First-order phase transitions produce abrupt changes to the character of both ground and excited electronic states. Here we conduct electronic compressibility measurements to map the spin phase diagram and Landau level (LL) energies of monolayerin a magnetic field. We resolve a sequence of first-order phase transitions between completely spin-polarized LLs and states with LLs of both spins. Unexpectedly, the LL gaps are roughly constant over a wide range of magnetic fields below the transitions, which we show reflects spin-polarized ground states with opposite spin excitations. These transitions also extend into compressible regimes, with a sawtooth boundary between full and partial spin polarization. We link these observations to the important influence of LL filling on the exchange energy beyond a smooth density-dependent contribution. Our results show thatrealizes a unique hierarchy of energy scales where such effects induce reentrant magnetic phase transitions tuned by density and magnetic field.
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Published by the American Physical Society 2024 Free, publicly-accessible full text available August 1, 2025 -
Many seemingly contradictory experimental findings concerning the superconducting state in Sr2RuO4 can be accounted for on the basis of a conjectured accidental degeneracy between two patterns of pairing that are unrelated to each other under the (D4h) symmetry of the crystal: a dx2-y2-wave (B1g) and a gxy(x2-y2)-wave (A2g) superconducting state. In this paper, we propose a generic multiband model in which the g-wave pairing involving the xz and yz orbitals arises from second-nearest-neighbor BCS channel effective interactions. Even if timereversal symmetry is broken in a d + ig state, such a superconductor remains gapless with a Bogoliubov Fermi surface that approximates a (vertical) line node. The model gives rise to a strain-dependent splitting between the critical temperature Tc and the time-reversal symmetry-breaking temperature TTRSB that is qualitatively similar to some of the experimental observations in Sr2RuO4.more » « less
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We study multi-valley electron gases in the low density (rs ≫ 1) limit. Here the ground-state is always a Wigner crystal (WC), with additional pseudo-spin order where the pseudo-spins are related to valley occupancies. Depending on the symmetries of the host semiconductor and the values of the parameters such as the anisotropy of the effective mass tensors, we find a striped or chiral pseudo-spin antiferromagnet, or a time-reversal symmetry breaking orbital loop-current ordered pseudo-spin ferromagnet. Our theory applies to the recently-discovered WC states in AlAs and in mono and bilayer transition metal dichalcogenides. We identify a set of interesting electronic liquid crystalline phases that could arise by continuous quantum melting of such WCs.
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We study a simple electron-phonon model on square and triangular versions of the Lieb lattice using an asymptotically exact strong coupling analysis. At zero temperature and electron density n 1/4 1 (one electron per unit cell), for various ranges of parameters in the model, we exploit a mapping to the quantum dimer model to establish the existence of a spin-liquid phase with Z(2) topological order (on the triangular lattice) and a multicritical line corresponding to a quantum critical spin liquid (on the square lattice). In the remaining part of the phase diagram, we find a host of charge-density-wave phases (valence-bond solids), a conventional s-wave superconducting phase, and with the addition of a small Hubbard U to tip the balance, a phonon-induced d-wave superconducting phase. Under a special condition, we find a hidden pseudospin SUo2 thorn symmetry that implies an exact constraint on the superconducting order parameters.more » « less
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Abstract We examine key aspects of the theory of the Bardeen–Cooper–Schrieffer (BCS) to Bose–Einstein condensation (BEC) crossover, focusing on the temperature dependence of the chemical potential,
μ . We identify an accurate method of determining the change ofμ in the cuprate high temperature superconductors from angle-resolved-photoemission data (along the ‘nodal’ direction), and show thatμ varies by less than a few percent of the Fermi energy over a range of temperatures from far below to several times above the superconducting transition temperature,T c . This shows, unambiguously, that not only are these materials always on the BCS side of the crossover (which is a phase transition in thed -wave case), but are nowhere near the point of the crossover (where the chemical potential approaches the band bottom). -
Perturbative considerations account for the properties of conventional metals, including the range of temperatures where the transport scattering rate is 1/ τ tr = 2 π λ T , where λ is a dimensionless strength of the electron–phonon coupling. The fact that measured values satisfy λ ≲ 1 has been noted in the context of a possible “Planckian” bound on transport. However, since the electron–phonon scattering is quasielastic in this regime, no such Planckian considerations can be relevant. We present and analyze Monte Carlo results on the Holstein model which show that a different sort of bound is at play: a “stability” bound on λ consistent with metallic transport. We conjecture that a qualitatively similar bound on the strength of residual interactions, which is often stronger than Planckian, may apply to metals more generally.more » « less
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Abstract What limits the value of the superconducting transition temperature ( T c ) is a question of great fundamental and practical importance. Various heuristic upper bounds on T c have been proposed, expressed as fractions of the Fermi temperature, T F , the zero-temperature superfluid stiffness, ρ s (0), or a characteristic Debye frequency, ω 0 . We show that while these bounds are physically motivated and are certainly useful in many relevant situations, none of them serve as a fundamental bound on T c . To demonstrate this, we provide explicit models where T c / T F (with an appropriately defined T F ), T c / ρ s (0), and T c / ω 0 are unbounded.more » « less