philip phillips

The density fluctuation spectrum captures many fundamental properties of strange metals. Using momentum-resolved electron energy-loss spectroscopy (M-EELS), we recently showed that the density response of the strange metal Bi2Sr2CaCu2O8+x (Bi-2212) at large momentum, q, exhibits a constant-in-frequency continuum [Mitrano, PNAS 115, 5392 (2018); Husain, PRX 9, 041062 (2019)] reminiscent of the marginal Fermi liquid (MFL) hypothesis of the late 1980s [Varma, PRL 63, 1996 (1989)]. However, reconciling this observation with infrared (IR) optics experiments, which show a well-defined plasmon excitation at q ∼ 0, has been challenging. Here we report M-EELS measurements of Bi-2212 using 4× improved momentum resolution, allowing us to reach the optical limit. For momenta q < 0.04 r.l.u., the M-EELS data show a plasmon feature that is quantitatively consistent with IR optics. For q > 0.04 r.l.u., the spectra become incoherent with an MFL-like, constant-in-frequency form. We speculate that, at finite frequency, ω, and nonzero q, some attribute of this Planckian metal randomizes the probe electron, causing it to lose information about its own momentum.

G08. 00003: Interaction-driven Mott Transition with a non-degenerate Ground State in the Orbital Hatsugai-Kohmoto model

The density fluctuation spectrum captures many fundamental properties of strange metals. Using momentum-resolved electron energy loss spectroscopy (M-EELS), we recently showed that the density response of the strange metal Bi 2 Sr 2 CaCu 2 O 8+ x (Bi-2212) at large momentum, q, exhibits a constant-in-frequency continuum [M. Mitrano et al., Proc. Natl. Acad. Sci. USA 115, 5392 (2018); AA Husain et al., Phys. Rev. X 9, 041062 (2019)] reminiscent of the marginal Fermi liquid (MFL) hypothesis of the late 1980s [CM Varma et al., Phys. Rev. Lett. 63, 1996 (1989)]. However, reconciling this observation with infrared (IR) optics experiments, which show a well-defined plasmon excitation at q∼ 0, has been challenging. Here we report M-EELS measurements of Bi-2212 using 4× improved momentum resolution, allowing us to reach the optical limit. For momenta q< 0.04 reciprocal lattice unites (rlu), the M-EELS data …

The Hatsugai-Kohmoto (HK) model provides a rich testbed for Mott physics due to its exact diagonalizability in momentum space [1, 2]. Extending HK to multiple orbitals introduces non-trivial commutators between the interaction and kinetic terms. We show that the orbital HK model exhibits Dynamical Spectral Weight Transfer under doping away from the Mott insulating phase, which indicates the presence of delocalized modes. We postulate that the orbital HK model provides a bridge between band HK and the Hubbard model due to the introduction of momentum state mixing.

We show here that numerous examples abound where changing topology does not necessarily close the bulk insulating charge gap as demanded in the standard non-interacting picture. From extensive determinantal and dynamical cluster quantum Monte Carlo simulations of the half-filled and quarter-filled Kane-Mele-Hubbard model, we show that for sufficiently strong interactions at either half- or quarter-filling, a transition between topological and trivial insulators occurs without the closing of a charge gap. To shed light on this behavior, we illustrate that an exactly solvable model reveals that while the single-particle gap remains, the many-body gap does in fact close. These two gaps are the same in the non-interacting system but depart from each other as the interaction turns on. We purport that for interacting systems, the proper probe of topological phase transitions is the closing of the many-body rather than the single-particle gap.

The strange metal phase with T-linear resistivity is believed to be related to Planckian dissipation. Our recent experiments using momentum-resolved EELS (M-EELS), from the strange metal Bi 2.1 Sr 1.9 CaCu 2 O 8+ x (Bi-2212), show that at q< 0.04 rlu, the spectra have a well-defined plasmon consistent with IR optics. At q> 0.04 rlu, the M-EELS data behave like a featureless continuum [1-3]. Here, using improved resolution, we report the new, finite-q measurements of the charge susceptibility on Bi-2212 at low energy, where ω~ k B T. The spectra show ω/T-scaling with a power law exponent α~-0.92. Above T c, the M-EELS spectra could be fit well by a CFT 0+ 1 theory with conformal dimension δ~ 0.06, which significantly deviates from the Marginal Fermi Liquid form (δ= 0.5). The M-EELS spectra can also be described in terms of relaxational dynamics where the charge dynamics exhibits Planckian dissipation.

Theoretical physics suffered a major loss with the death of my dear friend Jan Zaanen on January 18. This note is my remembrance of him.

Based on the surprising observation that all the correlation functions in the local-in-momentum space Hatsugai-Kohmoto (HK) model fall off exponentially with distance, we show that the -orbital extension has a fundamental connection with the Hubbard model. Namely, for -orbitals on each of the -points and dynamics between the orbital degrees of freedom, the -orbital HK model is equivalent to covering the Brillouin zone with Hubbard clusters each containing -sites all connected via twisted boundary conditions. The thermodynamic limit arises from the -state starting point of the HK model. While the Hubbard model necessarily results when , we show that already at , standard results for the Hubbard model emerge, such as the onset of an insulating state regardless of the strength of the interactions, double occupancy, dynamical spectral weight transfer, charge neutral excitations leading to the algebraic temperature dependence of the specific heat, all with a fraction of the computational time of more advanced cluster methods and making it possible to obtain analytical insights. Consequently, the -orbital HK model offers a new tool for strongly correlated quantum matter.

All the physical properties of strange metals can be unified under the marginal Fermi liquid (MFL) phenomenology, which conjectures that the itinerant electrons are coupled to a continuum of charge fluctuations, of unknown origin. Recently, we reported momentum-resolved electron energy-loss spectroscopy (M-EELS) measurements from the strange metal Bi 2.1 Sr 1.9 CaCu 2 O8+ x (Bi-2212, T c= 91K) demonstrating that such a continuum exists. However, these measurements* seem* to contradict IR ellipsometry studies showing that this material exhibits a conventional plasmon at Ω p~ 1 eV. Here, we use Jain and Allen's model of a layered conductor to make a quantitative comparison between IR data and M-EELS measurements at small momentum, q. We show that the M-EELS data are quantitatively consistent with IR for q< 0.02 rlu, indicating that both probes see the same plasmon in the small q limit. For q> 0 …

Motivated by recent experimental work on moiré systems in a strong magnetic field, we compute the compressibility as well as the spin correlations and Hofstadter spectrum of spinful electrons on a honeycomb lattice with Hubbard interactions using the determinantal quantum Monte Carlo method. While the interactions in general preserve quantum and anomalous Hall states, emergent features arise corresponding to an antiferromagnetic insulator at half-filling and other incompressible states following the Chern sequence ± (2N + 1). These odd integer Chern states exhibit strong ferromagnetic correlations and arise spontaneously without any external mechanism for breaking the spin-rotation symmetry. Analogs of these magnetic states should be observable in general interacting quantum Hall systems. In addition, the interacting Hofstadter spectrum is qualitatively similar to the experimental data at …

We establish the Hatsugai-Kohmoto model as a stable quartic fixed point (distinct from Wilson-Fisher) by computing the β function in the presence of perturbing local interactions. In vicinity of the half-filled doped Mott state, the β function vanishes for all local interactions regardless of their sign. The only flow away from the HK model is through the superconducting channel which lifts the spin degeneracy as does any ordering tendency. The superconducting instability is identical to that established previously [Phillips et al., Nat. Phys. 16, 1175 (2020)]. A corollary of this work is that any system in which the spectral weight bifurcates into lower and upper bands such as the Hubbard model with repulsive interactions flows into the HK stable fixed point in the vicinity of half-filling. Consequently, although the HK model has all-to-all interactions, the bifurcation of the spectral weight is stable as nothing local destroys it. The …

In 1956, David Pines predicted the existence of an acoustic plasmon, dubbed a''demon'', in metals with multiple and sufficiently distinct charge carrier species. Despite extensive searches, demons have not been observed directly until recent momentum-resolved electron energy-loss spectroscopy (M-EELS) experiments on Sr 2 RuO 4. Here, we discuss the necessary and sufficient conditions for the existence of demons as a stable collective excitation in multiband metals. We derive expressions for the dispersion, intensity, and damping rate of the demon in terms of band structure parameters, thereby going beyond the heuristic assumptions in Pines' original arguments. Furthermore, we comment on the suitability of various experimental probes, including M-EELS, optical spectroscopy, and X-ray spectroscopy, for detecting demons, and we make predictions for other materials where demons may be present and …

While the recent advances in topology have led to a classification scheme for electronic bands described by the standard theory of metals, a similar scheme has not emerged for strongly correlated systems such as Mott insulators in which a partially filled band carries no current. By including interactions in the topologically nontrivial Haldane model, we show that a quarter-filled state emerges with a nonzero Chern number provided the interactions are sufficiently large. We first motivate this result on physical grounds and then by two methods: Analytically by solving exactly a model in which interactions are local in momentum space and then numerically through the corresponding Hubbard model. All methods yield the same result: For sufficiently large interaction strengths, the quarter-filled Haldane model is a ferromagnetic topological Mott insulator with a Chern number of unity. Possible experimental realizations in …

In non-interacting systems, bands from non-trivial topology emerge strictly at half-filling and exhibit either the quantum anomalous Hall or spin Hall effects. Here we show using determinantal quantum Monte Carlo and an exactly solvable strongly interacting model that these topological states now shift to quarter filling. A topological Mott insulator is the underlying cause. The peak in the spin susceptibility is consistent with a possible ferromagnetic state at T = 0. The onset of such magnetism would convert the quantum spin Hall to a quantum anomalous Hall effect. While such a symmetry-broken phase typically is accompanied by a gap, we find that the interaction strength must exceed a critical value for this to occur. Hence, we predict that topology can obtain in a gapless phase but only in the presence of interactions in dispersive bands. These results explain the recent quarter-filled quantum anomalous Hall effects …

We introduce interactions into two general models for quantum spin Hall physics. Although the traditional picture is that such physics appears at half-filling, we show that in the presence of strong interactions, the quarter-filled state instead exhibits the quantum spin Hall effect with spin Chern number C s= 1. A topological Mott insulator is the underlying cause that lies outside the standard mathbbZ 2 topological classification. An intermediate interacting regime emerges that exhibits a quantum anomalous Hall effect when the lower band isflat'. This state transitions to the quantum spin Hall effect once the interactions are sufficiently large. We show that this intermediate regime is consistent with the simultaneous observation in transition metal dichalcogenide moir'e materials of a quantum anomalous Hall phase at quarter filling and a quantum spin Hall effect at half-filling. The valley coherence seen in such moir'e …

The ATLAS experiment is currently preparing for an upgrade of the inner tracking detector for the High-Luminosity LHC. The new tracker, ITk, employs an all-silicon detector with outer strip layers. The building block of the ITk strip barrel is the stave which consists of a low-mass support structure hosting the common electrical, optical and cooling services as well as 28 silicon modules. In this contribution, we outline the challenging aspects of the stave pre-production testing phase at Brookhaven National Laboratory. The electrical characterization of these staves, hosting the final design of all ASICs, will be discussed in detail.

The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a ‘demon’, could exist in three-dimensional (3D) metals containing more than one species of charge carrier. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral and do not couple to light, so have never been detected in an equilibrium, 3D metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals, optical properties of metal nanoparticles, soundarons in Weyl semimetals and high-temperature superconductivity in, for example, metal hydrides,, –. Here, we present evidence for a demon in Sr2RuO4 from momentum-resolved electron energy-loss spectroscopy. Formed of electrons in the β and γ …

The Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity described all superconductors until the 1986 discovery of the high-temperature counterpart in the cuprate ceramic materials. This discovery has challenged conventional wisdom as these materials are well known to violate the basic tenets of the Landau Fermi liquid theory of metals, crucial to the BCS solution. Precisely what should be used to replace Landau's theory remains an open question. The natural question arises: What is the simplest model for a non-Fermi liquid that yields tractable results. Our work builds on an overlooked symmetry that is broken in the normal state of generic models for the cuprates and hence serves as a fixed point. A surprise is that this fixed point also exhibits Cooper's instability. However, the resultant superconducting state differs drastically from that of the standard BCS theory. For example the famous Hebel-Slichter …

We show exactly that standard “invariants” advocated to define topology for noninteracting systems deviate strongly from the Hall conductance whenever the excitation spectrum contains zeros of the single-particle Green’s function, G, as in general strongly correlated systems. Namely, we show that if the chemical potential sits atop the valence band, the “invariant” changes without even accessing the conduction band but by simply traversing the band of zeros that might lie between the two bands. Since such a process does not change the many-body ground state, the Hall conductance remains fixed. This disconnect with the Hall conductance arises from the replacement of the Hamiltonian, h (k), with G− 1 in the current operator, thereby laying plain why perturbative arguments fail.

Pair density waves (PDWs) are superconducting states formed by Cooper pairs of electrons containing a nonzero center-of-mass momentum. They are characterized by a spatially modulated order parameter and may occur in a variety of emerging quantum materials such as cuprates, transition-metal dichalcogenides (TMDs), and Kagome metals. Despite extensive theoretical and numerical studies seeking PDWs in a variety of lattices and interacting settings, there is currently no exact mechanism that spontaneously favors a modulated solution of the superconducting order parameter. Here, we study the problem of two electrons subject to an anisotropic attractive potential. We solve the two-body Schrödinger wave equation exactly to determine the pair binding energy as a function of the center-of-mass momentum. We find that a modulated (finite momentum) pair is favored over a homogeneous (zero momentum …