# SIMES Publications

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**"Spectroscopic fingerprint of charge order melting driven by quantum fluctuations in a cuprate"** — W. S. Lee: Ke-Jin J. Zhou, M. Hepting, J. Li, A. Nag, A. C. Walters, M. Garcia-Fernandez, H. C. Robarts, M. Hashimoto, H. Lu, B. Nosarzewski, D. Song, H. Eisaki, Z. X. Shen, B. Moritz, J. Zaanen & T. P. Devereaux; Nature Physics, 08/31/20.

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Ke-Jin J. Zhou, M. Hepting, J. Li, A. Nag, A. C. Walters, M. Garcia-Fernandez, H. C. Robarts, M. Hashimoto, H. Lu, B. Nosarzewski, D. Song, H. Eisaki, Z. X. Shen, B. Moritz, J. Zaanen & T. P. Devereaux

**Abstract**

Copper oxide high-*T*_{C} superconductors possess a number of exotic orders that coexist with or are proximal to superconductivity. Quantum fluctuations associated with these orders may account for the unusual characteristics of the normal state, and possibly affect the superconductivity^{1,2,3,4}. Yet, spectroscopic evidence for such quantum fluctuations remains elusive. Here, we use resonant inelastic X-ray scattering to reveal spectroscopic evidence of fluctuations associated with a charge order^{5,6,7,8,9,10,11,12,13,14} in nearly optimally doped Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}. In the superconducting state, while the quasielastic charge order signal decreases with temperature, the interplay between charge order fluctuations and bond-stretching phonons in the form of a Fano-like interference increases, an observation that is incompatible with expectations for competing orders. Invoking general principles, we argue that this behaviour reflects the properties of a dissipative system near an order–disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations, which melt charge order.

**"Ferromagnetic order beyond the superconducting dome in a cuprate superconductor"** — Tarapada Sarkar: D. S. Wei, J. Zhang, N. R. Poniatowski, P. R. Mandal, A. Kapitulnik, Richard L. Greene; Science, 05/01/20.

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D. S. Wei, J. Zhang, N. R. Poniatowski, P. R. Mandal, A. Kapitulnik, Richard L. Greene

**Abstract**

According to conventional wisdom, the extraordinary properties of the cuprate high-temperature superconductors arise from doping a strongly correlated antiferromagnetic insulator. The highly overdoped cuprates—whose doping lies beyond the dome of superconductivity—are considered to be conventional Fermi liquid metals. We report the emergence of itinerant ferromagnetic order below 4 kelvin for doping beyond the superconducting dome in thin films of electron-doped La_{2–x}Ce_{x}CuO_{4} (LCCO). The existence of this ferromagnetic order is evidenced by negative, anisotropic, and hysteretic magnetoresistance, hysteretic magnetization, and the polar Kerr effect, all of which are standard signatures of itinerant ferromagnetism in metals. This surprising result suggests that the overdoped cuprates are strongly influenced by electron correlations.

**"Disorder-induced suppression of charge density wave order: STM study of Pd-intercalated ErTe**_{3"} — Alan Fang: Joshua A. W. Straquadine, Ian R. Fisher, Steven A. Kivelson, and Aharon Kapitulnik; Physical Review B, 12/24/19.

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Joshua A. W. Straquadine, Ian R. Fisher, Steven A. Kivelson, and Aharon Kapitulnik

**Abstract**

Pd-intercalated ErTe_{3} is studied as a model system to explore the effect of increasing disorder on an incommensurate two-component charge density wave (CDW). The ordering vectors of the CDW components lie along the two in-plane principal axes of the nearly tetragonal crystal structure. Using scanning tunneling microscopy (STM), we show that introducing Pd intercalants (i.e., disorder) induces CDW dislocations, which appear to be associated with each CDW component separately. Increasing Pd concentration has a stronger effect on the secondary CDW order, manifested in a higher density of dislocations, and thus increases the anisotropy (nematic character) of the CDW. Suggestive evidence of Bragg glass phases at weak disorder is also discussed.

**"Thermal diffusivity above the Mott-Ioffe-Regel limit"** — Jiecheng Zhang: Erik D. Kountz, Eli M. Levenson-Falk, Dongjoon Song, Richard L. Greene, and Aharon Kapitulnik; Physical Review B, 12/24/19.

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Erik D. Kountz, Eli M. Levenson-Falk, Dongjoon Song, Richard L. Greene, and Aharon Kapitulnik

**Abstract**

We present high-resolution thermal diffusivity measurements on several near optimally doped electron- and hole-doped cuprate systems in a temperature range that passes through the Mott-Ioffe-Regel limit, above which the quasiparticle picture fails. Our primary observations are that the inverse thermal diffusivity is linear in temperature and can be fitted to *D*^{−1} _{Q} = *aT* + *b*. The slope *a* is interpreted through the Planckian relaxation time *τ* ≈ h¯/*k*_{B}T and a thermal diffusion velocity *v*_{B}, which is close, but larger than the sound velocity. The intercept *b* represents a crossover diffusion constant that separates coherent from incoherent quasiparticles. These observations suggest that both phonons and electrons participate in the thermal transport, while reaching the Planckian limit for relaxation time.

**"Thermalization and possible signatures of quantum chaos in complex crystalline materials"** — Jiecheng Zhang: Erik D. Kountz, Kamran Behnia , and Aharon Kapitulnik; Proceedings of the National Academy of Sciences, 09/12/19.

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Erik D. Kountz, Kamran Behnia , and Aharon Kapitulnik

**Abstract**

Analyses of thermal diffusivity data on complex insulators and on strongly correlated electron systems hosted in similar complex crystal structures suggest that quantum chaos is a good description for thermalization processes in these systems, particularly in the high-temperature regime where the many phonon bands and their interactions dominate the thermal transport. Here we observe that for these systems diffusive thermal transport is controlled by a universal Planckian timescale *τ*∼*ℏ*/*k*_{B}*T* and a unique velocity *v*_{E}. Specifically, *v*_{E}≈*v*_{ph} for complex insulators, and *v*_{ph}≲*v*_{E}≪*v*_{F}v in the presence of strongly correlated itinerant electrons (*v*_{ph} and *v*_{F} are the phonon and electron velocities, respectively). For the complex correlated electron systems we further show that charge diffusivity, while also reaching the Planckian relaxation bound, is largely dominated by the Fermi velocity of the electrons, hence suggesting that it is only the thermal (energy) diffusivity that describes chaos diffusivity.

**"A lower bound to the thermal diffusivity of insulators"** — Kamran Behnia: Aharon Kapitulnik; Journal of Physics: Condensed Matter, 07/11/19.

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Aharon Kapitulnik

**Abstract**

It has been known for decades that thermal conductivity of insulating crystals becomes proportional to the inverse of temperature when the latter is comparable to, or higher than, the Debye temperature. This behavior has been understood as resulting from Umklapp scattering among phonons. We put under scrutiny the magnitude of the thermal diffusion constant in this regime and find that it does not fall below a threshold set by the square of sound velocity times the Planckian time (). The conclusion, based on scrutinizing the ratio in cubic crystals with high thermal resistivity, appears to hold even in glasses where Umklapp events are not conceivable. Explaining this boundary, reminiscent of a recently-noticed limit for charge transport in metals, is a challenge to theory.

**"Colloquium: Anomalous metals: Failed superconductors"** — Aharon Kapitulnik: Steven A. Kivelson, Boris Spivak; Reviews of Modern Physics, 01/28/19.

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Steven A. Kivelson, Boris Spivak

**Abstract**

The observation of metallic ground states in a variety of two-dimensional electronic systems poses a fundamental challenge for the theory of electron fluids. Here evidence is analyzed for the existence of a regime, called the “anomalous metal regime,” in diverse 2D superconducting systems driven through a quantum superconductor to metal transition by tuning physical parameters such as the magnetic field, the gate voltage in the case of systems with a metal-oxide semiconductor field-effect transistor (MOSFET) geometry, or the degree of disorder. The principal phenomenological observation is that in the anomalous metal, as a function of decreasing temperature, the resistivity first drops as if the system were approaching a superconducting ground state, but then saturates at low temperatures to a value that can be orders of magnitude smaller than the Drude value. The anomalous metal also shows a giant positive magnetoresistance. Thus, it behaves as if it were a “failed superconductor.” This behavior is observed in a broad range of parameters. It will be moreover exhibited, by theoretical solution of a model of superconducting grains embedded in a metallic matrix, that as a matter of principle such anomalous metallic behavior can occur in the neighborhood of a quantum superconductor to metal transition. However, it will be also argued that the robustness and ubiquitous nature of the observed phenomena are difficult to reconcile with any existing theoretical treatment and speculate about the character of a more fundamental theoretical framework.

**"Polar Kerr Effect from Time-Reversal Symmetry Breaking in the Heavy-Fermion Superconductor PrOs**_{4}Sb_{12"} — E. M. Levenson-Falk: E. R. Schemm, Y. Aoki, M. B. Maple, and A. Kapitulnik; Physical Review Letters, 05/04/18.

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E. R. Schemm, Y. Aoki, M. B. Maple, and A. Kapitulnik

**Abstract**

We present polar Kerr effect measurements of the filled skutterudite superconductor PrOs_{4}Sb_{12}. Simultaneous ac susceptibility measurements allow us to observe the superconducting transition under the influence of heating from the optical beam. A nonzero Kerr angle *θ*_{K} develops below the superconducting transition, saturating at ∼300 nrad at low temperatures. This result is repeated across several measurements of multiple samples. By extrapolating the measured *θ*_{K}(*T*) to zero optical power, we are able to show that the Kerr angle onset temperature in one set of measurements is consistent with the transition to the *B* phase at *T*_{C}_{2}. We discuss the possible explanations for this result and its impact on the understanding of multiphase and inhomogeneous superconductivity in PrOs_{4}Sb_{12}.

**"Bursting at the seams: Rippled monolayer bismuth on NbSe**_{2"} — Alan Fang: Carolina Adamo, Shuang Jia, Robert J. Cava, Shu-Chun Wu, Claudia Felser and Aharon Kapitulnik; Science Advances, 04/13/18.

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Carolina Adamo, Shuang Jia, Robert J. Cava, Shu-Chun Wu, Claudia Felser and Aharon Kapitulnik

**Abstract**

Bismuth, one of the heaviest semimetals in nature, ignited the interest of the materials physics community for its potential impact on topological quantum material systems that use its strong spin-orbit coupling and unique orbital hybridization. In particular, recent theoretical predictions of unique topological and superconducting properties of thin bismuth films and interfaces prompted intense research on the growth of submonolayers to a few monolayers of bismuth on different substrates. Similar to bulk rhombohedral bismuth, the initial growth of bismuth films on most substrates results in buckled bilayers that grow in either the (111) or (110) directions, with a lattice constant close to that of bulk Bi. By contrast, we show a new growth pattern for bismuth monolayers on NbSe_{2}. We find that the initial growth of Bi can form a strongly bonded commensurate layer, resulting in a compressively strained two-dimensional (2D) triangular lattice. We also observed unique pattern of 1D ripples and domain walls is observed. The single layer of bismuth also introduces strong marks on the electronic properties at the surface.

**"Superconductor to weak-insulator transitions in disordered Tantalum Nitride films"** — Nicholas P. Breznay: Mihir Tendulkar, Li Zhang, Sang-Chul Lee, and Aharon Kapitulnik; Phys Rev B, 10/31/17.

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Mihir Tendulkar, Li Zhang, Sang-Chul Lee, and Aharon Kapitulnik

**Abstract**

We study the two-dimensional superconductor-insulator transition (SIT) in thin films of tantalum nitride. At zero magnetic field, films can be disorder-tuned across the SIT by adjusting thickness and film stoichiometry; insulating films exhibit classical hopping transport. Superconducting films exhibit a magnetic-field-tuned SIT, whose insulating ground state at high field appears to be a quantum-corrected metal. Scaling behavior at the field-tuned SIT shows classical percolation critical exponents *zν* ≈ 1.3, with a corresponding critical field *H*_{c} << *H*_{c}_{2}, the upper critical field. The Hall effect exhibits a crossing point near *H*_{c}, but with a nonuniversal critical value *ρ*^{c}_{xy} comparable to the normal-state Hall resistivity. We propose that high-carrier-density metals will always exhibit this pattern of behavior at the boundary between superconducting and (trivially) insulating ground states.