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Register a new userUnveiling the role of competing fluctuations at an unconventional quantum critical point
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Quantum critical points (QCPs) are widely accepted as a source of a diverse set of collective quantum phases of matter. A central question is how the order parameters of phases near a QCP interact and determine the fundamental character of the critical dynamics which drive the quantum critical behavior. One of the most interesting proposals for the quantum critical behavior that occurs in correlated electron systems is that the behavior may arise from local, as opposed to long wavelength, critical fluctuations of the order parameter. The local criticality is believed to give rise to energy over temperature ($E/T$) scaling of the dynamic susceptibility with a fractional exponent near the quantum critical point (QCP). Here we show that $E/T$ scaling is indeed observed for CeCu$_{6-x}$Ag$_x$ but on closer inspection, the fluctuations can be separated into two components, implying that multiple order parameters play an important role in the unconventional critical behavior. Additionally, when the fluctuations corresponding to the magnetically ordered side of the phase diagram are separated, they are found to be three dimensional and to obey the scaling behavior expected for long wavelength fluctuations near an itinerant antiferromagnetic QCP.
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We report measurements of in-plane electrical and thermal transport properties in the limit $T rightarrow 0$ near the unconventional quantum critical point in the heavy-fermion metal $beta$-YbAlB$_4$. The high Kondo temperature $T_K$ $simeq$ 200 K in this material allows us to probe transport extremely close to the critical point, at unusually small values of $T/T_K < 5 times 10^{-4}$. Here we find that the Wiedemann-Franz law is obeyed at the lowest temperatures, implying that the Landau quasiparticles remain intact in the critical region. At finite temperatures we observe a non-Fermi liquid T-linear dependence of inelastic scattering processes to energies lower than those previously accessed. These processes have a weaker temperature dependence than in comparable heavy fermion quantum critical systems, and suggest a new temperature scale of $T sim 0.3 K$ which signals a sudden change in character of the inelastic scattering.
Quantum criticality is a central concept in condensed matter physics, but the direct observation of quantum critical fluctuations has remained elusive. Here we present an x-ray diffraction study of the charge density wave (CDW) in 2H-NbSe2 at high pressure and low temperature, where we observe a broad regime of order parameter fluctuations that are controlled by proximity to a quantum critical point. X-rays can track the CDW despite the fact that the quantum critical regime is shrouded inside a superconducting phase, and, in contrast to transport probes, allow direct measurement of the critical fluctuations of the charge order. Concurrent measurements of the crystal lattice point to a critical transition that is continuous in nature. Our results confirm the longstanding expectations of enhanced quantum fluctuations in low dimensional systems, and may help to constrain theories of the quantum critical Fermi surface.
Strange metal behavior is ubiquitous to 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.
Ferropericlase, (Mg,Fe)O is one of the most abundant minerals of the Earths lower mantle. The high-spin (HS) to low-spin (LS) transition in the Fe2+ ions can dramatically alter the physical and chemical properties of (Mg,Fe)O in the deep mantle, thereby changing our understanding of the Earths deep interior. To establish a fundamental understanding of the ground electronic state of iron, the electronic and magnetic states of Fe2+ in (Mg0.75,Fe0.25)O have been investigated by transmission (TMS) and synchrotron (NFS) Mossbauer spectroscopy at high pressures and low temperatures (down to 5 K). The results show that the ground electronic state of Fe2+ at the critical pressure Pc of the spin transition and close to T=0 is determined by a quantum critical point Pq (T = 0, Pc) where the energy difference between the HS and LS states (an energy gap for the spin fluctuation) is zero. The deviation from T=0 leads to the thermal excitation for the HS or LS state, suggesting a strong influence on the magnetic and hence the physical properties of the material. Combining these with theoretical calculations, the results indicate that the existence of the quantum critical point at zero temperature affects not only the low-temperature physical properties, but also the strong temperature/pressure-dependent properties at conditions relevant to the middle layer of the lower mantle.
In multi-band metals quasi-particles arising from different atomic orbitals coexist at a common Fermi surface. Superconductivity in these materials may appear due to interactions within a band (intra-band) or among the distinct metallic bands (inter-band). Here we consider the suppression of superconductivity in the intra-band case due to hybridization. The fluctuations at the superconducting quantum critical point (SQCP) are obtained calculating the response of the system to a fictitious space and time dependent field, which couples to the superconducting order parameter. The appearance of superconductivity is related to the divergence of a generalized susceptibility. For a single band superconductor this coincides with the textit{Thouless criterion}. For fixed chemical potential and large hybridization, the superconducting state has many features in common with breached pair superconductivity with unpaired electrons at the Fermi surface. The T=0 phase transition from the superconductor to the normal state is in the universality class of the density-driven Bose-Einstein condensation. For fixed number of particles and in the strong coupling limit, the system still has an instability to the normal sate with increasing hybridization.
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