arXiv subjects

Vikram Nagarajan

Publications and source records attributed to Vikram Nagarajan.

13 recordsLinked to original sources

arXiv2026

Eclipsing Kitaev: off-diagonal exchange governs the correlated high-field phases of $β$-Li$_2$IrO$_3$

We report a high-field thermodynamic study of the hyperhoneycomb Kitaev material $β$-Li$_2$IrO$_3$, using magnetotropic susceptibility to resolve its low-temperature field-angle phase diagram across the principal crystallographic planes in magnetic fields up to $60$ T. Rather than evolving directly from the low-field incommensurate state into a polarized regime, the system exhibits a strongly direction-dependent sequence of correlated phases. Most notably, for fields in the $ac$-plane, we identify an additional high-field phase that is absent in the other principal planes and exists only within a restricted region of field-angle space. This phase structure is naturally explained by the competition between magnetic field and bond-directional exchange interactions. Using a symmetry-based description supported by microscopic calculations within the $J$-$K$-$Γ$ model, we show that off-diagonal $Γ$ exchange couples the ferromagnetic and staggered magnetic orders and thereby stabilizes the observed correlated high-field phases. The measured angular dependence of the critical fields is quantitatively captured by this theory, identifying $Γ$ exchange as the key interaction controlling the high-field response. These results clarify why the promise of a field-induced spin liquid -- the notion that suppressing magnetic order might reveal the underlying Kitaev physics -- remains unfulfilled in candidate materials: even when the Kitaev interaction is large, off-diagonal exchange stabilizes symmetry-constrained correlated phases that instead preempt the polarized state.

cond-mat.str-el↗

arXiv2026

Anisotropic Kitaev Spin Glass in Li$_{2}$Ru$_{x}$Ir$_{1-x}$O$_{3}$

Kitaev iridates have emerged as an important class of spin-orbit-entangled quantum materials in which bond-directional exchange interactions generate strong magnetic frustration and unconventional correlated states. Rather than realizing simple ordered magnets, members of the alpha,beta,gamma-Li2IrO3 family exhibit fragile and highly anisotropic magnetic behavior, including incommensurate counter-rotating order, strong field sensitivity, and pressure-driven electronic reconstruction. These phenomena place the iridates in close proximity to the Kitaev quantum spin liquid (QSL) regime, while simultaneously revealing the importance of competing interactions, lattice distortions, and spin-orbit-assisted hopping processes beyond the ideal Kitaev limit. Understanding how chemical substitution perturbs these competing interactions provides a route toward probing the underlying frustrated magnetic state. Here, we study single crystals of beta-Li2RuxIr1-xO3 with dilute Ru substitution, x<~10%. Through a combination of magnetometry, resonant elastic X-ray scattering, ac-heat capacity, and muon spin relaxation/rotation, we show that weak magnetic disorder continuously suppresses the incommensurate antiferromagnetic ground state without stabilizing an alternative long-range ordered phase. Instead, the system evolves into a bulk static spin glass characterized by slow relaxation, aging behavior, and frozen local magnetic fields. Despite the loss of long-range magnetic order, the glassy state retains substantial directional anisotropy without significant distortion to the honeycomb lattice, suggesting that disorder freezes spins within a still-active bond-directional exchange environment. In this picture, dilute Ru substitution provides a controlled pathway into an anisotropic Kitaev spin glass regime that preserves essential fingerprints of the underlying Kitaev exchange network.

cond-mat.str-el↗

arXiv2024

Gapped commensurate antiferromagnetic response in a strongly underdoped model cuprate superconductor

It is a distinct possibility that spin fluctuations are the pairing interactions in a wide range of unconventional superconductors. In the case of the high-transition-temperature (high-$T_c$) cuprates, in which superconductivity emerges upon doping an antiferromagnetic Mott-insulating state, spin correlations might furthermore drive unusual pseudogap phenomena. Here we use polarized and unpolarized magnetic neutron scattering to study the simple tetragonal cuprate $\mathrm{HgBa}_{2}\mathrm{CuO}_{4+δ}$ at very low doping ($T_c \approx 55$ K, hole concentration $p \approx 0.064$). In stark contrast to prior results for other underdoped cuprates, we find no evidence of incommensurate spin-density-wave, charge-spin stripe, or $q = 0$ magnetic order. Instead, the antiferromagnetic response in both the superconducting and pseudogap states is gapped below $Δ_\mathrm{AF} \approx 6$ meV, commensurate over a wide energy range, and disperses above about 55 meV. Given the documented model nature of $\mathrm{HgBa}_{2}\mathrm{CuO}_{4+δ}$, which exhibits high structural symmetry and minimal point disorder effects, we conclude that the observed behavior signifies the unmasked response of the quintessential $\mathrm{CuO}_{2}$ planes near the Mott-insulating state. These results for $\mathrm{HgBa}_{2}\mathrm{CuO}_{4+δ}$ can therefore be expected to serve as a benchmark for a refined theoretical understanding of the cuprates.

cond-mat.supr-con↗

arXiv2024

Collapse of susceptibility and non-trivial spin dynamics in the hyper-honeycomb magnet $β$-Li$_2$IrO$_3$ under high pressure

We present high-pressure (2 GPa) $^7$Li nuclear magnetic resonance (NMR) measurements on single crystals of the hyper-honeycomb Kitaev magnet $β$-Li$_2$IrO$_3$. The spectra show evidence for a structural phase transition around 200 K and a coexistence of phases, consistent with the results of other measurement techniques. The NMR spectra and line shift measurements demonstrate a strong suppression of the local magnetic susceptibility at high pressure. However, the spin-lattice relaxation ($1/T_1$) shows a clear power-law temperature dependence. This is inconsistent with a gapped singlet ground state of dimers and tetramers, as was previously proposed, and is instead evocative of a more exotic quantum spin liquid-like ground state.

cond-mat.str-el↗

arXiv2023

Low Resistance Ohmic Contact to P-type Monolayer WSe2

Advanced microelectronics in the future may require semiconducting channel materials beyond silicon. Two-dimensional (2D) semiconductors, characterized by their atomically thin thickness, hold immense promise for high-performance electronic devices at the nanometer scale with lower heat dissipation. One challenge for achieving high-performance 2D semiconductor field effect transistors (FET), especially for p-type materials, is the high electrical contact resistance present at the metal-semiconductor interface. In conventional bulk semiconductors, low resistance ohmic contact is realized through heavy substitutional doping with acceptor or donor impurities at the contact region. The strategy of substitutional doping, however, does not work for p-type 2D semiconductors such as monolayer tungsten diselenide (WSe$_2$).In this study, we developed highly efficient charge-transfer doping with WSe$_2$/$α$-RuCl$_3$ heterostructures to achieve low-resistance ohmic contact for p-type WSe$_2$ transistors. We show that a hole doping as high as 3$\times$10$^{13}$ cm$^{-2}$ can be achieved in the WSe$_2/α$-RuCl$_3$ heterostructure due to its type-III band alignment. It results in an Ohmic contact with resistance lower than 4 k Ohm $μ$m at the p-type monolayer WSe$_2$/metal junction. at room temperature. Using this low-resistance contact, we demonstrate high-performance p-type WSe$_2$ transistors with a saturation current of 35 $μ$A$\cdot$ $μ$m$^{-1}$ and an I$_{ON}$/I$_{OFF}$ ratio exceeding 10$^9$ It could enable future microelectronic devices based on 2D semiconductors and contribute to the extension of Moore's law.

cond-mat.mes-hall↗

arXiv2023

Electronic transport mechanisms in a thin crystal of the Kitaev candidate $α$-RuCl$_3$ probed through guarded high impedance measurements

$α$-RuCl$_3$ is considered to be the top candidate material for the experimental realization of the celebrated Kitaev model. It is however known that additional interactions beyond the Kitaev model trigger in $α$-RuCl$_3$, a long-range zigzag antiferromagnetic ground state. In this work, we investigate a nanoflake of $α$-RuCl$_3$ through guarded high impedance measurements aimed at reaching through electronic transport, the regime where the system turns into a zigzag antiferromagnet. We investigated a variety of temperatures (\SI{1.45}{\kelvin} - \SI{175}{\kelvin}) and out-of-plane magnetic fields ranging up to \SI{11}{\tesla}. We found a clear signature of a structural phase transition at $\approx 160$\,K as reported for thin crystals of $α$-RuCl$_3$, as well as a thermally activated behavior at temperatures above $\approx 30$\,K with a characteristic activation energy significantly smaller than the energy gap that we observe for $α$-RuCl$_3$ bulk crystals through our Angle Resolved Photoemission Spectroscopy (ARPES) experiments. Additionally we found that below $\approx 30$\,K, transport is ruled by Efros-Shklovskii (ES) VRH. These observations point to the presence of Coulomb impurities in our thin crystals. Most importantly, our data shows that below the magnetic ordering transition known for bulk $α$-RuCl$_3$ ($\approx 7$\,K), there is a clear deviation from VRH or thermal activation transport mechanisms. Our work demonstrates the possibility of reaching through specialized high impedance measurements, the thrilling ground states predicted for $α$-RuCl$_3$ at low temperatures in the frame of the Kitaev model, and informs about the transport mechanisms in this material in a wide temperature range as well as on important characteristic quantities such as the localization length of the impurities in a thin $α$-RuCl$_3$ crystal.

cond-mat.str-el↗

arXiv2022

Nonvolatile Electric Field Control of Thermal Magnons in the Absence of an Applied Magnetic Field

Spin transport through magnetic insulators has been demonstrated in a variety of materials and is an emerging pathway for next-generation spin-based computing. To modulate spin transport in these systems, one typically applies a sufficiently strong magnetic field to allow for deterministic control of magnetic order. Here, we make use of the well-known multiferroic magnetoelectric, BiFeO3, to demonstrate non-volatile, hysteretic, electric-field control of thermally excited magnon current in the absence of an applied magnetic field. These findings are an important step toward magnon-based devices, where electric-field-only control is highly desirable.

cond-mat.mtrl-sci↗

arXiv2021

Magnon-spinon dichotomy in the Kitaev hyperhoneycomb $β$-Li$_2$IrO$_3$

The family of edge-sharing tri-coordinated iridates and ruthenates has emerged in recent years as a major platform for Kitaev spin liquid physics, where spins fractionalize into emergent magnetic fluxes and Majorana fermions with Dirac-like dispersions. While such exotic states are usually pre-empted by long-range magnetic order at low temperatures, signatures of Majorana fermions with long coherent times have been predicted to manifest at intermediate and higher energy scales, similar to the observation of spinons in quasi-1D spin chains. Here we present a Resonant Inelastic X-ray Scattering study of the magnetic excitations of the hyperhoneycomb iridate $β$-Li$_2$IrO$_3$ under a magnetic field with a record-high-resolution spectrometer. At low-temperatures, dispersing spin waves can be resolved around the predicted intertwined incommensurate spiral and field-induced zigzag orders, whose excitation energy reaches a maximum of 16meV. A 2T magnetic field softens the dispersion around ${\bf Q}=0$. The behavior of the spin waves under magnetic field is consistent with our semiclassical calculations for the ground state and the dynamical spin structure factor, which further predicts that the ensued intertwined uniform states remain robust up to very high fields (100 T). Most saliently, the low-energy magnon-like mode is superimposed by a broad continuum of excitations, centered around 35meV and extending up to 100meV. This high-energy continuum survives up to at least 300K -- well above the ordering temperature of 38K -- and gives evidence for pairs of long-lived Majorana fermions of the proximate Kitaev spin liquid.

cond-mat.str-el↗

arXiv2020

Evidence for freezing of charge degrees of freedom across a critical point in CeCoIn$_5$

The presence of a quantum critical point separating two distinct zero-temperature phases is thought to underlie the `strange' metal state of many high-temperature superconductors. The nature of this quantum critical point, as well as a description of the resulting strange metal, are central open problems in condensed matter physics. In large part, the controversy stems from the lack of a clear broken symmetry to characterize the critical phase transition, and this challenge is no clearer than in the example of the unconventional superconductor CeCoIn$_5$. Through Hall effect and Fermi surface measurements of CeCoIn$_5$, in comparison to ab initio calculations, we find evidence for a critical point that connects two Fermi surfaces with different volumes without apparent symmetry-breaking, indicating the presence of a transition that involves an abrupt localization of one sector of the charge degrees of freedom. We present a model for the anomalous electrical Hall resistivity of this material based on the conductivity of valence charge fluctuations.

cond-mat.str-el↗

arXiv2020

Magnetoresistance scaling, disorder, `hot spots' and the origin of $T$-linear resistivity in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

The scaling of $H$-linear magnetoresistance in field and temperature was measured in under-doped (x = 0.19) and optimally-doped (x=0.31)~BaFe$_2$(As$_{1-x}$P$_x$)$_2$. We analyze the data based on an orbital model in the presence of strongly anisotropic quasiparticle spectra and scattering time due to antiferromagnetism. The magnetoresistance is dominated by the properties of small regions of the Fermi surface called `hot spots' where antiferromagnetic excitations induce a large quasiparticle scattering rate. Approximate temperature-magnetic field scaling relations are derived and shown to be consistent with the experimental data. We argue that these results link the origin of linear-in-temperature resistivity to hot spots arising from an antiferromagnetic critical point, and magnetoresistance measurements provide a route to quantify this link.

cond-mat.str-el↗

arXiv2020

Competition between magnetic order and charge localization in Na$_2$IrO$_3$ thin crystal devices

Spin orbit assisted Mott insulators such as sodium iridate (Na$_2$IrO$_3$) have been an important subject of study in the recent years. In these materials, the interplay of electronic correlations, spin-orbit coupling, crystal field effects and a honeycomb arrangement of ions bring exciting ground states, predicted in the frame of the Kitaev model. The insulating character of Na$_2$IrO$_3$ has hampered its integration to an electronic device, desirable for applications, such as the manipulation of quasiparticles interesting for topological quantum computing. Here we show through electronic transport measurements supported by Angle Resolved Photoemission Spectroscopy (ARPES) experiments, that electronic transport in Na$_2$IrO$_3$ is ruled by variable range hopping and it is strongly dependent on the magnetic ordering transition known for bulk Na$_2$IrO$_3$, as well as on external electric fields. Electronic transport measurements allow us to deduce a value for the localization length and the density of states in our Na$_2$IrO$_3$ thin crystals devices, offering an alternative approach to study insulating layered materials.

cond-mat.mes-hall↗

arXiv2019

Hidden spin-orbital order in the Kitaev hyperhoneycomb $β$-Li$_2$IrO$_3$

We report the existence of a phase transition at high temperature in the 3D Kitaev candidate material, $β$-Li$_2$IrO$_3$. We show that the transition is bulk, intrinsic and orders a tiny magnetic moment with a spatially anisotropic saturation moment. We show that even though this transition is global, it does not freeze the local Ir moments, which order at much lower temperatures into an incommensurate state. Rather, the ordered moment has an orbital origin that is coupled to spin correlations, likely of a Kitaev origin. The separate ordering of spin-correlated orbital moments and of local Ir moments reveals a novel way in which magnetic frustration in Kitaev systems can lead to coexisting magnetic states.

cond-mat.str-el↗

arXiv2017

Percolative nature of the dc paraconductivity in the cuprate superconductors

We present an investigation of the planar direct-current (dc) paraconductivity of the model cuprate material HgBa$_2$CuO$_{4+δ}$ in the underdoped part of the phase diagram. The simple quadratic temperature-dependence of the Fermi-liquid normal-state resistivity enables us to extract the paraconductivity above the macroscopic $T_c$ with great accuracy. The paraconductivity exhibits unusual exponential temperature dependence, with a characteristic temperature scale that is distinct from $T_c$. In the entire temperature range where it is discernable, the paraconductivity is quantitatively explained by a simple superconducting percolation model, which implies that underlying gap disorder dominates the emergence of superconductivity.

cond-mat.supr-con↗