arXiv subjects

Ilija K. Nikolov

Publications and source records attributed to Ilija K. Nikolov.

9 recordsLinked to original sources

arXiv2026

Thermally melted quadrupolar order and intrinsically quantum phases in 5$d^1$ double perovskites

Spin-orbit-coupled $d^1$ double perovskites exhibit a rich interplay of spin, orbital, and quadrupolar degrees of freedom (DOF), giving rise to competing magnetic and multipolar phases. Although quantum mean-field theories predict many exotic phases, their stability against thermal fluctuations and reproducibility within a classical framework remain an open question. Here, we surpass the mean-field limitations by deploying large-scale classical Monte Carlo simulations on the projected $j=3/2$ manifold of the FCC lattice, allowing complex ordering patterns to emerge spontaneously without preassigned magnetic symmetries. Our thermodynamic mapping reveals that thermal fluctuations melt the intermediate-temperature quadrupolar phase over part of the phase diagram, while it survives intact elsewhere. Crucially, by systematically isolating the boundary between classical and quantum stability, we demonstrate that while the four-sublattice antiferromagnetic and ferromagnetic (FM) phases are robustly classical, the coplanar canted FM[110] state completely destabilizes. This identifies the FM[110] phase as an intrinsically quantum state born out of quantum fluctuations. Our results demonstrate that the dominant magnetic phases are robust within a classical description, where the essential physics of the system is captured by weakly entangled, short-range correlated DOF and highlight the role of thermal fluctuations in determining the stability of different types of magnetic and quadrupolar order.

cond-mat.str-el↗

arXiv2026

Emergence of Subdominant Quadrupolar Order in the 5$d^2$ Multipolar Mott Insulator Ba$_2$CaOsO$_6$

Relativistic $5d$ Mott insulators host a rich variety of exotic hidden multipolar order, whose microscopic nature remains heavily debated. A broken local point-symmetry (BLPS) phase followed by antiferromagnetism was previously observed in the doped compound Ba$_2$Na$_{0.1}$Ca$_{0.9}$OsO$_6$, where mixed Os valence places the system toward the $5d^2$ end of the $5d^1$-$5d^2$ regime. In contrast, pristine $5d^2$ Ba$_2$CaOsO$_6$ has shown a single octupolar transition with average cubic symmetry, raising the question of whether BLPS arises from chemical disorder in the doped systems. Here, we discover the BLPS phase in pristine Ba$_2$CaOsO$_6$ emerging at $T^* \approx 50$ K, precisely coincident with the octupolar transition. Beyond standard ${}^{43}$Ca nuclear magnetic resonance (NMR) spectral analysis, our spin-echo nutation measurements detect a finite electric field gradient, demonstrating spontaneous symmetry breaking despite the cubic average structure. While our NMR data support the previously identified octupolar order, the BLPS phase reveals a subdominant quadrupolar contribution, establishing that the hidden order in Ba$_2$CaOsO$_6$ is intrinsically multicomponent. Such mixed multipolar states lie beyond conventional theoretical descriptions of the $5d^2$ ground state. Our results establish local symmetry breaking as an intrinsic property of the clean $5d^2$ limit and provide a new constraint on microscopic theories of strongly spin-orbit-coupled Mott insulators.

cond-mat.str-el↗

arXiv2026

Comment on: Microscopic signatures of an imaginary charge density wave in a kagome metal

Recently, Suetsugu $\textit{at al.}$ reported microscopic signatures of an imaginary charge density wave (iCDW) and TRSB in CsV$_3$Sb$_5$ at $T^* \approx 120$ K, attributing nuclear quadrupole resonance (NQR) linewidths and asymmetric Zeeman-perturbed NQR (Zp-NQR) lineshapes to chiral loop currents in [arXiv:2605.05101; https://doi.org/10.1038/s41567-026-03339-8]. In this Comment, we demonstrate that these observations are not evidence of exotic physics but are instead fully explained by crystalline mosaicity and pre-transitional CDW fluctuations. Through exact diagonalization of the full nuclear-spin Hamiltonian, we show that the observed asymmetric lineshapes arise naturally from an angular mosaic spread in the Zp-NQR regime, completely reproducing the data without the need for internal magnetic fields. Furthermore, we reveal that the reported emergent local field ($h_{\text{loc}}$) is an artifact resulting from unjustified and unreported fitting constraints, and the use of an approximate perturbative model. Strikingly, we point out that this local-field model required for the proposed iCDW produces features that are absent in the data. Addressing the recent Reply by Suetsugu $\textit{at al.}$ [arXiv:2608.24927v1], we show that these core issues remain unresolved: the linewidth follows a standard Curie-Weiss law with no phase transition at $T^*$, and their definition of mosaicity overlooks well-documented strain-mediated lattice distortions. Most critically, when the full Hamiltonian is solved exactly and the artificial fitting constraints are removed, the inferred loop-current signature vanishes ($h_{\text{loc}} \to 0$). We therefore conclude that the claim of an iCDW state in in Ref. [arXiv:2605.05101; https://doi.org/10.1038/s41567-026-03339-8] is unsupported by the data, arising instead from incomplete spectral simulations and inadequate theoretical modeling.

cond-mat.str-el↗

arXiv2026

Competing lattice structures induced by Sn substitution in CsV$_3$Sb$_5$

Understanding the effect of chemical substitution on competing phases of kagome metals is crucial for disentangling the interplay between local structural distortions and electronic instabilities. In \cvs, Sn substitution strongly modifies the electronic phase diagram, yet the microscopic mechanism driving this remains unclear. Here, we combine $^{121}$Sb nuclear quadrupole resonance (NQR) measurements and density functional theory calculations to investigate the atomic-scale effects of Sn substitution in CsV$_3$Sb$_{5-x}$Sn$_x$. At low Sn concentrations, the observed satellite NQR peaks exhibit signatures of local structural distortion induced by Sn substitution, qualitatively consistent with our computational analysis. These impurity-induced features persist across the entire experimentally investigated doping range, up to $x$ = 0.65, and remain observable up to room temperature. For $x=1$, the fully doped idealized case, the estimated dynamical instabilities of the kagome lattice suggest the stabilization of two nearly energy-degenerate equilibrium structures characterized by V-trimers, distinguished by a zero- or $π$-phase shift between adjacent layers along the $c$-axis. Together, these results show that Sn substitution drives a complex interplay between local impurity-induced distortions and competing structural instabilities in vanadium-based kagome compounds.

cond-mat.str-el↗

arXiv2026

Orbital glass conceals missing magnetic entropy in a relativistic Mott insulator

Coupling between different degrees of freedom (DOF) in an electronic material leads to exotic phases of matter characterized by complex and competing order parameters as well as emergent excitations. Building a microscopic understanding of these order parameters and their mutual relationship is hindered by the fact that different orders often mask each others' response to conventional experimental probes. Here, we reveal how to disentangle responses from distinct orders that arise from the coupling between the spin and orbital DOF. Our method uses a phase sensitive technique that measures ground state properties by independently resolving interactions of different symmetries. This allows us to directly detect an orbital glass state caused by competing interactions in the $5d^1$ relativistic Mott insulator Ba$_2$NaOsO$_6$. We observe short-range orbital order up to 380 K and a dramatic increase of orbital dispersion near the magnetic phase transition. This orbital dispersion generates a directional ordering, $\textit{i.e.}$, it forms an orbital nematic state which breaks the rotational symmetry of the crystal. We establish that the orbital nematic state induces the magnetic ordering. The presence of this short-range orbital order well above the magnetic phase transition solves the long-standing puzzle of missing entropy in this material.

cond-mat.str-el↗

arXiv2025

Observation of ubiquitous charge correlations and hidden quantum critical point in hole-doped kagome superconductors

The interplay between superconductivity and charge-density wave (CDW) order, and its evolution with carrier density, is central to the physics of many quantum materials, notably high-$T_c$ cuprates and kagome metals. Hole-doped kagome compounds exhibit puzzling double-dome superconductivity and, as chemical substitution inevitably introduces quenched disorder, their properties remain poorly understood. Here, by leveraging the sensitivity of nuclear quadrupole resonance to local and static orderings, we uncover new features, primarily the incipient and fragmented CDW phases, in the charge landscape of CsV$_3$Sb$_{5-x}$Sn$_x$. Static CDW puddles are observed well above the transition temperature, a hallmark of pinning by defects. Their doping and temperature evolution indicate that, in the absence of disorder, the inverse Star-of-David $π$-shifted (ISD-$π$) CDW order would vanish near $x=0.12$, between the two superconducting domes. This critical doping represents a hidden quantum critical point. Nevertheless, the ISD-$π$ pattern persists well beyond previous reports, although its volume fraction is progressively reduced up to the critical doping at which it saturates. We establish that carrier doping promotes fragmentation of the ISD-$π$ order, whereas randomness preserves the ISD-$π$ patches.

cond-mat.str-el↗

arXiv2023

Multi-modal spectroscopy of order parameter distributions

We present a multi-modal spectroscopic paradigm that enables independent measurement of charge and spin degrees of freedom (DOF) in strongly correlated materials. This spin-based technique probes symmetry-specific Hamiltonian parameters by analyzing how the time delay between applied pulses ($τ$) affects the response. We demonstrate ways in which charge DOF that couple through the quadrupolar interaction (inversion symmetric) can be independently measured even in the presence of large magnetic noise (inversion asymmetric). The method quantifies both the strength of the interactions and its distribution (noise). We provide protocols to directly and independently measure the distribution of interaction strengths, even when the average value of the interaction is zero. By independently measuring distributions of different forms of disorder, this methodology can elucidate which microscopic symmetry drives a phase transition. We discuss potential applications to study complex phase transitions in strongly interacting quantum materials.

cond-mat.mes-hall↗

arXiv2022

Spin Squeezing as a Probe of Emergent Quantum Orders

Nuclear magnetic resonance (NMR) experiments can reveal local properties in materials, but are often limited by the low signal-to-noise ratio. Spin squeezed states have an improved resolution below the Heisenberg limit in one of the spin components, and have been extensively used to improve the sensitivity of atomic clocks, for example. Interacting and entangled spin ensembles with non-linear coupling are a natural candidate for implementing squeezing. Here, we propose measurement of the spin-squeezing parameter that itself can act as a local probe of emergent orders in quantum materials. In particular, we demonstrate how to investigate an anisotropic electric field gradient via its coupling to the nuclear quadrupole moment. While squeezed spin states are pure, the squeezing parameter can be estimated for both pure and mixed states. We evaluate the range of fields and temperatures for which a thermal-equilibrium state is sufficient to improve the resolution in an NMR experiment and probe relevant parameters of the quadrupole Hamiltonian, including its anisotropy.

quant-ph↗

arXiv2022

PULSEE: A software for the quantum simulation of an extensive set of magnetic resonance observables

We present an open-source software for the simulation of observables in magnetic resonance experiments, including nuclear magnetic/quadrupole resonance NMR/NQR and electron spin resonance (ESR), developed to assist experimental research in the design of new strategies for the investigation of fundamental quantum properties of materials, as inspired by magnetic resonance protocols that emerged in the context of quantum information science (QIS). The package introduced here enables the simulation of both standard NMR spectroscopic observables and the time-evolution of an interacting single-spin system subject to complex pulse sequences, i.e. quantum gates. The main purpose of this software is to facilitate in the development of much needed novel NMR-based probes of emergent quantum orders, which can be elusive to standard experimental probes. The software is based on a quantum mechanical description of nuclear spin dynamics in NMR/NQR experiments and has been widely tested on available theoretical and experimental results. Moreover, the structure of the software allows for basic experiments to easily be generalized to more sophisticated ones, as it includes all the libraries required for the numerical simulation of generic spin systems. In order to make the program easily accessible to a large user base, we developed a user-friendly graphical interface, Jupyter notebooks, and fully-detailed documentation. Lastly, we portray several examples of the execution of the code that illustrate the potential of a novel NMR paradigm, inspired by QIS, for efficient investigation of emergent phases in strongly correlated materials.

quant-ph↗