arXiv subjects

I. S. Tupitsyn

Publications and source records attributed to I. S. Tupitsyn.

At least 19 recordsLinked to original sources

arXiv2026

Emergent BEC mechanism in flat-band superconductors

The formation of bound bosonic pairs of fermions, followed by their Bose-Einstein (quasi) condensation (BEC), is a foundational mechanism of superconductivity. At low filling, flat-band superconductivity is well captured by this mechanism provided the flat band is separated from the occupied lower band by an energy gap. However, particularly high $T_c$ values are anticipated when the non-interacting flat and lower bands touch---as in the prototypical attractive Lieb-lattice model studied here---invalidating the conventional picture: while interactions might protect the bound state by opening a gap, no small parameter guarantees the separation of the bound-state energy from this gap or occupied-band excitations, leaving the pair's fate uncertain. Based on a controlled-precision numerical protocol---which we demonstrate to be essential in this fundamentally non-perturbative problem---we show that the BEC mechanism, underpinned by an interaction-induced gap, is generically robust and remarkably efficient: fermions doped into the flat band form bound pairs within this gap with an anomalously light effective mass, enabling an exceptionally high $T_c$.

cond-mat.supr-con↗

arXiv2026

Numerically Exact Study of Flat-Band Superconductivity

Current theories of high-temperature superconductivity in flat-band systems predict a linear dependence of the transition temperature on the attractive interaction, $T_c(U) = c|U|$. However, the value of $c$ and the full nonlinear $T_c(U)$ curve---with a maximum at large $|U|$---are known beyond mean-field and quantum geometry estimates only for systems with isolated flat bands. Using a controlled diagrammatic Monte Carlo technique, we trace the onset of superfluid response in the Lieb lattice with attractive Hubbard interaction. Focusing on the half-filled flat-band case, where the ordering mechanism differs fundamentally from both BCS and BEC (preformed Cooper pair) scenarios, we find that the pairing response diverges linearly with decreasing temperature over a broad range of $U$, leading to a sharp crossover to long-range correlations at a characteristic temperature $T_*$, which provides a controlled upper bound on $T_c$. The highest $T_*$ occurs when all three bands touch at a single momentum point, potentially corresponding to high $T_c$ values.

cond-mat.supr-con↗

arXiv2026

Avoided Stoner instability at a single ordinary Van Hove point

When the Fermi surface and the Brillouin zone boundary touch at a Van Hove point, mean-field analysis predicts a ferromagnetic (Stoner) instability at finite $T_{MF}$ for any coupling strength due to the divergent density of states. However, the predicted effect has not been observed experimentally. Several qualitative theoretical proposals have been put forward to explain why the mean-field prediction fails. Based on numerically exact results for the two-dimensional Hubbard model with an ordinary Van Hove singularity, we uncover the mechanisms behind the suppression of the ferromagnetic instability. We employ two diagrammatic Monte Carlo approaches: (i) the four-channel self-consistent approximation and (ii) numerically exact method of combinatorial summation of diagrams with controlled resummation of the truncated expansion. We find that the system avoids the Stoner instability down to temperatures an order of magnitude below $T_{MF}$ due to the combination of the downward renormalization of the effective coupling and the suppression of the density of states by the loss of the quasiparticle residue.

cond-mat.str-el↗

arXiv2024

Dielectric losses in metals

Bethe-Salpeter equation (BSE) in the self-consistent Hartree-Fock (HF) basis is often used for describing complex many-body effects in material science applications. Its exact solution on the real-frequency axis at finite temperature for polarization using the diagrammatic Monte Carlo method [Phys. Rev. B \textbf{109}, 045152, (2024)] captures effects of multiple Coulomb scattering of a single particle-hole excitation, but does not account for multiple pair excitations important for studying dielectric loses in metals at frequencies comparable to the plasmon mode. In this paper we report technical developments which allow one to efficiently compute the dielectric response in a wide frequency range from zero to a few Fermi energies without systematic bias at finite $T$. By applying it to the homogeneous electron gas we demonstrate how at small momenta the gap in the spectral density between the electron-hole and plasmon excitations, existing within the HF-BSE approach, is filled with two particle-hole excitations and is completely washed out already at temperature $T \sim \varepsilon_F/10$.

cond-mat.str-el↗

arXiv2024

Landau Damping in an Electron Gas

Material science methods aim at developing efficient computational schemes for describing complex many-body effects and how they are revealed in experimentally measurable properties. Bethe-Salpeter equation in the self-consistent Hartree-Fock basis is often used for this purpose, and in this paper we employ the real-frequency diagrammatic Monte Carlo framework for solving the ladder-type Bethe-Salpeter equation for the 3-point vertex function (and, ultimately, for the system's polarization) to study the effect of electron-hole Coulomb scattering on Landau damping in the homogeneous electron gas. We establish how this damping mechanism depends on the Coulomb parameter $r_s$ and changes with temperature between the correlated liquid and thermal gas regimes. In a broader context of dielectric response in metals, we also present the full polarization and the typical dependence of the exchange-correlation kernel on frequency at finite momentum and temperature within the same computational framework.

cond-mat.str-el↗

arXiv2024

Interaction-enhanced nesting in Spin-Fermion and Fermi-Hubbard models

The spin-fermion (SF) model postulates that the dominant coupling between low-energy fermions in near critical metals is mediated by collective spin fluctuations (paramagnons) peaked at the Néel wave vector, ${\bf Q}_N$, connecting hot spots on opposite sides of the Fermi surface. It has been argued that strong correlations at hot spots lead to a Fermi surface deformation (FSD) featuring flat regions and increased nesting. This conjecture was confirmed in the perturbative self-consistent calculations when the paramagnon propagator dependence on momentum deviation from ${\bf Q}_N$ is given by $χ^{-1} \propto |Δq|$. Using diagrammatic Monte Carlo (diagMC) technique we show that such a dependence holds only at temperatures orders of magnitude smaller than any other energy scale in the problem, indicating that a different mechanism may be at play. Instead, we find that a $χ^{-1} \propto |Δq|^{2}$ dependence yields a robust finite-$T$ scenario for achieving FSD. To link phenomenological and microscopic descriptions, we applied the connected determinant diagMC method to the $(t-t')$ Hubbard model and found that in this case: (i) the FSD is not very pronounced, and, instead, it is the lines of zeros of the renormalized dispersion relation that deform towards nesting; (ii) this phenomenon appears at large $U/t>5.5$ before the formation of electron and hole pockets; (iii) the static spin susceptibility is well described by $χ^{-1} \propto |Δq|^{2}$. Flat FS regions yield a non-trivial scenario for realizing a non-Fermi liquid state.

cond-mat.str-el↗

arXiv2024

Solving the Bethe-Salpeter Equation in Real Frequencies at Finite Temperature

Self-consistent Hartree-Fock approximation combined with solutions of the Bethe-Salpeter equation offers a powerful tool for studies of strong correlation effects arising in condensed matter models, nuclear physics, quantum field theories, and real materials. The standard finite-temperature approach would be to first solve the problem in the Matsubara representation and then apply numerical analytic continuation to the real-frequency axis to link theoretical results with experimental probes, but this ill-conditioned procedure often distorts important spectral features even for very accurate imaginary-frequency data. We demonstrate that the ladder-type finite-temperature Bethe-Salpeter equation in the Hartree-Fock basis for the 3-point vertex function and, ultimately, system's polarization can be accurately solved directly on the real frequency axis using the diagrammatic Monte Carlo technique and series resummation. We illustrate the method by applying it to the homogeneous electron gas model and demonstrate how multiple scattering events renormalize Landau damping.

cond-mat.str-el↗

arXiv2021

Real-frequency response functions at finite temperature

Building on previous developments, we show that the Diagrammatic Monte Carlo technique allows to compute finite temperature response functions directly on the real-frequency axis within any field-theoretical formulation of the interacting fermion problem. There are no limitations on the type and nature of the system's action or whether partial summation and self-consistent treatment of certain diagram classes are used. In particular, by eliminating the need for numerical analytic continuation from a Matsubara representation, our scheme allows to study spectral densities of arbitrary complexity with controlled accuracy in models with frequency-dependent effective interactions. For illustrative purposes we consider the problem of the plasmon line-width in a homogeneous electron gas (jellium).

cond-mat.str-el↗

arXiv2020

Signatures of multiple charge excitations in RIXS spectra of metals

We study how multiple charge excitations appear in the resonant inelastic x-ray scattering (RIXS) spectra of metals. The single excitations in the problem are the plasmons and electron-hole pairs, and multi-excitation processes are usually neglected. However, at small momentum transfer the multi-excitation contributions may dominate the signal and one needs to understand how to interpret the data. In particular, we demonstrate how to "decode" the total multi-excitation intensity and extract the plasmon dispersion. While our calculations are based on the random phase approximation, which does not allow to obtain quantitatively precise results in the entire region of parameters, we expect them to capture semi-qualitatively all features expected for charged Fermi-liquid states, including universal and singular properties of the RIXS spectra.

cond-mat.str-el↗

arXiv2019

Resonant Inelastic X-Ray Scattering in Metals: A Diagrammatic Approach

We develop a formalism to study the Resonant Inelastic X-ray Scattering (RIXS) response in metals based on the diagrammatic expansion for its cross section. The standard approach to the solution of the RIXS problem relies on two key approximations: short-range potentials and non-interacting conduction electrons. However, these approximations are inaccurate for charged particles in metals, where the long-range Coulomb interaction and dynamic screening effects are very important. In this work we study how to extract important information about collective excitations in the Coulomb plasma, plasmons and electron-hole pairs, from RIXS data. We find that single- and multi-plasmon excitations can easily be distinguished by positions of the corresponding peaks, singularities, and their intensities. We also discuss the hybrid processes, where plasmon emission is accompanied by excitation of electron-hole pairs, and study how they manifest themselves.

cond-mat.str-el↗

arXiv2015

Solutions of the Two Dimensional Hubbard Model: Benchmarks and Results from a Wide Range of Numerical Algorithms

Numerical results for ground state and excited state properties (energies, double occupancies, and Matsubara-axis self energies) of the single-orbital Hubbard model on a two-dimensional square lattice are presented, in order to provide an assessment of our ability to compute accurate results in the thermodynamic limit. Many methods are employed, including auxiliary field quantum Monte Carlo, bare and bold-line diagrammatic Monte Carlo, method of dual fermions, density matrix embedding theory, density matrix renormalization group, dynamical cluster approximation, diffusion Monte Carlo within a fixed node approximation, unrestricted coupled cluster theory, and multi-reference projected Hartree-Fock. Comparison of results obtained by different methods allows for the identification of uncertainties and systematic errors. The importance of extrapolation to converged thermodynamic limit values is emphasized. Cases where agreement between different methods is obtained establish benchmark results that may be useful in the validation of new approaches and the improvement of existing methods.

cond-mat.str-el↗

arXiv2014

Fidelity Threshold of the Surface Code Beyond Single-Qubit Error Models

The surface code is one the most promising alternatives for implementing fault-tolerant, large-scale quantum information processing. Its high threshold for single-qubit errors under stochastic noise is one of its most attrative features. We develop an exact formulation for the fidelity of the surface code that allows us to probe much further on this promise of strong protection. This formulation goes beyond the stochastic single-qubit error model approximation and can take into account both correlated errors and inhomogeneities in the coupling between physical qubits and the environment. For the case of a bit-flipping environment, we map the complete evolution after one quantum error correction cycle onto the problem of computing correlation functions of a two-dimensional Ising model with boundary fields. Exact results for the fidelity threshold of the surface code are then obtained for several relevant types of noise. Analytical predictions for a representative case are confirmed by Monte Carlo simulations.

quant-ph↗

arXiv2011

Decoherence in Crystals of Quantum Molecular Magnets

Decoherence in Nature has become one of the most pressing problems in physics. Many applications, including quantum information processing, depend on understanding it; and fundamental theories going beyond quantum mechanics have been suggested [1-3], where the breakdown of quantum theory appears as an 'intrinsic decoherence', mimicking environmental decoherence [4]. Such theories cannot be tested until we have a handle on ordinary environmental decoherence processes. Here we show that the theory for insulating electronic spin systems can make accurate predictions for environmental decoherence in molecular-based quantum magnets [5]. Experimental understanding of decoherence in molecular magnets has been limited by short decoherence times, which make coherent spin manipulation extremely difficult [6-9]. Here we reduce the decoherence by applying a strong magnetic field. The theory predicts the contributions to the decoherence from phonons, nuclear spins, and intermolecular dipolar interactions, for a single crystal of the Fe8 molecular magnet. In experiments we find that the decoherence time varies strongly as a function of temperature and magnetic field. The theoretical predictions are fully verified experimentally - there are no other visible decoherence sources. Our investigation suggests that the decoherence time is ultimately limited by nuclear spins, and can be extended up to about 500 microseconds, by optimizing the temperature, magnetic field, and nuclear isotopic concentrations.

cond-mat.mes-hall↗

arXiv2010

Tunneling and inversion symmetry in single-molecule magnets: the case of the Mn12 wheel molecule

We present a detailed study of the influence of various interactions on the spin quantum tunneling in a Mn12 wheel molecule. The effects of single-ion and exchange (spin-orbit) anisotropy are first considered, followed by an analysis of the roles played by secondary influences, e.g. disorder, dipolar and hyperfine fields, and magnetoacoustic interactions. Special attention is paid to the role of the antisymmetric Dzyaloshinski-Moriya (DM) interaction. This is done within the framework of a 12-spin microscopic model, and also using simplified dimer and tetramer approximations in which the electronic spins are grouped in 2 or 4 blocks, respectively. If the molecule is inversion symmetric, the DM interaction between the dimer halves must be zero. In an inversion symmetric tetramer, two independent DM vectors are allowed, but no new tunneling transitions are generated by the DM interaction. Experiments on the Mn12 wheel can only be explained if the molecular inversion symmetry is broken, and we explore this in detail using both models, focussing on the asymmetric disposition and rounding of Berry phase minima associated with quantum interference between states of opposite parity. A remarkable behavior exists for the `Berry phase zeroes' as a function of the directions of the internal DM vectors and the external transverse field. A rather drastic breaking of the molecular inversion-symmetry is required to explain the experiments; in the tetramer model this requires a reorientation of the DM vectors on one half of the molecule by nearly 180 degrees. This cannot be attributed to sample disorder. These results are of general interest for the quantum dynamics of tunneling spins, and lead to some interesting experimental predictions.

cond-mat.mtrl-sci↗

arXiv2009

Manifestation of Spin Selection Rules on the Quantum Tunneling of Magnetization in a Single Molecule Magnet

We present low temperature magnetometry measurements on a new Mn3 single-molecule magnet (SMM) in which the quantum tunneling of magnetization (QTM) displays clear evidence for quantum mechanical selection rules. A QTM resonance appearing only at elevated temperatures demonstrates tunneling between excited states with spin projections differing by a multiple of three: this is dictated by the C3 symmetry of the molecule, which forbids pure tunneling from the lowest metastable state. Resonances forbidden by the molecular symmetry are explained by correctly orienting the Jahn-Teller axes of the individual manganese ions, and by including transverse dipolar fields. These factors are likely to be important for QTM in all SMMs.

cond-mat.mes-hall↗

arXiv2009

Collective dynamics of interacting Ising spins: Exact results for the Bethe lattice

We study the low temperature dynamics in films made of molecular magnets, i. e. crystals composed of molecules having large electronic spin S in their ground state. The electronic spin dynamics is mediated by coupling to a nuclear spin bath; this coupling allows transitions for a small fraction of electronic spins between their two energy minima, Sz=+- S, under resonant conditions when the change of the Zeeman energy in magnetic dipolar field of other electronic spins is compensated by interaction with nuclear spins. Transitions of resonant spins can result in opening or closing resonances in their neighbors leading to the collective dynamics at sufficiently large density P0 of resonant spins. We formulate and solve the equivalent dynamic percolation problem for the Bethe lattice (BL) of spins interacting with z neighbors and find that depending on the density of resonant spins P0 and the number of neighbors z the system has either one (2 5) kinetic transitions at P_{0}=Pc1 ~ exp(-1/3)/(3z) and P0=Pc2 ~ exp(-1)/z. The former transition is continuous and associated with the formation of an infinite cluster of coupled resonant spins similarly to the static percolation transition occurring at P0 ~ 1/z. The latter transition, z>5, is discontinuous and associated with the instantaneous increase in the density of resonant spins from the small value ~ 1/z to near unity. Experimental implications of our results are discussed.

cond-mat.dis-nn↗

arXiv2008

Topological multicritical point in the Toric Code and 3D gauge Higgs Models

We report a new type of multicritical point that arises from competition between the Higgs and confinement transitions in a Z_2 gauge system. The phase diagram of the 3d gauge Higgs model has been obtained by Monte-Carlo simulation on large (up to 60^3) lattices. We find the transition lines continue as 2nd-order until merging into a 1st-order line. These findings pose the question of an effective field theory for a multicritical point involving noncommuting order parameters. A similar phase diagram is predicted for the 2-dimensional quantum toric code model with two external fields, h_z and h_x; this problem can be mapped onto an anisotropic 3D gauge Higgs model.

cond-mat.stat-mech↗

arXiv2007

Stability of Bose Einstein condensates of hot magnons in YIG

We investigate the stability of the recently discovered room temperature Bose-Einstein condensate (BEC) of magnons in Ytrrium Iron Garnet (YIG) films. We show that magnon-magnon interactions depend strongly on the external field orientation, and that the BEC in current experiments is actually metastable - it only survives because of finite size effects, and because the BEC density is very low. On the other hand a strong field applied perpendicular to the sample plane leads to a repulsive magnon-magnon interaction; we predict that a high-density magnon BEC can then be formed in this perpendicular field geometry.

cond-mat.other↗