arXiv subjects

Rico Pohle

Publications and source records attributed to Rico Pohle.

16 recordsLinked to original sources

arXiv2025

Kitaev Meets Affleck-Kennedy-Lieb-Tasaki: Competing Quantum Disorder in Spin-3/2 Honeycomb Systems

We investigate an S=3/2 quantum spin model on a two-dimensional honeycomb lattice that continuously interpolates between two paradigmatic quantum disordered states with distinct entanglement structures: the Kitaev quantum spin liquid and the Affleck-Kennedy-Lieb-Tasaki (AKLT) valence bond solid. Combining classical, semi-classical, and exact diagonalization approaches, we map out the ground-state phase diagram and elucidate the role of quantum fluctuations across the entire parameter range. While classical and semi-classical frameworks predict noncoplanar orders competing with a collinear N\'eel state, we find these phases to be fragile: once full quantum fluctuations are included, they melt into a quantum-entangled state characterized by suppressed spin correlations and enhanced entanglement entropy. Our findings highlight how competition between qualitatively different quantum disordered phases provides a fertile playground for unconventional phases emerging from their interplay and quantum fluctuations.

cond-mat.str-el↗

arXiv2025

Electron-phonon-coupled Langevin dynamics for strongly-correlated insulators

The Landau-Lifshitz-Gilbert (LLG) equations are widely used to study spin dynamics in Mott insulators. However, because energy damping is typically introduced phenomenologically, their validity for describing nonequilibrium processes and their connection to the microscopic origin of dissipation in real materials remains unclear. In this paper, we derive generalized stochastic LLG equations from first principles for spin-orbital coupled Mott insulators, explicitly incorporating the coupling between electronic degrees of freedom and lattice vibrations. Our approach is based on a path-integral formalism formulated along the Keldysh contour, which naturally accounts for dissipation and thermal fluctuations through interactions with a phonon bath and emergent stochastic noise. We benchmark our theoretical framework by numerically integrating the equations of motion for a two-orbital spin chain coupled to Einstein phonons. The resulting energy relaxation mimics realistic cooling dynamics, exhibits nontrivial transient behavior during thermalization, and accurately reproduces thermodynamic properties upon equilibration. We further demonstrate how electron-phonon coupling induces hybridization between electronic and phononic modes in the excitation spectrum and show that the conventional LLG equations are recovered as a limiting case of our microscopic theory. These results establish a robust and reliable framework for capturing dissipative spin dynamics in strongly correlated systems, both in and out of equilibrium.

cond-mat.str-el↗

arXiv2025

Nematic spin liquid in a spin-1 pyrochlore magnet and its realization in $\mathrm{NaCaNi}_2\mathrm{F}_7$

The search for spin liquids, magnetic phases which lie outside the Landau paradigm, remains one of the central challenges for modern condensed matter physics. For a long time, the prime candidates were thought to be spin-1/2 magnets, but recently examples have been identified in many spin-1 materials, including the pyrochlore NaCaNi$_2$F$_7$. Here we use numerical simulation to explore the spin liquid phases which arise in a minimal model of a spin-1 magnet on the pyrochlore lattice. We find this model supports seven distinct spin liquid phases, including one with nematic correlations. Through quantitative comparison with inelastic neutron scattering, we show that this nematic spin liquid provides a compelling scenario for NaCaNi$_2$F$_7$. These results suggest that the behaviour of spin liquids found in spin-1 pyrochlore magnets may be even richer than in materials with spin-1/2 moments.

cond-mat.str-el↗

arXiv2024

Classical $\mathbb{Z}_2$ spin liquid on the generalized four-color Kitaev model

While $U$(1) spin liquids have been extensively studied in both quantum and classical regimes, exact classical $\mathbb{Z}_2$ spin liquids arising from models with nearest-neighbor, bilinear spin interactions are still rare. In this Letter, we explore the four-color Kitaev model as a minimal model for stabilizing classical $\mathbb{Z}_2$ spin liquids across a broad family of tricoordinated lattices. By formulating a $\mathbb{Z}_2$ lattice gauge theory, we identify this spin liquid as being described by an emergent Gauss's law with effective charge-2 condensation, and deconfined fractionalized bond-charge excitations. We complement our findings with Monte Carlo simulations, revealing a crossover from a high-temperature paramagnet to a low-temperature liquid phase characterized by residual entropy, classical $\mathbb{Z}_2$ flux order, and diffuse spin structure factors.

cond-mat.str-el↗

arXiv2024

Eight-color chiral spin liquid in the $S=1$ bilinear-biquadratic model with Kitaev interactions

Multipolar spin systems provide a rich ground for the emergence of unexpected states of matter due to their enlarged spin degree of freedom. In this study, with a specific emphasis on $S=1$ magnets, we explore the interplay between spin nematic states and spin liquids. Based on the foundations laid in the prior work [R. Pohle et al., Phys. Rev. B 107, L140403 (2023)], we investigate the $S=1$ Kitaev model with bilinear-biquadratic interactions, which stabilizes, next to Kitaev spin liquid, spin nematic and triple-$q$ phases, also an exotic chiral spin liquid. Through a systematic reduction of the spin degree of freedom -- from $\mathbb{CP}^{2}$ to $\mathbb{CP}^{1}$ and ultimately to a discrete eight-color model -- we provide an intuitive understanding of the nature and origin of this chiral spin liquid. We find that the chiral spin liquid is characterized by an extensive ground-state degeneracy, bound by a residual entropy, extremely short-ranged correlations, a nonzero scalar spin chirality marked by $\mathbb{Z}_{2}$ flux order, and a gapped continuum of excitations. Our work contributes not only to the specific exploration of $S=1$ Kitaev magnets but also to the broader understanding of the importance of multipolar spin degree of freedom on the ground state and excitation properties in quantum magnets.

cond-mat.str-el↗

arXiv2023

Ground state of the $S$=1/2 pyrochlore Heisenberg antiferromagnet: A quantum spin liquid emergent from dimensional reduction

The quantum antiferromagnet on the pyrochlore lattice offers an archetypal frustrated system, which potentially realizes a quantum spin liquid characterized by the absence of standard spontaneous symmetry breaking even at zero temperature, unusually as an isotropic 3D system. Despite tremendous progress in the literature, however, the nature of the ground state of the fully quantum-mechanical spin Hamiltonian on the pyrochlore lattice still remains elusive. Here, we show that an unconventional type of quantum spin liquid is born out from the pyrochlore system after the self-organized dimensional reduction leading to confined states in 2D layers. This conclusion is obtained from state-of-the-art variational Monte Carlo (VMC) simulations at zero temperature. Quantum spin liquids triggered by the emergent dimensional reduction is an unexplored route of the spin-liquid formation. The dimensional reduction from 3D to 2D is a consequence of a conventional spontaneous symmetry breaking, while the resultant decoupling of layers enables the emergence of a 2D quantum spin liquid that is adiabatically disconnected from trivial product states and exhibits strong quantum entanglement. The stabilized quantum spin liquid exhibits an algebraic decay of correlations and vanishing excitation gap in the thermodynamic limit. The wave-function structure supports the fractionalization of the spin into spinons. This spin-liquid ground state persists in the presence of spin-orbit interactions, which expands the possibilities of realizing quantum spin liquids in real pyrochlore-structured materials.

cond-mat.str-el↗

arXiv2023

Gravitational wave analogues in spin nematics and cold atoms

Many large-scale phenomena in our Universe, such as gravitational waves, are challenging to reproduce in laboratory settings. However, parallels with condensed matter systems can provide alternative routes for experimental accessibility. Here we show how spin nematic phases provide a low-energy avenue for accessing the physics of linearized gravity, and in particular that their Goldstone modes are relativistically-dispersing massless spin-2 excitations, analogous to gravitational waves. We show at the level of the action that the low-energy effective field theory describing a spin nematic is in correspondence with that of linearized gravity. We then explicitly identify a microscopic model of a spin-1 magnet whose excitations in the low energy limit are relativistically dispersing, massless spin-2 Bosons which are in one-to-one correspondence with gravitational waves and, supported by simulation, outline a procedure for directly observing these analogue waves in a cold gas of $^{23}$Na atoms.

cond-mat.str-el↗

arXiv2023

Curie-law crossover in spin liquids

The Curie-Weiss law is widely used to estimate the strength of frustration in frustrated magnets. However, the Curie-Weiss law was originally derived as an estimate of magnetic correlations close to a mean-field phase transition, which -- by definition -- is absent in spin liquids. Instead, the susceptibility of spin liquids is known to undergo a Curie-law crossover between two magnetically disordered regimes. Here, we study the generic aspect of the Curie-law crossover by comparing a variety of frustrated spin models in two and three dimensions, using both classical Monte Carlo simulations and analytical Husimi tree calculations. Husimi tree calculations fit remarkably well the simulations for all temperatures and almost all lattices. We also propose a Husimi Ansatz for the reduced susceptibility $χT$, to be used in complement to the traditional Curie-Weiss fit in order to estimate the Curie-Weiss temperature $θ_{\rm cw}$. Applications to materials are discussed.

cond-mat.str-el↗

arXiv2023

Spin Nematics Meet Spin Liquids: Exotic Quantum Phases in the Spin-$1$ Bilinear-Biquadratic Model with Kitaev Interactions

Spin liquid crystals are magnetic analogs of liquid crystals, possessing properties of both liquids and solids, a typical example of which are spin nematics. Spin nematics share many features with spin liquids, and the interplay between them is a promising, but little explored, route to uncovering new phases of matter. Here, we address this question in the context of a spin-$1$ magnet on the honeycomb lattice, by considering a model with both biquadratic interactions, favouring spin-nematic states, and Kitaev-like interactions, supporting spin liquids. Accompanying these, where dipole and quadrupole moments compete, we find a plethora of exotic phases, including multiple-$q$ states with nonzero scalar spin chirality; a quasi-one-dimensional coplanar phase; a twisted conical phase; and a noncoplanar order state which gives way to a chiral spin liquid at finite temperature. The implication of these results for experiment is discussed.

cond-mat.str-el↗

arXiv2023

Variational Benchmarks for Quantum Many-Body Problems

The continued development of computational approaches to many-body ground-state problems in physics and chemistry calls for a consistent way to assess its overall progress. In this work, we introduce a metric of variational accuracy, the V-score, obtained from the variational energy and its variance. We provide an extensive curated dataset of variational calculations of many-body quantum systems, identifying cases where state-of-the-art numerical approaches show limited accuracy, and future algorithms or computational platforms, such as quantum computing, could provide improved accuracy. The V-score can be used as a metric to assess the progress of quantum variational methods toward a quantum advantage for ground-state problems, especially in regimes where classical verifiability is impossible.

quant-ph↗

arXiv2022

Low-Energy Excitations of Skyrmion Crystals in a Centrosymmetric Kondo-Lattice Magnet: Decoupled Spin-Charge Excitations and Nonreciprocity

We theoretically study spin and charge excitations of skyrmion crystals stabilized by conduction-electron-mediated magnetic interactions via spin-charge coupling in a centrosymmetric Kondo-lattice model by large-scale spin-dynamics simulations combined with the kernel polynomial method. We reveal clear segregation of spin and charge excitation channels and nonreciprocal nature of the spin excitations governed by the Fermi-surface geometry, which are unique to the skyrmion crystals in centrosymmetric itinerant hosts and can be a source of novel physical phenomena.

cond-mat.str-el↗

arXiv2022

Semi-classical simulation of spin-1 magnets

Theoretical studies of magnets have traditionally concentrated on either classical spins, or the extreme quantum limit of spin-1/2. However, magnets built of spin-1 moments are also intrinsically interesting, not least because they can support quadrupole, as well as dipole moments, on a single site. For this reason, spin-1 models have been extensively studied as prototypes for quadrupolar (spin-nematic) order in magnetic insulators, and Fe-based superconductors. At the same time, because of the presence of quadrupoles, the classical limit of a spin-1 moment is not an $O(3)$ vector, a fact which must be taken into account in describing their properties. In this Article we develop a method to simulate spin-1 magnets based on a $u(3)$ algebra which treats both dipole and quadrupole moments on equal footing. This approach is amenable to both classical and quantum calculations, and we develop the techniques needed to calculate thermodynamic properties through Monte Carlo simulations and classical low-temperature expansion, and dynamical properties, through "molecular dynamics" simulations and a multiple-boson expansion. As a case study, we present detailed analytic and numerical results for the thermodynamic properties of ferroquadrupolar order on the triangular lattice, and its associated dynamics. At low temperatures, we show that it is possible to "correct" for the effects of classical statistics in simulations, and extrapolate to the zero-temperature quantum results found in flavour-wave theory.

cond-mat.str-el↗

arXiv2018

Half moons are pinch points with dispersion

"Pinch points," singular features observed in (quasi-)elastic neutron scattering, are a widely discussed hallmark of spin liquids with an emergent gauge symmetry. Much less attention has been paid to "half moons," distinctive crescent patterns at finite energy, which have been observed in experiments on a number of pyrochlore magnets, and in a wide range of model calculations. Here we unify these two phenomena within a single framework, paying particular attention to the case of ordered, or field-saturated states, where pinch points and half moons can be found in bands of excitations above a gap. We find that half moons are nothing other than pinch points inscribed on a dispersing band. Molecular dynamics simulations of the kagome lattice antiferromagnet are used to explore how these bands evolve into the ground state and excitations of a classical spin liquid. We explicitly demonstrate that this theory can reproduce the pinch points and half moons observed in Nd$_2$Zr$_2$O$_7$.

cond-mat.str-el↗

arXiv2018

Symmetry and optical selection rules in graphene quantum dots

Graphene quantum dots (GQD's) have optical properties which are very different from those of an extended graphene sheet. In this Article we explore how the size, shape and edge--structure of a GQD affect its optical conductivity. Using representation theory, we derive optical selection rules for regular-shaped dots, starting from the symmetry properties of the current operator. We find that, where the x- and y-components of the current operator transform with the same irreducible representation (irrep) of the point group - for example in triangular or hexagonal GQD's - the optical conductivity is independent of the polarisation of the light. On the other hand, where these components transform with different irreps - for example in rectangular GQD's - the optical conductivity depends on the polarisation of light. We find that GQD's with non-commuting point-group operations - for example dots of rectangular shape - can be distinguished from GQD's with commuting point-group operations - for example dots of triangular or hexagonal shape - by using polarized light. We carry out explicit calculations of the optical conductivity of GQD's described by a simple tight--binding model and, for dots of intermediate size, \textcolor{blue}{($10 \lesssim L \lesssim 50\ \text{nm}$)} find an absorption peak in the low--frequency range of the spectrum which allows us to distinguish between dots with zigzag and armchair edges. We also clarify the one-dimensional nature of states at the van Hove singularity in graphene, providing a possible explanation for very high exciton-binding energies. Finally we discuss the role of atomic vacancies and shape asymmetry.

cond-mat.mtrl-sci↗

arXiv2017

How many spin liquids are there in Ca$_{10}$Cr$_7$O$_{28}$?

The search for novel phases of matter is a central theme of modern physics, with some of the most intriguing examples provided by the spin liquids found in magnets with competing, or "frustrated" interactions. Ca$_{10}$Cr$_7$O$_{28}$, a novel spin-$1/2$ magnet with a bilayer breathing-kagome lattice, has properties which differ from from any known spin liquid. However, understanding Ca$_{10}$Cr$_7$O$_{28}$ presents a significant challenge, because of its complex frustration. Here we use large-scale molecular-dynamics simulation to explore the origin of spin-liquid behaviour in Ca$_{10}$Cr$_7$O$_{28}$. We uncover qualitatively different behaviour on different timescales, and argue that ground state of Ca$_{10}$Cr$_7$O$_{28}$ is born out of a slowly-fluctuating "spiral spin liquid", while faster fluctuations echo the U(1) spin liquid found in the kagome antiferromagnet. These results provide a concrete scenario for spin-liquid behaviour in Ca$_{10}$Cr$_7$O$_{28}$, and highlight the possibility of spin liquids existing on multiple timescales.

cond-mat.str-el↗

arXiv2016

Reentrance of disorder in the anisotropic shuriken Ising model

For a material to order upon cooling is common sense. What is more seldom is for disorder to reappear at lower temperature, which is known as reentrant behavior. Such resurgence of disorder has been observed in a variety of systems, ranging from Rochelle salts to nematic phases in liquid crystals. Frustration is often a key ingredient for reentrance mechanisms. Here we shall study a frustrated model, namely the anisotropic shuriken lattice, which offers a natural setting to explore an extension of the notion of reentrance between magnetic disordered phases. By tuning the anisotropy of the lattice, we open a window in the phase diagram where magnetic disorder prevails down to zero temperature. In this region, the competition between multiple disordered ground states gives rise to a double crossover where both the low- and high-temperature regimes are less correlated than the intervening classical spin liquid. This reentrance of disorder is characterized by an entropy plateau, a multi-step Curie law crossover and a rather complex diffuse scattering in the static structure factor. Those results are confirmed by complementary numerical and analytical methods: Monte Carlo simulations, Husimi-tree calculations and an exact decoration-iteration transformation.

cond-mat.stat-mech↗