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1. Spin independence of the strongly enhanced effective mass in ultra-clean SiGe/Si/SiGe two-dimensional electron system

   https://doi.org/10.1038/s41598-023-44580-y  

   Melnikov, M. Yu. ; Shakirov, A. A. ; Shashkin, A. A. ; Huang, S. H. ; Liu, C. W. ; Kravchenko, S. V. ( October 2023 , Scientific Reports)

   The effective mass at the Fermi level is measured in the strongly interacting two-dimensional (2D) electron system in ultra-clean SiGe/Si/SiGe quantum wells in the low-temperature limit in tilted magnetic fields. At low electron densities, the effective mass is found to be strongly enhanced and independent of the degree of spin polarization, which indicates that the mass enhancement is not related to the electrons’ spins. The observed effect turns out to be universal for silicon-based 2D electron systems, regardless of random potential, and cannot be explained by existing theories.

    

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2. Coexistence of metallic and nonmetallic properties in the pyrochlore Lu2Rh2O7

   https://doi.org/10.1038/s41535-019-0148-1  

   Hallas, Alannah M. ; Sharma, Arzoo Z. ; Mauws, Cole ; Chen, Qiang ; Zhou, Haidong D. ; Ding, Cui ; Gong, Zizhou ; Tachibana, Makoto ; Sarte, Paul M. ; Attfield, J. Paul ; et al ( March 2019 , npj Quantum Materials)

   Transition metal oxides of the 4dand 5dblock have recently become the targets of materials discovery, largely due to their strong spin–orbit coupling that can generate exotic magnetic and electronic states. Here, we report the high-pressure synthesis of Lu₂Rh₂O₇, a new cubic pyrochlore oxide based on 4d⁵Rh⁴⁺, and characterizations via thermodynamic, electrical transport, and muon spin relaxation measurements. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetic contribution, while heat capacity shows an enhanced Sommerfeld coefficient,γ = 21.8(1) mJ/mol-Rh K². Muon spin relaxation measurements confirm that Lu₂Rh₂O₇remains paramagnetic down to 2 K. Taken in combination, these three measurements suggest that Lu₂Rh₂O₇is a correlated paramagnetic metal with a Wilson ratio ofR_W = 2.5. However, electric transport measurements present a striking contradiction as the resistivity of Lu₂Rh₂O₇is observed to monotonically increase with decreasing temperature, indicative of a nonmetallic state. Furthermore, although the magnitude of the resistivity is that of a semiconductor, the temperature dependence does not obey any conventional form. Thus, we propose that Lu₂Rh₂O₇may belong to the same novel class of non-Fermi liquids as the nonmetallic metal FeCrAs.

    

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3. Critical metallic phase in the overdoped random t - J model

   https://doi.org/10.1073/pnas.2206921119  

   Christos, Maine ; Joshi, Darshan G. ; Sachdev, Subir ; Tikhanovskaya, Maria ( July 2022 , Proceedings of the National Academy of Sciences)

   We investigate a model of electrons with random and all-to-all hopping and spin exchange interactions, with a constraint of no double occupancy. The model is studied in a Sachdev–Ye–Kitaev-like large-Mlimit with SU(M) spin symmetry. The saddle-point equations of this model are similar to approximate dynamic mean-field equations of realistic, nonrandom,t-Jmodels. We use numerical studies on both real and imaginary frequency axes, along with asymptotic analyses, to establish the existence of a critical non–Fermi-liquid metallic ground state at large doping, with the spin correlation exponent varying with doping. This critical solution possesses a time-reparameterization symmetry, akin to Sachdev–Ye–Kitaev (SYK) models, which contributes a linear-in-temperature resistivity over the full range of doping where the solution is present. It is therefore an attractive mean-field description of the overdoped region of cuprates, where experiments have observed a linear-Tresistivity in a broad region. The critical metal also displays a strong particle–hole asymmetry, which is relevant to Seebeck coefficient measurements. We show that the critical metal has an instability to a low-doping spin-glass phase and compute a critical doping value. We also describe the properties of this metallic spin-glass phase.

    

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4. Quantum oscillations of electrical resistivity in an insulator

   https://doi.org/10.1126/science.aap9607  

   Xiang, Z. ; Kasahara, Y. ; Asaba, T. ; Lawson, B. ; Tinsman, C. ; Chen, Lu ; Sugimoto, K. ; Kawaguchi, S. ; Sato, Y. ; Li, G. ; et al ( October 2018 , Science)

   In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here, we report a notable exception in an insulator—ytterbium dodecaboride (YbB₁₂). The resistivity of YbB₁₂, which is of a much larger magnitude than the resistivity in metals, exhibits distinct quantum oscillations. These unconventional oscillations arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals with a large effective mass. Quantum oscillations in the magnetic torque are also observed, albeit with a lighter effective mass.

    

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5. Magnetic field reveals vanishing Hall response in the normal state of stripe-ordered cuprates

   https://doi.org/10.1038/s41467-021-24000-3  

   Shi, Zhenzhong ; Baity, P. G. ; Terzic, J. ; Pokharel, Bal K. ; Sasagawa, T. ; Popović, Dragana ( December 2021 , Nature Communications)

   null (Ed.)

   Abstract The origin of the weak insulating behavior of the resistivity, i.e. $${\rho }_{xx}\propto {\mathrm{ln}}\,(1/T)$$ ρ x x ∝ ln ( 1 / T ) , revealed when magnetic fields ( H ) suppress superconductivity in underdoped cuprates has been a longtime mystery. Surprisingly, the high-field behavior of the resistivity observed recently in charge- and spin-stripe-ordered La-214 cuprates suggests a metallic, as opposed to insulating, high-field normal state. Here we report the vanishing of the Hall coefficient in this field-revealed normal state for all $$T\ <\ (2-6){T}_{{\rm{c}}}^{0}$$ T < ( 2 − 6 ) T c 0 , where $${T}_{{\rm{c}}}^{0}$$ T c 0 is the zero-field superconducting transition temperature. Our measurements demonstrate that this is a robust fundamental property of the normal state of cuprates with intertwined orders, exhibited in the previously unexplored regime of T and H . The behavior of the high-field Hall coefficient is fundamentally different from that in other cuprates such as YBa 2 Cu 3 O 6+ x and YBa 2 Cu 4 O 8 , and may imply an approximate particle-hole symmetry that is unique to stripe-ordered cuprates. Our results highlight the important role of the competing orders in determining the normal state of cuprates. 

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