Singular charge fluctuations at a magnetic quantum critical point

Abstract

Strange metal behavior is ubiquitous in correlated materials ranging from cuprate superconductors to bilayer graphene. There is increasing recognition that it arises from physics beyond the quantum fluctuations of a Landau order parameter which, in quantum critical heavy fermion antiferromagnets, may be realized as critical Kondo entanglement of spin and charge. The dynamics of the associated electronic delocalization transition could be ideally probed by optical conductivity, but experiments in the corresponding frequency and temperature ranges have remained elusive. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh 2 Si 2 , a model strange metal compound. We observe frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery implicates critical charge fluctuations as playing a central role in the strange metal behavior, thereby elucidating one of the longstanding mysteries of correlated quantum matter. arXiv:1808.02296v1 [cond-mat.str-el]

Terahertz conductivity of heavy-fermion systems from time-resolved spectroscopy

“…Our values of the scaling exponents are consistent, within experimental accuracy, with the values extracted in Ref. [37] for YbRh 2 Si 2 . The ω/T scaling points to the fact that the Kondo-breakdown scenario seems to be valid in the CeCu 6−x Au x system, in agreement with our earlier findings for the Kondo spectral-weight breakdown [28,29].…”

“…Near a Kondo-breakdown QCP, when the low-energy Fermi liquid scale T * vanishes, dynamical response quantities should obey ω/T scaling of the form σ (ω, T ) ∝ T −α f (hω/k B T ) with a universal scaling function f (x). This was theoretically expected [43,44] and experimentally found not only in CeCu 5.9 Au 0.1 for the dynamic magnetic response [31], but also more recently in YbRh 2 Si 2 for the optical conductivity [37]. Note that T * is to be distinguished from the Kondo lattice temperature T * K characterizing the onset of Kondo spin-screening and heavy quasiparticle formation.…”

“…The fit was pinned to the data at the highest frequency, ignoring all spectral features between 1.0 to 1.8 THz, to illustrate the physically meaningful situation that develops at low frequencies, i.e., ν 0.75 THz. The DC value of the optical conductivity is obtained from the extrapolation of the finite-frequency results to zero frequency [24,25,37]. Because of the large overall systematic error in the absolute value of the DC conductivity and the scattering rate, we revert to relative values for their temperature dependence by taking the ratio with respect to the high temperature values (here, with T = 145 K for CeCu 6 ) in the ensuing discussions.…”

“…4(a) and 4(b)] shows a very similar hightemperature Drude behavior as observed in the case of the heavy-fermion compound, CeCu 6 . However, for T < 10 K the low-frequency part of the conductivity shows deviation from the Drude behavior that presumably reflects the onset of the non-Fermi-liquid behavior [37,42] near the quantumcritical point [see the yellow-shaded region in Fig. 4(b)].…”

Quantum Gravity Emergence from Entanglement in a Multi-Fold Universe

“…This is what is sometimes called quantum matter [141], defined as forms of matter where the effects of entanglement are manifest on the macroscopic scale, and with entirely different physical properties than when no macroscopic entanglement takes place [141]. Quantum matter covers superconductors, superfluids, Einstein Bose condensate, strange metals [285] etc. A list can be found in [141].…”