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Register a new userA strange metal in a bosonic system
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Fermi liquid theory forms the basis for our understanding of the majority of metals, which is manifested in the description of transport properties that the electrical resistivity goes as temperature squared in the limit of zero temperature. However, the observations of strange metal states in various quantum materials, notably high-temperature superconductors, bring this spectacularly successful theoretical framework into crisis. When electron scattering rate 1/{tau} hits its limit, kBT/{hbar} where {hbar} is the reduced Plancks constant, T represents absolute temperature and kB denotes Boltzmanns constant, Planckian dissipation occurs and lends strange metals a surprising link to black holes, gravity, and quantum information theory. Here, we show the characteristic signature of strange metallicity arising unprecedentedly in a bosonic system. Our nanopatterned YBa2Cu3O7-{delta}(YBCO) film arrays reveal T-linear resistance as well as B-linear magnetoresistance over an extended temperature and magnetic field range in a quantum critical region in the phase diagram. Moreover, the slope of the T-linear resistance {alpha}_cp appears bounded by {alpha}_cp {approx} h/2e^2 [1/T]_c^onset where T_c^onset is the temperature at which Cooper pairs form, intimating a common scale-invariant transport mechanism corresponding to Planckian dissipation.In contrast to fermionic systems where the temperature and magnetic field dependent scattering rates combine in quadrature of {hbar}/{tau} {approx} {sqrt} (((k_B T)^2+({mu}_B B)^2)), both terms linearly combine in the present bosonic system, i.e. {hbar}/{tau} {approx} (k_B T+[{gamma}{mu}]_B B), where {gamma} is a constant. By extending the reach of strange metal phenomenology to a bosonic system, our results suggest that there is a fundamental principle governing their transport which transcends particle statistics.
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Some of the highest-transition-temperature superconductors across various materials classes exhibit linear-in-temperature `strange metal or `Planckian electrical resistivities in their normal state. It is thus believed by many that this behavior holds the key to unlock the secrets of high-temperature superconductivity. However, these materials typically display complex phase diagrams governed by various competing energy scales, making an unambiguous identification of the physics at play difficult. Here we use electrical resistivity measurements into the micro-Kelvin regime to discover superconductivity condensing out of an extreme strange metal state -- with linear resistivity over 3.5 orders of magnitude in temperature. We propose that the Cooper pairing is mediated by the modes associated with a recently evidenced dynamical charge localization-delocalization transition, a mechanism that may well be pertinent also in other strange metal superconductors.
The breakdown of the celebrated Fermi liquid theory in the strange metal phase is the central enigma of correlated quantum matter. Motivated by recent experiments reporting short-lived carriers, along with the ubiquitous observations of modulated excitations in the phase diagram of cuprates, we propose a model for this phase. We introduce bosons emerging from the remnants of a pair density wave as additional current carriers in the strange metal phase. These bosonic excitations are finite momentum Cooper pairs and thus carry twice the electronic charge, and its net spin can either be zero or one arising from the two spin-$1/2$ electrons. We show that such a model can capture the famous linear relationship of resistivity with temperature and manifests the Drude form of ac-conductivity with a Planckian dissipation rate. Furthermore, such bosons are incoherent and hence do not contribute to the Hall conductivity. The bosons emerging from the electron pairs of spin-triplet symmetry also reproduce the recently observed linear in-field magnetoresistance [P. Giraldo-Gallo et al., Science 361, 479 (2018); J. Ayres et al., arXiv: 2012.01208 (2020)].
We performed scanning tunneling spectroscopic experiments on hole-doped NdBa$_2$Cu$_3$O$_{7-delta}$. The d$I$/d$V$ curves obtained at 4.2 K are asymmetric with clear peak-dip and hump structures. Energy derivatives of these curves show peaks at energies beyond the dip features. Highly precise full potential bandstructure calculations confirm a featureless electronic density of states in that energy region. Our results indicate that tunneling electrons couple to a collective mode in the CuO$_2$ plane.
The normal state of cuprates is dominated by the strange metal phase that, near optimal doping, shows a linear temperature dependence of the resistivity persisting down to the lowest $T$, when superconductivity is suppressed. For underdoped cuprates this behavior is lost below the pseudogap temperature $T$*, where Charge Density Waves(CDW) together with other intertwined local orders characterize the ground state. Here we show that the $T$-linear resistivity of highly strained, ultrathin and underdoped YBa$_2$Cu$_3$O$_{7-delta}$ films is restored when the CDW amplitude, detected by Resonant Inelastic X-ray scattering, is suppressed. This observation points towards an intimate connection between the onset of CDW and the departure from $T$-linear resistivity in underdoped cuprates, a link that was missing until now. It also illustrates the potentiality of strain control to manipulate the ground state of quantum materials.
Anomalous metallic behavior, marked by a saturating finite resistivity much lower than the Drude estimate, has been observed in a wide range of two-dimensional superconductors. Utilizing the electrostatically gated LaAlO3/SrTiO3 interface as a versatile platform for superconductor-metal quantum phase transitions, we probe variations in the gate, magnetic field, and temperature to construct a phase diagram crossing from superconductor, anomalous metal, vortex liquid, to Drude metal states, combining longitudinal and Hall resistivity measurements. We find that the anomalous metal phases induced by gating and magnetic field, although differing in symmetry, are connected in the phase diagram and exhibit similar magnetic field response approaching zero temperature. Namely, within a finite regime of the anomalous metal state, the longitudinal resistivity linearly depends on field while the Hall resistivity diminishes, indicating an emergent particle-hole symmetry. The universal behavior highlights the uniqueness of the quantum bosonic metallic state, distinct from bosonic insulators and vortex liquids.
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