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1. Anomalous quantum criticality in the electron-doped cuprates

   https://doi.org/10.1073/pnas.1817653116  

   Mandal, P. R. ; Sarkar, Tarapada ; Greene, Richard L. ( March 2019 , Proceedings of the National Academy of Sciences)

   In the physics of condensed matter, quantum critical phenomena and unconventional superconductivity are two major themes. In electron-doped cuprates, the low critical field (H_C2) allows one to study the putative quantum critical point (QCP) at low temperature and to understand its connection to the long-standing problem of the origin of the high-T_Csuperconductivity. Here we present measurements of the low-temperature normal-state thermopower (S) of the electron-doped cuprate superconductor La_2−xCe_xCuO₄(LCCO) fromx= 0.11–0.19. We observe quantum critical$\mathit{S}/\mathit{T}$versus$\mathbf{l}\mathbf{n}\left(\mathbf{1}/\mathit{T}\right)$behavior over an unexpectedly wide doping rangex= 0.15–0.17 above the QCP (x= 0.14), with a slope that scales monotonically with the superconducting transition temperature (T_Cwith H = 0). The presence of quantum criticality over a wide doping range provides a window on the criticality. The thermopower behavior also suggests that the critical fluctuations are linked withT_C. Above the superconductivity dome, atx= 0.19, a conventional Fermi-liquid$\mathit{S}\propto \mathit{T}$behavior is found for$\mathit{T}\le$40 K.

    

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2. Quantum-critical scale invariance in a transition metal alloy

   https://doi.org/10.1038/s42005-020-00448-5  

   Nakajima, Yasuyuki ; Metz, Tristin ; Eckberg, Christopher ; Kirshenbaum, Kevin ; Hughes, Alex ; Wang, Renxiong ; Wang, Limin ; Saha, Shanta R. ; Liu, I-Lin ; Butch, Nicholas P. ; et al ( October 2020 , Communications Physics)

   Quantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe_1/3Co_1/3Ni_1/3)₂As₂, indicative of quantum criticality at zero temperature and applied magnetic field. Beyond a linear-in-temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system unhindered by a pairing instability.

    

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3. Precision adiabatic scanning calorimetry of a nematic – ferroelectric nematic phase transition

   https://doi.org/10.1080/02678292.2021.2007550  

   Thoen, Jan ; Korblova, Eva ; Walba, David M. ; Clark, Noel A. ; Glorieux, Christ ( December 2021 , Liquid Crystals)

   In high-resolution adiabatic scanning calorimetry (ASC) experiments, data for the temperature dependence of the specific enthalpy, h(T), and of the specific heat capacity, c(p)(T), are simultaneously obtained, from which the order of the phase transition and critical behaviour can be evaluated. ASC was applied to study the nematic to ferroelectric nematic phase transition (N-N-F) in the liquid crystal molecule 4-[(4-nitrophenoxy)carbonyl]phenyl 2,4-dimethoxybenzoate (RM734). The N-N-F was found to be very weakly first order with a latent heat Delta h = 0.115 +/- 0.005 J/g. The pretransitional specific heat capacity behaviour is substantially larger in the high-temperature N phase than in the low-temperature N-F phase. In both phases the power-law analysis of c(p)(T) resulted in a critical exponent alpha = 0.50 +/- 0.05 and amplitude ratio A(NF)/A(N) = 0.42 +/- 0.03. The very small latent heat and the value of alpha indicate that the N-N-F transition is close to a tricritical point. This is confirmed by a value of the order parameter exponent beta approximate to 0.25, recently obtained from electric polarisation measurements. Invoking two-scale-factor universality, it follows from the low value of A(NF)/A(N) ratio that the size of the critical fluctuations is much larger in the N-F phase than in the N phase. 

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4. Multi-band mass enhancement towards critical doping in a pnictide superconductor

   https://doi.org/10.1038/s41535-018-0144-x  

   Moir, Camilla Margaret ; Riggs, Scott Chandler ; Galvis, Jose Augusto ; Lian, Xiujun ; Giraldo-Gallo, Paula ; Chu, Jiun-Haw ; Walmsley, Philip ; Fisher, Ian Randal ; Shekhter, Arkady ; Boebinger, Gregory Scott ( February 2019 , npj Quantum Materials)

   Near critical doping, high-temperature superconductors exhibit multiple anomalies associated with enhanced electronic correlations and quantum criticality. Quasiparticle mass enhancement approaching optimal doping has been reported in quantum oscillation measurements in both cuprate and pnictide superconductors. Although the data are suggestive of enhanced interactions, the microscopic theory of quantum oscillation measurements near a quantum critical point is not yet firmly established. It is therefore desirable to have a direct thermodynamic measurement of quasiparticle mass. Here we report high-magnetic field measurements of heat capacity in the doped pnictide superconductor BaFe₂(As_1−xP_x)₂. We observe saturation of the specific heat at high magnetic field in a broad doping range above optimal doping which enables a direct determination of the electronic density of states recovered when superconductivity is suppressed. Our measurements find a strong total mass enhancement in the Fermi pockets that superconduct. This mass enhancement extrapolates to a mass divergence at a critical doping ofx = 0.28.

    

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5. Pristine quantum criticality in a Kondo semimetal

   https://doi.org/10.1126/sciadv.abf9134  

   Fuhrman, Wesley T. ; Sidorenko, Andrey ; Hänel, Jonathan ; Winkler, Hannes ; Prokofiev, Andrey ; Rodriguez-Rivera, Jose A. ; Qiu, Yiming ; Blaha, Peter ; Si, Qimiao ; Broholm, Collin L. ; et al ( May 2021 , Science Advances)

   The observation of quantum criticality in diverse classes of strongly correlated electron systems has been instrumental in establishing ordering principles, discovering new phases, and identifying the relevant degrees of freedom and interactions. At focus so far have been insulators and metals. Semimetals, which are of great current interest as candidate phases with nontrivial topology, are much less explored in experiments. Here, we study the Kondo semimetal CeRu 4 Sn 6 by magnetic susceptibility, specific heat, and inelastic neutron scattering experiments. The power-law divergence of the magnetic Grünesien ratio reveals that, unexpectedly, this compound is quantum critical without tuning. The dynamical energy over temperature scaling in the neutron response throughout the Brillouin zone and the temperature dependence of the static uniform susceptibility, indicate that temperature is the only energy scale in the criticality. Such behavior, which has been associated with Kondo destruction quantum criticality in metallic systems, could be generic in the semimetal setting. 

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