Quasiparticle Dispersion Research Articles

Neural network approach to quasiparticle dispersions in doped antiferromagnets

Numerically simulating large, spinful, fermionic systems is of great interest in condensed matter physics. However, the exponential growth of the Hilbert space dimension with system size renders exact quantum state parameterizations impractical. Owing to their representative power, neural networks often allow to overcome this exponential scaling. Here, we investigate the ability of neural quantum states (NQS) to represent the bosonic and fermionic t − J model – the high interaction limit of the Hubbard model – on various 1D and 2D lattices. Using autoregressive, tensorized recurrent neural networks (RNNs), we study ground state representations upon hole doping the half-filled system. Additionally, we propose a method to calculate quasiparticle dispersions, applicable to any network architecture or lattice geometry, and allowing to infer the low-energy physics from NQS. By analyzing the strengths and weaknesses of the RNN ansatz we shed light on the challenges and promises of NQS for simulating bosonic and fermionic systems.

Phase coexistence in a half-filled extended Hubbard model

We analyze the coexisting ordered phases that arise in the half-filled [Formula: see text]-t-U-V extended Hubbard Model on the square lattice when tackled within the Kotliar and Ruckenstein slave boson representation in the thermodynamical limit. Particular emphasis is put on the dependence of the quasi-particle dispersions, the gaps, the spin-resolved renormalization factors, and the distribution of double occupancy on the microscopical parameters of the model. Our calculations are performed in a parameter range that is most suitable for resistive switching. In particular, the main contributions to the gaps are shown to either arise from [Formula: see text] in the weak coupling regime, or from U and V when they are the largest scales. The gaps are thus from joint spin and charge origin, and so are the order parameters.

Hyperbolic Polaritons in Topological Nodal Ring Semimetals.

In mirror-symmetric systems, there is a possibility of the realization of extended gapless electronic states characterized as nodal lines or rings. Strain induced modifications to these states lead to the emergence of different classes of nodal rings with qualitatively different physical properties. Here we study optical response and the electromagnetic wave propagation in type I nodal ring semimetals, in which the low-energy quasiparticle dispersion is parabolic in momentum k_{x} and k_{y} and is linear in k_{z}. This leads to a highly anisotropic dielectric permittivity tensor in which the optical response is plasmonic in one spatial direction and dielectric in the other two directions. The resulting normal modes (polaritons) in the bulk material become hyperbolic over a broad frequency range, which is furthermore tunable by the doping level. The propagation, reflection, and polarization properties of the hyperbolic polaritons not only provide valuable information about the electronic structure of these fascinating materials in the most interesting region near the nodal rings but also pave the way to tunable hyperbolic materials with applications ranging from anomalous refraction and waveguiding to perfect absorption in ultrathin subwavelength films.

Untwisting Moiré Physics: Almost Ideal Bands and Fractional Chern Insulators in Periodically Strained Monolayer Graphene.

Moiré systems have emerged in recent years as a rich platform to study strong correlations. Here, we will propose a simple, experimentally feasible setup based on periodically strained graphene that reproduces several key aspects of twisted moiré heterostructures-but without introducing a twist. We consider a monolayer graphene sheet subject to a C_{2}-breaking periodic strain-induced pseudomagnetic field with period L_{M}≫a, along with a scalar potential of the same period. This system has almost ideal flat bands with valley-resolved Chern number ±1, where the deviation from ideal band geometry is analytically controlled and exponentially small in the dimensionless ratio (L_{M}/l_{B})^{2}, where l_{B} is the magnetic length corresponding to the maximum value of the pseudomagnetic field. Moreover, the scalar potential can tune the bandwidth far below the Coulomb scale, making this a very promising platform for strongly interacting topological phases. Using a combination of strong-coupling theory and self-consistent Hartree-Fock, we find quantum anomalous Hall states at integer fillings. At fractional filling, exact diagonaliztion reveals a fractional Chern insulator at parameters in the experimentally feasible range. Overall, we find that this system has larger interaction-induced gaps, smaller quasiparticle dispersion, and enhanced tunability compared to twisted graphene systems, even in their ideal limit.

Aperiodic Modulation of Graphene Driven by Oxygen-Induced Reconstruction of Rh(110).

Artificial nanostructuring of graphene has served as a platform to induce variations in its structural and electronic properties, fostering the experimental observation of a wide and fascinating phenomenology. Here, we present an approach to graphene tuning, based on Rh(110) surface reconstruction induced by oxygen atoms intercalation. The resulting nanostructured graphene has been characterized by scanning tunneling microscopy (STM) complemented by low-energy electron microscopy (LEEM), micro low-energy electron diffraction (μ-LEED), micro angle-resolved photoemission spectroscopy (μ-ARPES), and micro X-ray photoelectron spectroscopy (μ-XPS) measurements under ultrahigh vacuum (UHV) conditions at room temperature (RT). It is found that by fine-tuning the O2 exposure amount, a mixture of missing row surface reconstructions of the metal surface below the graphene layer can be induced. This atomic rearrangement under the graphene layer results in aperiodic patterning of the two-dimensional (2D) material. The electronic structure of the resulting nanostructured graphene is dominated by a linear dispersion of the Dirac quasiparticles, characteristic of its free-standing state but with a p-doping character. The local effects of the underlying missing rows on the interfacial chemistry and on the quasiparticle scattering processes in graphene are studied using atomically resolved STM images. The possibilities offered by this nanostructuring approach, which consists in inducing surface reconstructions under graphene, could provide a novel tuning strategy for this 2D material.

Spin conductivity spectrum and spin superfluidity in a binary Bose mixture

We investigate the spectrum of spin conductivity for a miscible two-component Bose-Einstein condensate (BEC) that exhibits spin superfluidity. By using the Bogoliubov theory, the regular part being the spin conductivity at finite ac frequency and the spin Drude weight characterizing the delta-function peak at zero frequency are analytically computed. We demonstrate that the spectrum exhibits a power-law behavior at low frequency, reflecting gapless density and spin modes specific to the binary BEC. At the phase transition points into immiscible and quantum-droplet states, the change in quasiparticle dispersion relations modifies the power law. In addition, the spin Drude weight becomes finite, indicating zero spin resistivity due to spin superfluidity. Our results also suggest that the Andreev-Bashkin drag density is accessible by measuring the spin conductivity spectrum.

Magnetic excitations in the helical Rashba superconductor

We investigate the magnetic excitation spectrum in the helical state of a noncentrosymmetric superconductor with inversion symmetry breaking and strong Rashba spin–orbit coupling. For this purpose we derive the general expressions of the dynamical spin response functions under the presence of strong Rashba splitting of conduction bands, superconducting gap and external field which lead to stabilization of Cooper pairs with finite overall momentum in a helical state. The latter is characterized by momentum space regions of paired and unpaired states with different quasiparticle dispersions. The magnetic response is determined by i) excitations within and between both paired and unpaired regions ii) anomalous coherence factors and iii) additional spin matrix elements due to helical Rashba spin texture of bands. We show that as a consequence typical correlated real space and spin space anisotropies appear in the dynamical susceptibility which would be observable as a characteristic fingerprint for a helical superconducting state in inelastic neutron scattering investigations.

Competition-induced sign reversal of Casimir-Lifshitz torque: Investigation of topological nodal-line semimetals

The dispersion of quasiparticles in topological node-line semimetals is significantly different in different directions. In a certain direction, the quasiparticles behave like relativistic particles with constant velocity. In other directions, they act as two-dimensional electron gas. The competition between relativistic and nonrelativistic dispersions can induce a sign reversal of Casimir-Lifshitz torque. Three different approaches can be applied to generate this sign reversal, i.e., tuning the anisotropic parameter or chemical potential in node-line semimetal, changing the distance between this material and substrate birefringence. Detailed calculations are illustrated for the system with topological node-line semimetal Ca$_3$P$_2$ and liquid crystal material 4-cyano-4-n-pentylcyclohexane-phenyl.

A machine learning route between band mapping and band structure

The electronic band structure and crystal structure are the two complementary identifiers of solid-state materials. Although convenient instruments and reconstruction algorithms have made large, empirical, crystal structure databases possible, extracting the quasiparticle dispersion (closely related to band structure) from photoemission band mapping data is currently limited by the available computational methods. To cope with the growing size and scale of photoemission data, here we develop a pipeline including probabilistic machine learning and the associated data processing, optimization and evaluation methods for band-structure reconstruction, leveraging theoretical calculations. The pipeline reconstructs all 14 valence bands of a semiconductor and shows excellent performance on benchmarks and other materials datasets. The reconstruction uncovers previously inaccessible momentum-space structural information on both global and local scales, while realizing a path towards integration with materials science databases. Our approach illustrates the potential of combining machine learning and domain knowledge for scalable feature extraction in multidimensional data.

Probing quantum many-body correlations by universal ramping dynamics

Ramping a physical parameter is one of the most common experimental protocols in studying a quantum system, and ramping dynamics has been widely used in preparing a quantum state and probing physical properties. Here, we present a novel method of probing quantum many-body correlation by ramping dynamics. We ramp a Hamiltonian parameter to the same target value from different initial values and with different velocities, and we show that the first-order correction on the finite ramping velocity is universal and path-independent, revealing a novel quantum many-body correlation function of the equilibrium phases at the target values. We term this method as the non-adiabatic linear response since this is the leading order correction beyond the adiabatic limit. We demonstrate this method experimentally by studying the Bose-Hubbard model with ultracold atoms in three-dimensional optical lattices. Unlike the conventional linear response that reveals whether the quasi-particle dispersion of a quantum phase is gapped or gapless, this probe is more sensitive to whether the quasi-particle lifetime is long enough such that the quantum phase possesses a well-defined quasi-particle description. In the Bose-Hubbard model, this non-adiabatic linear response is significant in the quantum critical regime where well-defined quasi-particles are absent. And in contrast, this response is vanishingly small in both superfluid and Mott insulators which possess well-defined quasi-particles. Because our proposal uses the most common experimental protocol, we envision that our method can find broad applications in probing various quantum systems.

Glass encapsulation of molecular-doped epitaxial graphene for quantum resistance metrology

The large Landau energy spacing, stemming from the linear energy-momentum dispersion of quasi-particles in graphene, allows an efficient realization of the quantum Hall effect at a small density of charge carriers. Promising scalable epitaxial graphene on silicon carbide (SiC), however, requires molecular doping, which is generally unstable under ambient conditions, to compensate for electron transfer from the SiC substrate. Here, we employed classical glass encapsulation common in organic electronics to passivate molecular-doped epitaxial graphene against water and oxygen molecules in air. We have investigated the stability of Hall quantization in a glass-encapsulated device for almost 1 year. The Hall quantization is maintained above a threshold magnetic field within 2 nΩ Ω−1 smaller than the measurement uncertainty of 3.5 nΩ Ω−1 through multiple thermal cycles for almost 1 year, while the ordinary unencapsulated device in air distinctly shows a relative deviation larger than 0.05% from the nominal quantized Hall resistance in 1 month.

Anharmonic thermodynamic properties and phase boundary across the postperovskite transition in MgSiO3

To address the effects of lattice anharmonicity across the perovskite to post-perovskite transition in MgSiO$_3$, we conduct calculations using the phonon quasiparticle (PHQ) approach. The PHQ is based on \textit{ab initio} molecular dynamics and, in principle, captures full anharmonicity. Free energies in the thermodynamic limit ($N \rightarrow \infty$) are computed using temperature-dependent quasiparticle dispersions within the phonon gas model. Systematic results on anharmonic thermodynamic properties and phase boundary are reported. Both the local density approximation (LDA) and the generalized gradient approximation (GGA) calculations are performed to provide confident constraints on these properties. Anharmonic effects are demonstrated by comparing results with those obtained using the quasiharmonic approximation (QHA). The inadequacy of the QHA is indicated by its overestimation of thermal expansivity and thermodynamic Gr\"{u}neisen parameter and its converged isochoric heat capacity in the high-temperature limit. The PHQ phase boundary has a Clapeyron slope ($dP/dT$) that increases with temperature. This result contrasts with the nearly zero curvature of the QHA phase boundary. Anharmonicity bends the phase boundary to lower temperatures at high pressures. Implications for the double-crossing of the phase boundary by the mantle geotherm are discussed.

Electronic structure of the highly conductive perovskite oxide SrMoO3

We use angle-resolved photoemission to map the Fermi surface and quasiparticle dispersion of bulklike thin films of ${\mathrm{SrMoO}}_{3}$ grown by pulsed laser deposition. The electronic self-energy deduced from our data reveals weak to moderate correlations in ${\mathrm{SrMoO}}_{3}$, consistent with our observation of well-defined electronic states over the entire occupied bandwidth. We further introduce spectral function calculations that combine dynamical mean-field theory with an unfolding procedure of density functional calculations and demonstrate good agreement of this approach with our experiments.

Electronic nature of the pseudogap in electron-doped Sr2IrO4

In high-temperature (Tc) cuprate superconductors, many exotic phenomena are rooted in the enigmatic pseudogap state, which has been interpreted as consisting of preformed Cooper pairs or competing orders or a combination thereof. Observation of pseudogap phenomenologically in electron-doped Sr2IrO4—the 5d electron counterpart of the cuprates, has spurred intense interest in the strontium iridates as a testbed for exploring the exotic physics of the cuprates. Here, we examine the pseudogap state of electron-doped Sr2IrO4 by angle-resolved photoemission spectroscopy (ARPES) and parallel theoretical modeling. Our analysis demonstrates that the pseudogap state of Sr2IrO4 appears without breaking the particle–hole symmetry or inducing spectral broadening which are telltale signatures of competing orders in the cuprates. We find quasiparticle dispersion and its temperature dependence in the pseudogap state of Sr2IrO4 to point to an electronic order with a zero scattering wave vector and limited correlation length. Particle–hole symmetric preformed Cooper pairs are discussed as a viable mechanism for such an electronic order. The potential roles of incommensurate density waves are also discussed.

Renormalization of Dispersion in Electron-Doped Bilayer Cuprate Superconductors

The renormalization of the electrons in cuprate superconductors is characterized by the kink in the quasiparticle dispersion. Here, the bilayer coupling effect on the quasiparticle dispersion kink in the electron-doped bilayer cuprate superconductors is studied based on the kinetic-energy-driven superconductivity. It is shown that the kink in the quasiparticle dispersion is present all around the electron Fermi surface, as the quasiparticle dispersion kink in the single-layer case. However, in comparison with the corresponding single-layer case, the kink effect in the quasiparticle dispersion at around the antinodal region becomes the most pronounced, indicating that the kink effect in the quasiparticle dispersion at around the antinodal region is enhanced by the bilayer coupling.

Fermi surface segmentation in the helical state of a Rashba superconductor

We investigate the quasiparticle excitations in the FFLO- type helical state of a superconductor with inversion-symmetry breaking and strong Rashba spin-orbit coupling. We restrict to a state with single finite momentum of Cooper pairs in the helical phase that is determined by minimization of the condensation energy. We derive the dependence of quasiparticle dispersions on the Rashba coupling strength and external field. It leads to a peculiar momentum-space segmentation of the corresponding Rashba Fermi surface sheets which has not yet been observed experimentally. We show that it may be directly visualized by the method of quasiparticle interference that identifies the critical points of the segmented sheets and can map their evolution with field strength, bias voltage and Rashba coupling. We also indicate a strategy how to determine the finite Cooper-pair momentum from experimental quantities. This investigation has the potential for a more detailed microscopic understanding of the helical superconducting state under the influence of Rashba spin-orbit coupling.

On the theory of nonhomogeneous nonequilibrium superconductivity in 2D systems with massless fermions

We analyze static and nonequilibrium superconducting properties of a 2D relativistic-like model system with local electron-electron interaction, Rashba spin-orbit interaction αR in presence of time-dependent in-plane magnetic field H(t). It is shown that similar to the 2D case with ordinary massive quasiparticle dispersion ε(k)∼|k|2 at large fields, such a system demonstrates a nonhomogeneous superconducting stripe phase with the order parameter Δ(r)=Δ(0)cos(2[μBB×r]n/ℏυF) (B is the magnetic induction, υF is the Fermi velocity, n is the normal to the plane, μB is the Bohr magneton, and αR≪υF) where the stripes are oriented along the B direction. In the considered system, the inter-stripe period L and the magnitude of the magnetic field B are related by a universal relation BL=ℏυF/μB≃0.714⋅10−4Tm. Contrary to the case of massive quasiparticles, where the condition αR∼υF can be, in principle, satisfied by increasing αR or by charge doping (Fermi velocity decreasing), in a relativistic-like system, where υF is doping-independent and one-two orders of magnitude larger than typical Fermi velocity in the “standard” 2D systems, the stripe phase can be the ground state at a rather low doping level. We also analyzed the nonequilibrium properties of the system with a focus on the melting of the stripe order (when the magnetic field is quenched to a lower value) and stripe dynamics (when the field is rotated by 90° degrees) and found several notable results. In particular, it was shown that the stripe domains melt according to law R∼1t at initial times, while at longer times they shrink exponentially. In the case of the flipped magnetic field, the stripe orientation gradually turns from x- to y-direction, and the intermediate “crossed-stripe” phase takes place during times of order of picoseconds. Such a crossed phase is built of periodic superconducting bubbles that potentially may have applications in modern ultrafast superconducting technologies.

Anomalous acoustoelectric effect induced by clapping modes in chiral superconductors

Clapping modes, which are relative amplitude and phase modes between two chiral components of Cooper pairs, are bosonic collective modes inherent to chiral superconductors. These modes behave as long-lived bosons with masses smaller than the threshold energy, $2|\Delta|$, for decay into unbound fermion pairs. Here, we clarify that the real/imaginary clapping modes in chiral superconductors directly couple to acoustic wave propagation when the weak particle-hole asymmetry of the normal state quasiparticle dispersion is taken into account. The clapping modes driven by an acoustic wave generate an alternating electric current, that is, the acoustoelectric effect in superconductors. Significantly, the clapping modes give rise to a transverse electric current. When the sound velocity is comparable to the Fermi velocity, as in heavy fermion compounds, the transverse current is resonantly enhanced at energy below the threshold for continuum excitations. This resonance provides a smoking-gun evidence of chiral superconductivity.

Cooper quartet correlations in infinite symmetric nuclear matter

We investigate the quartet correlations in four-component fermionic systems at the thermodynamic limit within a variational many-body theory. The Bardeen-Cooper-Schrieffer (BCS)-type variational wave function is extended to the systems with the coexistence of pair and quartet correlations at zero temperature. Special attention is paid to the application of the present framework to an alpha-particle condensation in symmetric nuclear matter, where the coexistence of deuteron and alpha condensations is anticipated. We also discuss how physical properties, such as quasiparticle dispersion, can be modified by the pair and quartet correlations and show a hierarchical structure of in-medium cluster formations in infinite nuclear matter. The present results may also contribute to the interdisciplinary understanding of fermionic condensations beyond the BCS paradigm in many-body systems.

Emergent {{{{{{{mathcal{PT}}}}}}}}-symmetry breaking of collective modes with topological critical phenomena

The spontaneous breaking of parity-time ({{{{{{{mathcal{PT}}}}}}}}) symmetry yields rich critical behavior in non-Hermitian systems, and has stimulated much interest, albeit most previous studies were performed within the single-particle or mean-field framework. Here, by studying the collective excitations of a Fermi superfluid with {{{{{{{mathcal{PT}}}}}}}}-symmetric spin-orbit coupling, we uncover an emergent {{{{{{{mathcal{PT}}}}}}}}-symmetry breaking in the Anderson-Bogoliubov (AB) collective modes, even as the superfluid ground state retains an unbroken {{{{{{{mathcal{PT}}}}}}}} symmetry. The critical point of the transition is marked by a non-analytic kink in the speed of sound, which derives from the coalescence and annihilation of the AB mode and its hole partner, reminiscent of the particle-antiparticle annihilation. The system consequently becomes immune to low-frequency external perturbations at the critical point, a phenomenon associated with the spectral topology of the complex quasiparticle dispersion. This critical phenomenon offers a fascinating route toward perturbation-free quantum states.