Subir Sachdev
Cambridge
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Wiley Online Library,
PNAS,
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Physical Review B
Herchel Smith Professor of Physics, Harvard University
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Articles by Subir Sachdev
Ferrimagnetism of ultracold fermions in a multiband Hubbard system
Editor’s summary Copper oxide superconductors and other strongly interacting systems are thought to be described by the Hubbard model. This model, which accounts for particle hopping and interactions, can be simulated using cold atoms in optical lattices. However, incorporating multiple energy bands, which is thought to be important for an accurate description of the physics, is tricky. Lebrat et al.
Fermionic Spinon Theory of the Hourglass Spin Excitation Spectrum of the Cuprates
We present a theory for the spin fluctuation spectrum of the hole-doped cuprates in a ground state with a period 4 unidirectional charge density wave (“stripe”) order. Motivated by recent experimental evidence for a fractionalized Fermi liquid (FL*) description of the intermediate temperature pseudogap metal, we employ a theory of fermionic spinons, which are confined with the onset of stripe order at low temperatures.
Probing the Kitaev honeycomb model on a neutral-atom quantum computer - Nature
Abstract Quantum simulations of many-body systems are among the most promising applications of quantum computers1. In particular, models based on strongly correlated fermions are central to our understanding of quantum chemistry and materials problems2, and can lead to exotic, topological phases of matter3,4. However, owing to the non-local nature of fermions, such models are challenging to simulate with qubit devices5.
Quantum coarsening and collective dynamics on a programmable simulator - Nature
Abstract Understanding the collective quantum dynamics of non-equilibrium many-body systems is an outstanding challenge in quantum science. In particular, dynamics driven by quantum fluctuations are important for the formation of exotic quantum phases of matter1, fundamental high-energy processes2, quantum metrology3,4 and quantum algorithms5.
Deconfined quantum criticality of nodal $d$-wave superconductivity, N\'eel order, and charge order on the square lattice at half-filling
We consider a SU(2) lattice gauge theory on the square lattice, with a single fundamental complex fermion and a single fundamental complex boson on each lattice site. Projective symmetries of the gauge-charged fermions are chosen so that they match with those of the spinons of the -flux spin liquid. Global symmetries of all gauge-invariant observables are chosen to match with those of the particle-hole symmetric electronic Hubbard model at half-filling.
Hidden orders and phase transitions for the fully packed quantum loop model on the triangular lattice - Communications Physics
Abstract Quantum loop and dimer models are prototypical correlated systems with local constraints, which are not only intimately connected to lattice gauge theories and topological orders but are also widely applicable to the broad research areas of quantum materials and quantum simulation. Employing our sweeping cluster quantum Monte Carlo algorithm, we reveal the complete phase diagram of the triangular-lattice fully packed quantum loop model.
Impurity spin dynamics in 2D antiferromagnets and superconductors
We discuss the universal theory of localized impurities in the paramagnetic state of 2D antiferromagnets where the spin gap is assumed to be significantly smaller than a typical exchange energy. We study the impurity spin susceptibility near the host quantum transition from a gapped paramagnet to a Néel state, and we compute the impurity-induced damping of the spin-1 mode of the gapped antiferromagnet. Under suitable conditions our results apply also to d-wave superconductors.
Conductance and thermopower fluctuations in interacting quantum dots
We model an interacting quantum dot of electrons by a Hamiltonian with random and all-to-all single-particle hopping (of root-mean-square strength t) and two-particle interactions (of root-mean-square strength J). For t≪J, such a model has a regime exhibiting the nonquasiparticle physics of the Sachdev-Ye-Kitaev (SYK) model at temperatures Ecoh≪T≪J, and that of a renormalized Fermi liquid at T≪Ecoh, where Ecoh=t2/J.
Equilibrium Dynamics of Infinite-Range Quantum Spin Glasses in a Field
Popular Summary Recent progress in quantum technologies has today enabled the study of complex optimization problems. Using a variety of quantum simulators, qubits (or, equivalently, quantum spins) can be arranged in arbitrary geometries as required while the interactions between them encode the problem of interest.
Localization of overdamped bosonic modes and transport in strange metals | Proceedings of the National Academy of Sciences
Significance Almost all higher-temperature superconductor materials exhibit a “strange metal” regime above the critical temperature for superconductivity. The important problem of theoretically computing the critical temperature for superconductivity therefore requires a complete theory of the strange metal. We investigate the subtle consequences of multielectron quantum entanglement in the presence of impurities at spatially random positions in a strange metal.
Emergence of nodal Bogoliubov quasiparticles across the transition from the pseudogap metal to the d-wave superconductor - npj Quantum Materials
Abstract We model the pseudogap state of the hole- and electron-doped cuprates as a metal with hole and/or electron pocket Fermi surfaces. In the absence of long-range antiferromagnetism, such Fermi surfaces violate the Luttinger requirement of enclosing the same area as free electrons at the same density.
Connecting the Many-Body Chern Number to Luttinger's Theorem through St\ifmmode \check{r}\else \v{r}\fi{}eda's Formula
Relating the quantized Hall response of correlated insulators to many-body topological invariants is a key challenge in topological quantum matter. Here, we use Středa’s formula to derive an expression for the many-body Chern number in terms of the single-particle interacting Green’s function and its derivative with respect to a magnetic field.
Clustering neural quantum states via diffusion maps
We discuss and demonstrate an unsupervised machine-learning procedure to detect topological order in quantum many-body systems. Using a restricted Boltzmann machine to define a variational Ansatz for the low-energy spectrum, we sample wave functions with probability decaying exponentially with their variational energy; this defines our training dataset that we use as input to a diffusion map scheme.
Theory of shot noise in strange metals
We extend the theory of shot noise in coherent metals to shot noise in strange metals without quasiparticle excitations. This requires a generalization of the Boltzmann equation with a noise source to distribution functions which depend independently on the excitation momentum and energy. We apply this theory to a model of a strange metal with linear in temperature (T) resistivity, describing a Fermi surface with a spatially random Yukawa coupling to a critical boson.
Nodal band-off-diagonal superconductivity in twisted graphene superlattices - Nature Communications
Abstract The superconducting state and mechanism are among the least understood phenomena in twisted graphene systems. Recent tunneling experiments indicate a transition between nodal and gapped pairing with electron filling, which is not naturally understood within current theory.
Disordered Quantum Critical Fixed Points from Holography
Using holographic duality, we present an analytically controlled theory of quantum critical points without quasiparticles, at finite disorder and finite charge density. These fixed points are obtained by perturbing a disorder-free quantum critical point with relevant disorder whose operator dimension is perturbatively close to Harris marginal. We analyze these fixed points both using field theoretic arguments, and by solving the bulk equations of motion in holography.
Engineering SYK Interactions in Disordered Graphene Flakes under Realistic Experimental Conditions
Abstract We model interactions following the Sachdev-Ye-Kitaev (SYK) framework in disordered graphene flakes up to 300 000 atoms in size (∼100nm in diameter) subjected to an out-of-plane magnetic field B of 5–20 Tesla within the tight-binding formalism.
Spin density wave, Fermi liquid, and fractionalized phases in a theory of antiferromagnetic metals using paramagnons and bosonic spinons
Abstract The pseudogap metal phase of hole-doped cuprates can be described by small Fermi surfaces of electronlike quasiparticles, which enclose a volume violating the Luttinger relation. This violation requires the existence of additional fractionalized excitations which can be viewed as fractionalized remnants of the paramagnon.
A model of d-wave superconductivity, antiferromagnetism, and charge order on the square lattice | Proceedings of the National Academy of Sciences
M. Kyle et al., Nodal quasiparticles and antinodal charge ordering in Ca2−x2−xNaxxCuO22Cl22. Science, 307, 901–904 (2005). N. Doiron-Leyraud et al., Quantum oscillations and the Fermi surface in an underdoped high-Tcc superconductor. Nature 447, 565–568 (2007). N. Harrison, S. E. Sebastian, Protected nodal electron pocket from multiple-Q ordering in underdoped high temperature superconductors. Phys. Rev. Lett. 106, 226402 (2011). A. Allais, D. Chowdhury, S.
Superconductivity of non-Fermi liquids described by Sachdev-Ye-Kitaev models
Abstract We investigate models of electrons in the Sachdev-Ye-Kitaev class with random and all-to-all electron hopping, electron spin exchange, and Cooper-pair hopping. An attractive on-site interaction between electrons leads to superconductivity at low temperatures. Depending on the relative strengths of the hopping and spin exchange, the normal state at the critical temperature is either a Fermi-liquid or a non-Fermi liquid.
Triangular lattice quantum dimer model with variable dimer density - Nature Communications
Abstract Quantum dimer models are known to host topological quantum spin liquid phases, and it has recently become possible to simulate such models with Rydberg atoms trapped in arrays of optical tweezers. Here, we present large-scale quantum Monte Carlo simulation results on an extension of the triangular lattice quantum dimer model with terms in the Hamiltonian annihilating and creating single dimers.
Correlated Insulators, Semimetals, and Superconductivity in Twisted Trilayer Graphene
Abstract Motivated by recent experiments indicating strong superconductivity and intricate correlated insulating and flavor-polarized physics in mirror-symmetric twisted-trilayer graphene, we study the effects of interactions in this system close to the magic angle, using a combination of analytical and numerical methods. We identify asymptotically exact correlated many-body ground states at all integer filling fractions ν of the flat bands.
Large-$N$ theory of critical Fermi surfaces
Abstract We describe the large-N saddle point, and the structure of fluctuations about the saddle point, of a theory containing a sharp, critical Fermi surface in two spatial dimensions. The theory describes the onset of Ising order in a Fermi liquid, and closely related theories apply to other cases with critical Fermi surfaces.
Quantum connections
Theoretical high-energy and condensed-matter physics share various ideas and tools. New connections between the two have been established through quantum information, providing exciting prospects for theoretical advances and even potential experimental studies. Six scientists discuss different directions of research in this inter-disciplinary field. New connections Access options Subscribe to Journal Get full journal access for 1 year $99.00 only $8.25 per issue All prices are NET prices.
Superconductivity, correlated insulators, and Wess-Zumino-Witten terms in twisted bilayer graphene
Superconductivity, correlated insulators, and Wess–Zumino–Witten terms in twisted bilayer graphene Maine Christos, Subir Sachdev, Mathias S. Scheurer Proceedings of the National Academy of Sciences Nov 2020, 202014691; DOI: 10.1073/pnas.2014691117
Thermoelectric power of Sachdev-Ye-Kitaev islands: Probing Bekenstein-Hawking entropy in quantum matter experiments
Abstract The Sachdev-Ye-Kitaev (SYK) model describes electrons with random and all-to-all interactions and realizes a many-body state without quasiparticle excitations and a nonvanishing extensive entropy S0 in the zero-temperature limit. Its low-energy theory coincides with the low-energy theory of near-extremal charged black holes with Bekenstein-Hawking entropy S0.
Gauge theories for the thermal Hall effect
Abstract We consider the thermal Hall effect of fermionic matter coupled to emergent gauge fields in 2+1 dimensions. While the low-temperature thermal Hall conductivity of bulk topological phases can be connected to chiral edge states and a gravitational anomaly, there is no such interpretation at nonzero temperatures above 2+1-dimensional quantum critical points.
Viewpoint: Graphene Reveals Its Strange Side
Subir Sachdev, Department of Physics, Harvard University, Cambridge, MA, USA February 18, 2020• Physics 13, 20 Experiments on magic-angle graphene reveal a “strange metal” phase and transport behavior consistent with so-called Planckian dissipation. Figure 1: The resistivity of magic-angle graphene—under certain conditions—increases linearly with temperature.
String Theory Helps to Explain Quantum Phases of Matter
Newly discovered states of matter embody what Einstein called “spooky action at a distance.” They defy explanation, but lately answers have come from a seemingly unrelated corner of physics: string theory Several years ago I found myself where I would never have expected: at a conference of string theorists. My own field is condensed matter: the study of materials such as metals and superconductors, which we cool in the laboratory to temperatures near absolute zero.
Holographic Quantum Matter
Overview Author(s) Praise Summary A comprehensive overview of holographic methods in quantum matter, written by pioneers in the field. This book, written by pioneers in the field, offers a comprehensive overview of holographic methods in quantum matter. It covers influential developments in theoretical physics, making the key concepts accessible to researchers and students in both high energy and condensed matter physics.
[1705.07896] Thermal diffusivity and chaos in metals without quasiparticles
Abstract: We study the thermal diffusivity $D_T$ in models of metals without quasiparticle excitations (`strange metals'). The many-body quantum chaos and transport properties of such metals can be efficiently described by a holographic representation in a gravitational theory in an emergent curved spacetime with an additional spatial dimension.
Emergent gauge fields and the high-temperature superconductors
AbstractThe quantum entanglement of many states of matter can be represented by electric and magnetic fields, much like those found in Maxwell’s theory. These fields ‘emerge’ from the quantum structure of the many-electron state, rather than being fundamental degrees of freedom of the vacuum. I review basic aspects of the theory of emergent gauge fields in insulators in an intuitive manner.
Quantum phase transitions out of the heavy Fermi liquid
We review recent work on the instability of the heavy Fermi liquid state (FL) of the Kondo lattice towards a magnetic metal in which the local moments are not part of the Fermi sea. Using insights drawn from the theory of deconfined quantum criticality of insulating antiferromagnets, we discuss the possibility of a direct second-order transition between the heavy Fermi liquid and such a magnetic metal.
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