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1. Emergent charge order near the doping-induced Mott-insulating quantum phase transition in Sr3Ru2O7

   https://doi.org/10.1038/s42005-019-0138-4  

   Leshen, Justin ; Kavai, Mariam ; Giannakis, Ioannis ; Kaneko, Yoshio ; Tokura, Yoshi ; Mukherjee, Shantanu ; Lee, Wei-Cheng ; Aynajian, Pegor ( March 2019 , Communications Physics)

   Search for novel electronically ordered states of matter emerging near quantum phase transitions is an intriguing frontier of condensed matter physics. In ruthenates, the interplay between Coulomb correlations among the 4delectronic states and their spin-orbit interactions, lead to complex forms of electronic phenomena. Here we investigate the double layered Sr₃(Ru_1−xMn_x)₂O₇and its doping-induced quantum phase transition from a metal to an antiferromagnetic Mott insulator. Using spectroscopic imaging with the scanning tunneling microscope, we visualize the evolution of the electronic states in real- and momentum-space. We find a partial-gap at the Fermi energy that develops with doping to form a weak Mott insulating state. Near the quantum phase transition, we discover a spatial electronic reorganization into a commensurate checkerboard charge order. These findings bear a resemblance to the universal charge order in the pseudogap phase of cuprates and demonstrate the ubiquity of charge order that emanates from doped Mott insulators.
2. Disorder induced power-law gaps in an insulator–metal Mott transition

   https://doi.org/10.1073/pnas.1808056115  

   Wang, Zhenyu ; Okada, Yoshinori ; O’Neal, Jared ; Zhou, Wenwen ; Walkup, Daniel ; Dhital, Chetan ; Hogan, Tom ; Clancy, Patrick ; Kim, Young-June ; Hu, Y. F. ; et al ( October 2018 , Proceedings of the National Academy of Sciences)

   A correlated material in the vicinity of an insulator–metal transition (IMT) exhibits rich phenomenology and a variety of interesting phases. A common avenue to induce IMTs in Mott insulators is doping, which inevitably leads to disorder. While disorder is well known to create electronic inhomogeneity, recent theoretical studies have indicated that it may play an unexpected and much more profound role in controlling the properties of Mott systems. Theory predicts that disorder might play a role in driving a Mott insulator across an IMT, with the emergent metallic state hosting a power-law suppression of the density of states (with exponent close to 1; V-shaped gap) centered at the Fermi energy. Such V-shaped gaps have been observed in Mott systems, but their origins are as-yet unknown. To investigate this, we use scanning tunneling microscopy and spectroscopy to study isovalent Ru substitutions in Sr₃(Ir_1-xRu_x)₂O₇(0 ≤x≤ 0.5) which drive the system into an antiferromagnetic, metallic state. Our experiments reveal that many core features of the IMT, such as power-law density of states, pinning of the Fermi energy with increasing disorder, and persistence of antiferromagnetism, can be understood as universal features of a disordered Mott system near an IMT and suggest that V-shaped gapsmore »may be an inevitable consequence of disorder in doped Mott insulators.

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3. The influence of crystal thickness and interlayer interactions on the properties of heavy ion irradiated MoS ₂

   https://doi.org/10.1088/2053-1583/ab817b  

   Isherwood, Liam H. ; Hennighausen, Zachariah ; Son, Seok-Kyun ; Spencer, Ben F. ; Wady, Paul T. ; Shubeita, Samir M. ; Kar, Swastik ; Casiraghi, Cinzia ; Baidak, Aliaksandr ( May 2020 , 2D Materials)

   Ion irradiation is a versatile tool to introduce controlled defects into two-dimensional (2D) MoS₂on account of its unique spatial resolution and plethora of ion types and energies available. In order to fully realise the potential of this technique, a holistic understanding of ion-induced defect production in 2D MoS₂crystals of different thicknesses is mandatory. X-ray photoelectron spectroscopy, electron diffraction and Raman spectroscopy show that thinner MoS₂crystals are more susceptible to radiation damage caused by 225 keV Xe⁺ions. However, the rate of defect production in quadrilayer and bulk crystals is not significantly different under our experimental conditions. The rate at which S atoms are sputtered as a function of radiation exposure is considerably higher for monolayer MoS₂, compared to bulk crystals, leading to MoO₃formation. P-doping of MoS₂is observed and attributed to the acceptor states introduced by vacancies and charge transfer interactions with adsorbed species. Moreover, the out-of-plane vibrational properties of irradiated MoS₂crystals are shown to be strongly thickness-dependent: in mono- and bilayer MoS₂, the confinement of phonons by defects results in a blueshift of the$A_{1g}$mode. Whereas, a redshift is observed in bulk crystals due to attenuation of the effective restoring forces acting on S atoms caused by vacanciesmore »in adjacent MoS₂layers. Consequently, the$A_{1g}$frequency of tri- and quadrilayer crystals is statistically invariant on account oft competition between phonon confinement effects and interlayer interactions. The$A_{1g}$linewidth is observed to decrease in bi- and trilayer crystals after low dose irradiation and is attributed to layer decoupling. This work shows that there is a complex interplay between defect production, crystal thickness and interlayer interactions in MoS₂. Our results demonstrate that ion irradiation is an effective tool to modulate the electronic, vibrational and structural properties of MoS₂, which may prove beneficial for practical applications.

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4. 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.
5. Doping and temperature evolutions of optical response of Sr3(Ir1-xRux)2O7

   https://doi.org/10.1038/s41598-020-79263-5  

   Ahn, Gihyeon ; Schmehr, J. L. ; Porter, Z. ; Wilson, S. D. ; Moon, S. J. ( December 2020 , Scientific Reports)

   Abstract We report on optical spectroscopic study of the Sr 3 (Ir 1- x Ru x ) 2 O 7 system over a wide doping regime. We find that the changes in the electronic structure occur in the limited range of the concentration of Ru ions where the insulator–metal transition occurs. In the insulating regime, the electronic structure associated with the effective total angular momentum J eff  = 1/2 Mott state remains robust against Ru doping, indicating the localization of the doped holes. Upon entering the metallic regime, the Mott gap collapses and the Drude-like peak with strange metallic character appears. The evolution of the electronic structure registered in the optical data can be explained in terms of a percolative insulator–metal transition. The phonon spectra display anomalous doping evolution of the lineshapes. While the phonon modes of the compounds deep in the insulating and metallic regimes are almost symmetric, those of the semiconducting compound with x  = 0.34 in close proximity to the doping-driven insulator–metal transition show a pronounced asymmetry. The temperature evolution of the phonon modes of the x  = 0.34 compound reveals the asymmetry is enhanced in the antiferromagnetic state. We discuss roles of the S  = 1 spins of the Rumore »ions and charge excitations for the conspicuous lineshape asymmetry of the x  = 0.34 compound.« less