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Coherent cellular dynamical mean-field theory: a real-space quantum embedding approach to disorder in strongly correlated electron...

Coherent cellular dynamical mean-field theory: a real-space quantum embedding approach to disorder in strongly correlated electron systems

Patrick Tscheppe, Marcel Klett, Henri Menke, Sabine Andergassen, Niklas Enderlein, Philipp Hansmann, Thomas Sch\"afer
https://arxiv.org/abs/2503.10364 https://arxiv.org/pdf/2503.10364 https://arxiv.org/html/2503.10364

arXiv:2503.10364v1 Announce Type: new
Abstract: We formulate a quantum embedding algorithm in real-space for the simultaneous theoretical treatment of nonlocal electronic correlations and disorder, the coherent cellular dynamical mean-field theory (C-CDMFT). This algorithm combines the molecular coherent potential approximation with the cellular dynamical mean-field theory. After a pedagogical introduction to quantum embedding theories for disordered and interacting electron systems, and a detailed discussion of its work flow, we present first results from C-CDMFT for the half-filled two-dimensional Anderson-Hubbard model on a square lattice: (i) the analysis of its Mott metal-insulator transition as a function of disorder strength, and (ii) the impact of different types of disorder on its magnetic phase diagram. For the latter, by means of a "disorder diagnostics", we are able to precisely identify the contributions of different disorder configurations to the system's magnetic response.

Dynamical response theory of interacting Majorana fermions and its application to generic Kitaev quantum spin liquids in a fieldPeng Rao,...

Dynamical response theory of interacting Majorana fermions and its application to generic Kitaev quantum spin liquids in a field

Peng Rao, Roderich Moessner, Johannes Knolle
https://arxiv.org/abs/2503.10330 https://arxiv.org/pdf/2503.10330 https://arxiv.org/html/2503.10330

arXiv:2503.10330v1 Announce Type: new
Abstract: Motivated by the appearance of Majorana fermions in a broad range of correlated and topological electronic systems, we develop a general method to compute the dynamical response of interacting Majorana fermions in the random-phase approximation (RPA). This can be applied self-consistently on top of Majorana mean-field theory (MFT) backgrounds, thereby in particular providing a powerful tool to analyse $\textit{generic}$ behaviour in the vicinity of (various heavily studied) exactly soluble models. Prime examples are quantum spin liquids (QSL) with emergent Majorana excitations, with the celebrated exact solution of Kitaev. We employ the RPA to study in considerable detail phase structure and dynamics of the extended Kitaev honeycomb $KJ\Gamma$-model, with and without an applied field. First, we benchmark our method with Kitaev's exactly soluble model, finding a remarkable agreement. The interactions between Majorana fermions even turn out to mimic the effect of local $\mathbb{Z}_2$ flux excitations, which we explain analytically. Second, we show how small non-Kitaev couplings $J$ and $\Gamma$ induce Majorana bound states, resulting in sharp features in the dynamical structure factor in the presence of fractionalisation: such 'spinon excitons' naturally appear, and can coexist and interact with the broad Majorana continuum. Third, for increasing couplings or field, our theory predicts instabilities of the KQSL triggered by the condensation of the sharp modes. From the high symmetry momenta of the condensation we can deduce which magnetically ordered phases surround the KQSL, in good agreement with previous finite-size numerics. We discuss implications for experiments and the broad range of applicability of our method to other QSL and Majorana systems.

Observation of the dimer-singlet phase in the one-dimensional S = 1/2 Heisenberg antiferromagnet Cu(Ampy)ClBr (Ampy= C6H8N2 =...

Observation of the dimer-singlet phase in the one-dimensional S = 1/2 Heisenberg antiferromagnet Cu(Ampy)ClBr (Ampy= C6H8N2 = 2-(Aminomethyl)pyridine)

Saikat Nandi, Monika Jawale, Sanjay Bachhar, Rahul Kumar, Marlis Schuller, Rabindranath Bag, J. Wilkinson, J\"org Sichelschmidt, A. Sundaresan, Sara Haravifard, N. B\"uttgen, A. V. Mahajan
https://arxiv.org/abs/2503.10290 https://arxiv.org/pdf/2503.10290 https://arxiv.org/html/2503.10290

arXiv:2503.10290v1 Announce Type: new
Abstract: Spin-1/2 Heisenberg antiferromagnetic frustrated spin chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr (Ampy= C6H8N2 = 2-(Aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, 1H nuclear magnetic resonance (NMR), electron spin resonance (ESR), and muon spin relaxation (muSR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu2+ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu2+ spins as reflected by a Curie-Weiss temperature of about -9 K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the 1H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field muSR evidences lack of any static magnetism. We attribute these experimental results to the stabilization of a dimer-singlet phase in the presence of a next-near neighbor interaction and of a randomness in the exchange coupling driven by Cl/Br mixing.

Formulation of spin Nernst effect for spin-nonconserving insulating magnetsShinnosuke Koyama, Joji Nasuhttps://arxiv.org/abs/2503.10281...

Formulation of spin Nernst effect for spin-nonconserving insulating magnets

Shinnosuke Koyama, Joji Nasu
https://arxiv.org/abs/2503.10281 https://arxiv.org/pdf/2503.10281 https://arxiv.org/html/2503.10281

arXiv:2503.10281v1 Announce Type: new
Abstract: The spin Nernst effect, an antisymmetric response of a spin current to a temperature gradient, has attracted attention as spin transport phenomenon arising from the topologically nontrivial band structure of carriers. This effect can occur not only in itinerant electron systems but also in localized electron systems that emerge due to electronic correlations. In such systems, elementary excitations behaving as bosons govern spin transport, and magnetic interactions originating from spin-orbit coupling can induce a topologically nontrivial band structure. However, such magnetic interactions can potentially break spin conservation, thereby preventing conventional spin currents from being conserved. In this study, starting from a general localized electron model, we formulate the spin Nernst effect in terms of a conserved spin current that remains applicable even in spin-nonconserving systems. To address the torque term appearing in the conserved spin current, we adopt semiclassical theory and derive an expression for the spin Nernst coefficient for bosonic systems. This coefficient consists of two terms originating from the Berry curvature and the quantum metric, which are distinctly different from the spin Berry curvature obtained in an approach that neglects the torque term. We apply our framework to two specific quantum spin models, the Kitaev-Heisenberg and Shastry-Sutherland models, and calculate the temperature dependence of the spin Nernst coefficient. We find that this quantity significantly differs from that obtained by neglecting the torque term in both models. Furthermore, we clarify that the impact of the torque term on the spin Nernst effect strongly depends on the model parameters, suggesting that our formulation based on the conserved spin current is essential for understanding this effect in insulating magnets.

Quantum Spin Liquid phases in Kitaev MaterialsPo-Hao Chou, Chung-Yu Mou, Chung-Hou Chung, Sungkit Yiphttps://arxiv.org/abs/2503.10226...

Quantum Spin Liquid phases in Kitaev Materials

Po-Hao Chou, Chung-Yu Mou, Chung-Hou Chung, Sungkit Yip
https://arxiv.org/abs/2503.10226 https://arxiv.org/pdf/2503.10226 https://arxiv.org/html/2503.10226

arXiv:2503.10226v1 Announce Type: new
Abstract: We develop a gauge-invariant renormalized mean-field theory (RMFT) method, to reliably find the quantum spin liquid (QSL) states and their field response in realistic Kitaev materials. Remarkably, while our RMFT reproduces previous results based on more complicated numerical methods, it also predicts several new stable QSL states. In particular, since Kitaev spin liquid(KSL) is no longer a saddle point solution, a new exotic 2-cone state distinct from the KSL, is found to describe the experimental observations well, and hence the candidate state be realized in the Kitaev material, {\alpha}-RuCl3. We further explore the mechanism for the suppression of the observed thermal Hall conductivity at low temperatures within the fermionic framework, and show the theoretical polarangle dependence of fermionic gap that can distinguish the found 2-cone state from the KSL state in further experiment.

Oscillate and Renormalize: Fast Phonons Reshape the Kondo Effect in Flat Band SystemsLiam L. H. Lau, Andreas Gleis, Daniel Kaplan, Premala...

Oscillate and Renormalize: Fast Phonons Reshape the Kondo Effect in Flat Band Systems

Liam L. H. Lau, Andreas Gleis, Daniel Kaplan, Premala Chandra, Piers Coleman
https://arxiv.org/abs/2503.09989 https://arxiv.org/pdf/2503.09989 https://arxiv.org/html/2503.09989

arXiv:2503.09989v1 Announce Type: new
Abstract: We examine the interplay between electron correlations and phonons in an Anderson-Holstein impurity model with an Einstein phonon. When the phonons are slow compared to charge fluctuations (frequency $\omega_0 \ll U/2$, the onsite Coulomb scale $U/2$), we demonstrate analytically that the expected phonon-mediated reduction of interactions is completely suppressed, even in the strong coupling regime. This suppression arises from the oscillator's inability to respond to rapid charge fluctuations, manifested as a compensation effect between the polaronic cloud and the excited-state phonons associated with valence fluctuations. We identify a novel frozen mixed valence phase, above a threshold dimensionless electron-phonon coupling $\alpha^*$ when the phonons are slow, where the static phonon cloud locks the impurity into specific valence configurations, potentially explaining the puzzling coexistence of mixed valence behavior and insulating properties in materials like rust. Conversely, when the phonon is fast ($\omega_0 \gtrsim U/2$), the system exhibits conventional polaronic behavior with renormalized onsite interactions effectively $U_{\text{eff}}$ due to phonon mediated attraction, with additional satellite features in the local spectral function due to phonon excitations. Using numerical renormalization group (NRG) calculations, a fully dynamic renormalization technique, we confirm these behaviors in both regimes. These findings have important implications for strongly correlated systems where phonon energy scales may be comparable to the Coulomb scale, such as in twisted bilayer graphene, necessitating careful consideration of interaction renormalizations in theoretical models.

New perspectives on Density-Matrix Embedding TheoryAlicia Negre, Fabian Faulstich, Raehyun Kim, Thomas Ayral, Lin Lin, Eric...

New perspectives on Density-Matrix Embedding Theory

Alicia Negre, Fabian Faulstich, Raehyun Kim, Thomas Ayral, Lin Lin, Eric Canc\`es
https://arxiv.org/abs/2503.09881 https://arxiv.org/pdf/2503.09881 https://arxiv.org/html/2503.09881

arXiv:2503.09881v1 Announce Type: new
Abstract: Quantum embedding methods enable the study of large, strongly correlated quantum systems by (usually self-consistent) decomposition into computationally manageable subproblems, in the spirit of divide-and-conquer methods. Among these, Density Matrix Embedding Theory (DMET) is an efficient approach that enforces self-consistency at the level of one-particle reduced density matrices (1-RDMs), facilitating applications across diverse quantum systems. However, conventional DMET is constrained by the requirement that the global 1-RDM (low-level descriptor) be an orthogonal projector, limiting flexibility in bath construction and potentially impeding accuracy in strongly correlated regimes. In this work, we introduce a generalized DMET framework in which the low-level descriptor can be an arbitrary 1-RDM and the bath construction is based on optimizing a quantitative criterion related to the maximal disentanglement between different fragments. This yields an alternative yet controllable bath space construction for generic 1-RDMs, lifting a key limitation of conventional DMET. We demonstrate its consistency with conventional DMET in appropriate limits and exploring its implications for bath construction, downfolding (impurity Hamiltonian construction), low-level solvers, and adaptive fragmentation. We expect that this more flexible framework, which leads to several new variants of DMET, can improve the robustness and accuracy of DMET.

Crystal field splittings and magnetic ground state of the Jeff = 1/2 square lattice antiferromagnets YbBi2ClO4 and YbBi2IO4Pyeongjae Park,...

Crystal field splittings and magnetic ground state of the Jeff = 1/2 square lattice antiferromagnets YbBi2ClO4 and YbBi2IO4

Pyeongjae Park, Qianli Ma, G. Sala, S. Calder, Douglas L. Abernathy, Matthew B. Stone, Andrew F. May, Andrew D. Christianson
https://arxiv.org/abs/2503.09739 https://arxiv.org/pdf/2503.09739 https://arxiv.org/html/2503.09739

arXiv:2503.09739v1 Announce Type: new
Abstract: We report on the crystal field level splitting and magnetic ground state of the Jeff = 1/2 square lattice antiferromagnets YbBi2ClO4 and YbBi2IO4 using powder inelastic neutron scattering (INS) and neutron diffraction measurements. Both compounds exhibit a well-isolated $\Gamma_{7}$ doublet ground state under a tetragonal crystal field environment, confirming a robust Jeff = 1/2 picture with slight XY-type anisotropic character in the g-tensor. Notably, the ground state wave functions closely resemble the $\Gamma_{7}$ doublet expected in the perfect cubic limit, consistent with the nearly cubic ligand configuration of eight O2- ions surrounding Yb3+. Below TN =0.21 K, YbBi2IO4 exhibits a stripe long-range magnetic order characterized by an ordering wave vector qm = (1/2, 0, 0) or its symmetry-equivalent (0, 1/2, 0), with magnetic moments aligned along qm. The ordered moment is approximately 79 % of the classical prediction, significantly larger than expected from the isotropic J1-J2 model, suggesting the possible involvement of exchange anisotropy in explaining this observation. We show that symmetry-allowed XXZ and bond-dependent anisotropic exchange terms in a square lattice can play a critical role in stabilizing the stripe order and suppressing the moment reduction as observed. These findings establish YbBi2ClO4 and YbBi2IO4 as unique platforms for exploring rich Jeff = 1/2 magnetism from two less investigated perspectives: (i) on a square lattice and (ii) within a (nearly) cubic ligand environment.

Disentangling anomaly-free symmetries of quantum spin chainsSahand Seifnashri, Wilbur Shirleyhttps://arxiv.org/abs/2503.09717...

Disentangling anomaly-free symmetries of quantum spin chains

Sahand Seifnashri, Wilbur Shirley
https://arxiv.org/abs/2503.09717 https://arxiv.org/pdf/2503.09717 https://arxiv.org/html/2503.09717

arXiv:2503.09717v1 Announce Type: new
Abstract: We clarify the lore that anomaly-free symmetries are either on-site or can be transformed into on-site symmetries. We prove that any finite, internal, anomaly-free symmetry in a 1+1d lattice Hamiltonian system can be disentangled into an on-site symmetry by introducing ancillas and applying conjugation via a finite-depth quantum circuit. We provide an explicit construction of the disentangling circuit using Gauss's law operators and emphasize the necessity of adding ancillas. Our result establishes the converse to a generalized Lieb-Schultz-Mattis theorem by demonstrating that any anomaly-free symmetry admits a trivially gapped Hamiltonian.

Generalized Peierls substitution for Wannier obstructions: response to disorder and interactionsShuai A. Chen, Roderich Moessner, Tai Kai...

Generalized Peierls substitution for Wannier obstructions: response to disorder and interactions

Shuai A. Chen, Roderich Moessner, Tai Kai Ng
https://arxiv.org/abs/2503.09709 https://arxiv.org/pdf/2503.09709 https://arxiv.org/html/2503.09709

arXiv:2503.09709v1 Announce Type: new
Abstract: We study the interplay between quantum geometry, interactions, and external fields in complex band systems. When Wannier obstructions preclude a description based solely on atomic-like orbitals,this complicates the prediction of electromagnetic responses particularly in the presence of disorder and interactions. In this work, we introduce a generalized Peierls substitution framework based on Lagrange multipliers to enforce the constraints of the Wannier obstruction in the band of interest. Thus we obtain effective descriptions of interactions and disorder in the presence of non-trivial quantum geometry of that band. We apply our approach to examples including the diamagnetic response in flat-band superconductors and delocalization effects in flat-band metals caused by interactions and disorder.

Correlated flat-band physics in a bilayer kagome metal based on compact molecular orbitalsMounica Mahankali, Fang Xie, Yuan Fang, Lei Chen,...

Correlated flat-band physics in a bilayer kagome metal based on compact molecular orbitals

Mounica Mahankali, Fang Xie, Yuan Fang, Lei Chen, Shouvik Sur, Silke Paschen, Jean C. Souza, Moshe Haim, Ambikesh Gupta, Nurit Avraham, Haim Beidenkopf, Hengxin Tan, Binghai Yan, Qimiao Si
https://arxiv.org/abs/2503.09706 https://arxiv.org/pdf/2503.09706 https://arxiv.org/html/2503.09706

arXiv:2503.09706v1 Announce Type: new
Abstract: Flat bands, when located close to the Fermi energy, can considerably enhance the influence of electron correlations on the low energy physics in kagome and other frustrated-lattice metals. A major challenge in describing the interaction effects in such bulk materials is that the flat band is often intermixed with a large number of other bands. Here we show that the recently introduced notion of compact molecular orbitals (CMOs) enable a path forward in describing the dominant effect of the Coulomb interactions in spite of the complexity of the bandstructure. Our materials-based analysis allows for the understanding of the scanning-tunneling-microscopy experiment [J. C. Souza et al., preprint (2024)] of the bilayer kagome metal Ni$_3$In in terms of the CMO notion. From the resulting CMO, an effective Anderson lattice model can be set up. This CMO-based approach enables the calculation of correlation effects that is difficult to do based on the atomic orbitals. Furthermore, it suggests an enriched phase diagram for the strange metal physics of the kagome metal, which can be tested by future experiments. We discuss the implications of our results for the general correlation physics of flat band systems and beyond.

Altermagnets with topological order in Kitaev bilayersAayush Vijayvargia, Ezra Day-Roberts, Antia S. Botana, Onur...

Altermagnets with topological order in Kitaev bilayers

Aayush Vijayvargia, Ezra Day-Roberts, Antia S. Botana, Onur Erten
https://arxiv.org/abs/2503.09705 https://arxiv.org/pdf/2503.09705 https://arxiv.org/html/2503.09705

arXiv:2503.09705v1 Announce Type: new
Abstract: Building on recent advancements in altermagnetism, we develop a highly-frustrated magnetic model with Kitaev-like interactions that integrates key aspects of both quantum spin liquids and altermagnets. While the ground state is a gapless quantum spin liquid, our analysis indicates that an altermagnetic local order emerges upon the introduction of additional interactions that gap the excitation spectrum and give rise to a $\mathbb{Z}_2 $ topological order. This magnetically-fragmented topological altermagnet has fractionalized fermionic excitations with momentum-dependent splitting, in stark contrast to both standard altermagnets and Kitaev spin liquids. In addition, we discover two more altermagnetic phases, including a pseudo-altermagnet that exhibits splitting in the absence of a local order and a half-altermagnet that possesses only one type of fractionalized excitations, similar to a half-metal. We discuss experimental approaches for detecting these phases, including layer-dependent spin and heat transport. Our results highlight the rich physics that can arise due to the interplay between altermagnetism and fractionalized excitations in quantum magnets.

Resolving the Kagome Origin of the Strange Metallicity in Ni$_3$InJean C. Souza, Moshe Haim, Ambikesh Gupta, Mounica Mahankali, Fang Xie,...

Resolving the Kagome Origin of the Strange Metallicity in Ni$_3$In

Jean C. Souza, Moshe Haim, Ambikesh Gupta, Mounica Mahankali, Fang Xie, Yuan Fang, Lei Chen, Shiang Fang, Hengxin Tan, Minyong Han, Caolan John, Jingxu Zheng, Yiwen Liu, Binghai Yan, Joseph G. Checkelsky, Qimiao Si, Nurit Avraham, Haim Beidenkopf
https://arxiv.org/abs/2503.09704 https://arxiv.org/pdf/2503.09704 https://arxiv.org/html/2503.09704

arXiv:2503.09704v1 Announce Type: new
Abstract: Strong correlations promote singular properties such as strange metallicity, which shows considerable commonality across quantum materials platforms. Understanding the mechanism for such emerging universality is an outstanding challenge, given that the underlying degrees of freedom can be complex and varied. Progress may be made in flat band systems, especially kagome and other frustrated-lattice metals with active flat bands. These systems show strange metal behavior that bears a striking resemblance to what happens in heavy-fermion metals. Here, in scanning tunneling spectroscopy of kagome metal Ni$_3$In, we find a zero-bias peak-dip structure whose variation with magnetic field and temperature tracks the evolution of the strange metal properties. We identify the origin of the peak as compact molecular orbitals formed by destructive interference over the kagome sites, resulting in emergent $f$-shell-like localized moments. Using quasi-particle interference, we visualize their interaction with the Dirac light bands. We thus unveil the essential microscopic ingredients of the $d$-electron-based kagome metals that, while distinct from the atomic orbitals of the $f$-electron-based heavy fermion materials, are responsible for a shared phenomenology between the two types of systems. Our findings provide a new window to uncover and interconnect the essential and yet diverse microscopic building blocks in disparate families of quantum materials that drive a convergence towards a universal understanding in the regime of amplified quantum fluctuations.

Fluctuation corrections to the free energy of strongly correlated electron systemsDavid Riegler, Jannis Seufert, Ronny Thomale, Peter...

Fluctuation corrections to the free energy of strongly correlated electron systems

David Riegler, Jannis Seufert, Ronny Thomale, Peter W\"olfle
https://arxiv.org/abs/2503.09696 https://arxiv.org/pdf/2503.09696 https://arxiv.org/html/2503.09696

arXiv:2503.09696v1 Announce Type: new
Abstract: We determine the free energy of strongly correlated electron systems in the example of the Hubbard model by calculating the contribution of spin and charge fluctuations to the Gutzwiller approximation mean field result. We employ the slave boson formulation of Kotliar and Ruckenstein in its spin-rotation invariant form in the usual continuous time approximation of the functional integral representation, corrected by "high frequency contributions" (SRIKR+). Previous method-related shortcomings are shown to be overcome when the correct operator ordering for the renormalized kinetic energy is used. The results for the ground state energy in the paramagnetic phase are in very good agreement with state-of-the-art results obtained by methods such as density matrix embedded theory (DMET), quantum Monte Carlo (QMC) and others. The leading low temperature behavior of the free energy allows to extract the quasiparticle effective mass, in particular its enhancement near a continuous phase transition into an ordered state. Our work demonstrates that the SRIKR+ method is competitive with the best available alternative methods and equips the slave-boson approach with an improved synoptic power to explore strongly correlated electron systems.

Modified large-$N$ approach to gapless spin liquids, magnetic orders, and dynamics: Application to triangular lattice...

Modified large-$N$ approach to gapless spin liquids, magnetic orders, and dynamics: Application to triangular lattice antiferromagnets

Anjishnu Bose, Kathleen Hart, Ruairidh Sutcliffe, Arun Paramekanti
https://arxiv.org/abs/2503.09695 https://arxiv.org/pdf/2503.09695 https://arxiv.org/html/2503.09695

arXiv:2503.09695v1 Announce Type: new
Abstract: Recent work has shown that the triangular lattice spin-$1/2$ $J_1$-$J_2$ Heisenberg and XXZ antiferromagnets may exhibit coplanar or supersolid orders proximate to a gapless Dirac spin liquid phase. We explore a distinct $SU(2N)\!\!\times\!\!SU(M)$ fermionic parton approach, complemented by variational Monte Carlo calculations for the spin-$1/2$ model, to study the phase diagram of these models. We also calculate their dynamical spin response including parton interactions within a random phase approximation, and discuss implications for neutron scattering on triangular lattice cobaltates Ba$_3$CoSb$_2$O$_9$, Na$_2$BaCo(PO$_4$)$_2$, K$_2$Co(SeO$_3$)$_2$, Rb$_2$Co(SeO$_3$)$_2$, and Yb-based magnet KYbSe$_2$.