disordered phase in these compounds, that are distinct from (partially) polarized states. Recently, a cobaltate Na$_2$Co$_2$TeO$_6$ has emerged as a promising Kitaev candidate with high-spin $t^{5}_{2g}e^2_g$ configuration and spin-orbit entangled $J_{\rm eff} = 1/2$ honeycomb lattice system. There are intensive studies on field-induced magnetic states and phase transitions under in-plane magnetic fields. In this study, we propose an intermediate disordered phase induced by an out-of-plane field along the $c$-axis, through high-field magnetization and magnetocaloric effect measurements. To explain the high-field behavior of Na$_2$Co$_2$TeO$_6$, we develop an effective $K$-$J$-$Γ$-$Γ^{\prime}$ spin model featuring a dominant AFM Kitaev interaction. This framework uncovers an intermediate quantum spin liquid phase, establishing the material as a unique platform for exploring Kitaev physics and field-induced quantum-disordered states.

cond-mat.str-el↗

arXiv2019

Topological electronic structure and intrinsic magnetization in MnBi$_4$Te$_7$: a Bi$_2$Te$_3$-derivative with a periodic Mn sublattice

Combinations of non-trivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances material candidates are emerging. Yet, a compound with a band-inverted electronic structure and an intrinsic net magnetization remains unrealized. MnBi$_2$Te$_4$ is a candidate for the first antiferromagnetic topological insulator and the progenitor of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series. For $n$ = 1, we confirm a non-stoichiometric composition proximate to MnBi$_4$Te$_7$ and establish an antiferromagnetic state below 13 K followed by a state with net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topological surface state on the MnBi$_4$Te$_7$(0001) surface, analogous to the non-magnetic parent compound Bi$_2$Te$_3$. Our results render MnBi$_4$Te$_7$ as a band-inverted material with an intrinsic net magnetization and a complex magnetic phase diagram providing a versatile platform for the realization of different topological phases.

cond-mat.mtrl-sci↗

arXiv2019

Quantum Spin Liquid in a depleted triangular lattice Iridate K$_x$Ir$_y$O$_2$

We report discovery of a new iridate family K$_x$Ir$_y$O$_2$ with depleted triangular lattice planes made up of edge sharing IrO$_6$ octahedra separated by K planes. Such a material interpolates between the triangular and honeycomb lattices and is a new playground for Kitaev physics. The materials are Mott insulators with $y = 1 - x/4$. Physical property measurements for the $x \approx 0.85$ material are reported. Using magnetic susceptibility $χ$ versus temperature $T$ measurements we find $S_{eff} = 1/2$ moments interacting strongly with a Weiss temperature $θ\approx - 180$~K and no magnetic order or spin freezing down to $T = 1.8$~K\@. Heat capacity shows a broad maximum around $30$~K which is insensitive to magnetic fields and a $T$-linear low temperature behaviour with $γ\sim 10$~mJ/mol~K$^2$. These results are consistent with a gapless QSL state in K$_{0.85}$Ir$_{0.79}$O$_2$.

cond-mat.str-el↗

arXiv2018

Pressure-driven collapse of the relativistic electronic ground state in a honeycomb iridate

The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC). Such a localized electronic state carries an effective total angular momentum of $J_{eff}=1/2$. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these $J_{eff}=1/2$ moments are expected to be coupled through a special bond-dependent magnetic interaction, which is a necessary condition for the realization of a Kitaev quantum spin liquid. However, this relativistic electron picture is challenged by an alternate description, in which itinerant electrons are confined to a benzene-like hexagon, keeping the system insulating despite the delocalized nature of the electrons. In this quasi-molecular orbital (QMO) picture, the honeycomb iridates are an unlikely choice for a Kitaev spin liquid. Here we show that the honeycomb iridate Li2IrO3 is best described by a $J_{eff}=1/2$ state at ambient pressure, but crosses over into a QMO state under the application of small (~ 0.1 GPa) hydrostatic pressure. This result illustrates that the physics of iridates is extremely rich due to a delicate balance between electronic bandwidth, spin-orbit coupling, crystal field, and electron correlation.

cond-mat.str-el↗

arXiv2018

Dynamic Surface Modification due to Effusion of Na in Na$_2$IrO$_3$

The honeycomb lattice iridate Na$_2$IrO$_3$ shows frustrated magnetism and can potentially display Kitaev-like exchange interactions. Recently, it was shown that the electronic properties of the surface of crystalline Na$_2$IrO$_3$ can be tuned by Ar plasma treatment in a controlled manner leading to various phases of matter ranging from a fully gapped to a metallic surface, where the possibility of a charge-density wave (CDW) like transition is also expected. Here, through direct imaging with an atomic force microscope (AFM) in air, we show that the surface of crystalline Na$_2$IrO$_3$ evolves rapidly as elemental Na effuses out of the interleave planes to the surface and undergoes sublimation thereby disappearing from the surface gradually over time. Using conductive AFM we recorded a series of topographs and surface current maps simultaneously and found that the modification of the surface leads to change in the electronic properties in a dynamic fashion until the whole system reaches a dynamic equilibrium. These observations are important in the context of the exotic electronic and magnetic properties that the surface of Na$_2$IrO$_3$ displays.

cond-mat.mtrl-sci↗

arXiv2017

Density wave like transport anomalies in surface doped Na$_{2}$IrO$_3$

We report that the surface conductivity of Na$_{2}$IrO$_3$ crystal is extremely tunable by high energy Ar plasma etching and can be tuned from insulating to metallic with increasing etching time. Temperature dependent electrical transport for the metallic samples show signatures of first order phase transitions which are consistent with charge or spin density wave like phase transitions recently predicted theoretically. Additionally, grazing-incidence small-angle x-ray scattering (GISAXS) reveal that the room temperature surface structure of Na$_{2}$IrO$_3$ does not change after plasma etching.

cond-mat.str-el↗

arXiv2017

First order density wave like transitions in surface doped Na$_2$IrO$_3$

We demonstrate that the surface of the honeycomb lattice iridate Na$_2$IrO$_3$ is extremely tunable by plasma etching. We have succeeded in turning the surface of Na$_2$IrO$_3$ metallic by Ar plasma etching which leads to the removal of Na from the surface. The surface structure does not change in this process as revealed by grazing incidence small angle x-ray scattering (GISAXS). The sheet resistance $R_s$ can be reduced by several orders of magnitude by varying the etching duration. Temperature dependent $R_s(T)$ for the metallic samples show signatures of spin or charge density wave transitions with abrupt changes in $R_s$. Thermal hysteresis between cooling and warming measurements across the transition indicates a first order transition. For the most metallic sample $R_s(T)$ data at low temperatures follow a $T^2$ behaviour suggesting normal Fermi liquid behaviour.

cond-mat.str-el↗

arXiv2016

First-order magneto-structural transition in single crystals of the honeycomb lattice Ruthenate Li$_{2}$RuO$_3$

Li$_2$RuO$_3$ is known to crystallize in either $C2/m$ or $P2_1/m$ structures at room temperature. We report the first single crystal growth of Li$_2$RuO$_3$ and Na substituted crystals (Li$_{0.95}$Na$_{0.05}$)$_{2}$RuO$_3$ crystallizing in the $P2_1/m$ structure where a magneto-structural transition is observed at high temperatures. Using high temperature ($T \leq 1000$~K) magnetic susceptibility $χ$ measurements we study the magnetic anisotropy across the magneto-structural transition. Our results show for the first time that for Li$_2$RuO$_3$ the magnetic and structural transitions most likely occur at slightly different temperatures. The structural transition which is first order-like occurs first ($T \approx 570$~K) and drives the magnetic transition ($T \approx 540$~K). Rather surprisingly, just $5\%$ Na substitution for Li affects the magneto-structural transition in an interesting way. The first order transition temperature stays $\approx 540$~K, the magnetic anisotropy is reversed, and the Ru-Ru dimerization pattern changes from two short and four long Ru-Ru bonds per honeycomb in an armchair pattern for Li$_2$RuO$_3$ to four short and two long bonds per honeycomb in (Li$_{0.95}$Na$_{0.05}$)$_{2}$RuO$_3$ which can be viewed as two inter-penetrating armchair patterns.

cond-mat.str-el↗

arXiv2016

Raman Signatures of Strong Kitaev Exchange Correlations in (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ : Experiments and Theory

Inelastic light scattering studies on single crystals of (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ ($x = 0, 0.05$ and $0.15$) show a polarization independent broad band at $\sim $~2750 cm$^{-1}$ with a large band-width $\sim 1800$~cm$^{-1}$. For Na$_2$IrO$_3$ the broad band is seen for temperatures $ \leq 200$~K and persists inside the magnetically ordered state. For Li doped samples, the intensity of this mode increases, shifts to lower wave-numbers and persists to higher temperatures. Such a mode has recently been predicted (Knolle et.al.) as a signature of the Kitaev spin liquid. We assign the observation of the broad band to be a signature of strong Kitaev-exchange correlations. The fact that the broad band persists even inside the magnetically ordered state suggests that dynamically fluctuating moments survive even below $T_{N}$. This is further supported by our mean field calculations. The Raman response calculated in mean field theory shows that the broad band predicted for the spin liquid state survives in the magnetically ordered state near the zigzag-spin liquid phase boundary. A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter to be $J_K\approx 57$~meV.

cond-mat.str-el↗

arXiv2015

Fragile magnetic order in the honeycomb lattice Iridate Na$_{2}$IrO$_3$ revealed by magnetic impurity doping

We report the structure, magnetic, and thermal property measurements on single crystalline and polycrystalline samples of Ru substituted honeycomb lattice iridate Na$_2$Ir$_x$Ru$_{1-x}$O$_3$ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5). The evolution of magnetism in Na$_2$Ir$_x$Ru$_{1-x}$O$_3$ has been studied using dc and ac magnetic susceptibility, and heat capacity measurements. The parent compound Na$_2$IrO$_3$ is a spin-orbit driven Mott insulator with magnetic order of reduced moments below $T_N = 15$~K\@. In the Ru substituted samples the antiferromagnetic long range state is replaced by a spin glass like state even for the smallest substitution suggesting that the magnetic order in Na$_2$IrO$_3$ is extremely fragile. We argue that these behaviors indicate the importance of nearest-neighbor magnetic exchange in the parent Na$_2$IrO$_3$. Additionally, all samples show insulating electrical transport.

cond-mat.str-el↗

arXiv2015

Direct Evidence for Dominant Bond-directional Interactions in a Honeycomb Lattice Iridate Na2IrO3

Heisenberg interactions are ubiquitous in magnetic materials and have been prevailing in modeling and designing quantum magnets. Bond-directional interactions offer a novel alternative to Heisenberg exchange and provide the building blocks of the Kitaev model, which has a quantum spin liquid (QSL) as its exact ground state. Honeycomb iridates, A2IrO3 (A=Na,Li), offer potential realizations of the Kitaev model, and their reported magnetic behaviors may be interpreted within the Kitaev framework. However, the extent of their relevance to the Kitaev model remains unclear, as evidence for bond-directional interactions remains indirect or conjectural. Here, we present direct evidence for dominant bond-directional interactions in antiferromagnetic Na2IrO3 and show that they lead to strong magnetic frustration. Diffuse magnetic x-ray scattering reveals broken spin-rotational symmetry even above Neel temperature, with the three spin components exhibiting nano-scale correlations along distinct crystallographic directions. This spin-space and real-space entanglement directly manifests the bond-directional interactions, provides the missing link to Kitaev physics in honeycomb iridates, and establishes a new design strategy toward frustrated magnetism.

cond-mat.mtrl-sci↗