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1. Strong pairing in two dimensions: pseudogaps, domes, and other implications

   https://doi.org/10.1088/1367-2630/ab890b  

   Wang, Xiaoyu ; Chen, Qijin ; Levin, K. ( June 2020 , New Journal of Physics)

   This paper addresses the transition from the normal to the superfluid state in strongly correlated two dimensional fermionic superconductors and Fermi gases. We arrive at the Berezinskii–Kosterlitz–Thouless (BKT) temperatureT_BKTas a function ofattractivepairing strength by associating it with the onset of ‘quasi-condensation’ in the normal phase. Our approach builds on a criterion for determining the BKT transition temperature for atomic gases which is based on a well established quantum Monte Carlo analysis of the phase space density. This latter quantity, when derived from BCS–BEC crossover theory for fermions, leads to non-monotonic behavior forT_BKTas a function of the attractive interaction or inverse scattering length. In Fermi gases, this implies a robust superconducting dome followed by a long tail from the flat BEC asymptote, rather similar to what is observed experimentally. For lattice systems we find thatT_BKThas an absolute maximum of the order of 0.1E_F. We discuss how our results compare with those derived from the Nelson–Kosterlitz criterion based on the mean field superfluid density and the approach to the transition from below. While there is agreement in the strict mean-field BCS regime at weak coupling, we find that at moderate pairing strength bosonic excitations cause a substantial increase inT_BKTfollowed by an often dramatic decrease before the system enters the BEC regime.

    

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2. Anisotropic Properties of Quasi‐1D In 4 Se 3 : Mechanical Exfoliation, Electronic Transport, and Polarization‐Dependent Photoresponse

   https://doi.org/10.1002/adfm.202106459  

   Vorobeva, Nataliia S. ; Lipatov, Alexey ; Torres, Angel ; Dai, Jun ; Abourahma, Jehad ; Le, Duy ; Dhingra, Archit ; Gilbert, Simeon J. ; Galiy, Pavlo V. ; Nenchuk, Taras M. ; et al ( September 2021 , Advanced Functional Materials)

   Theoretical and experimental investigations of various exfoliated samples taken from layered In₄Se₃crystals are performed. In spite of the ionic character of interlayer interactions in In₄Se₃and hence much higher calculated cleavage energies compared to graphite, it is possible to produce few‐nanometer‐thick flakes of In₄Se₃by mechanical exfoliation of its bulk crystals. The In₄Se₃flakes exfoliated on Si/SiO₂have anisotropic electronic properties and exhibit field‐effect electron mobilities of about 50 cm² V⁻¹ s⁻¹at room temperature, which are comparable with other popular transition metal chalcogenide (TMC) electronic materials, such as MoS₂and TiS₃. In₄Se₃devices exhibit a visible range photoresponse on a timescale of less than 30 ms. The photoresponse depends on the polarization of the excitation light consistent with symmetry‐dependent band structure calculations for the most expectedaccleavage plane. These results demonstrate that mechanical exfoliation of layered ionic In₄Se₃crystals is possible, while the fast anisotropic photoresponse makes In₄Se₃a competitive electronic material, in the TMC family, for emerging optoelectronic device applications.

    

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3. Two superconducting states with broken time-reversal symmetry in FeSe 1−x S x

   https://doi.org/10.1073/pnas.2208276120  

   Matsuura, Kohei ; Roppongi, Masaki ; Qiu, Mingwei ; Sheng, Qi ; Cai, Yipeng ; Yamakawa, Kohtaro ; Guguchia, Zurab ; Day, Ryan P. ; Kojima, Kenji M. ; Damascelli, Andrea ; et al ( May 2023 , Proceedings of the National Academy of Sciences)

   Iron-chalcogenide superconductors FeSe_1−xS_xpossess unique electronic properties such as nonmagnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an ultranodal pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here, we report muon spin relaxation (μSR) measurements in FeSe_1−xS_xsuperconductors for0≤x≤0.22covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperatureT_cfor all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-fieldμSR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase (x>0.17). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The TRS breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe_1−xS_x, which calls for the theory of microscopic origins that account for the relation between nematicity and superconductivity.

    

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4. Spatial Mapping of Hot‐Spots at Lateral Heterogeneities in Monolayer Transition Metal Dichalcogenides

   https://doi.org/10.1002/adma.201808244  

   Yasaei, Poya ; Murthy, Akshay A. ; Xu, Yaobin ; dos Reis, Roberto ; Shekhawat, Gajendra S. ; Dravid, Vinayak P. ( April 2019 , Advanced Materials)

   Lateral heterogeneities in atomically thin 2D materials such as in‐plane heterojunctions and grain boundaries (GBs) provide an extrinsic knob for manipulating the properties of nano‐ and optoelectronic devices and harvesting novel functionalities. However, these heterogeneities have the potential to adversely affect the performance and reliability of the 2D devices through the formation of nanoscopic hot‐spots. In this report, scanning thermal microscopy (SThM) is utilized to map the spatial distribution of the temperature rise within monolayer transition metal dichalcogenide (TMD) devices upon dissipating a high electrical power through a lateral interface. The results directly demonstrate that lateral heterojunctions between MoS₂and WS₂do not largely impact the distribution of heat dissipation, while GBs of MoS₂appreciably localize heating in the device. High‐resolution scanning transmission electron microscopy reveals that the atomic structure is nearly flawless around heterojunctions but can be quite defective near GBs. The results suggest that the interfacial atomic structure plays a crucial role in enabling uniform charge transport without inducing localized heating. Establishing such structure–property‐processing correlation provides a better understanding of lateral heterogeneities in 2D TMD systems which is crucial in the design of future all‐2D electronic circuitry with enhanced functionalities, lifetime, and performance.

    

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5. 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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