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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.

    

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2. Signature of a randomness-driven spin-liquid state in a frustrated magnet

   https://doi.org/10.1038/s42005-022-00879-2  

   Khatua, J. ; Gomilšek, M. ; Orain, J. C. ; Strydom, A. M. ; Jagličić, Z. ; Colin, C. V. ; Petit, S. ; Ozarowski, A. ; Mangin-Thro, L. ; Sethupathi, K. ; et al ( April 2022 , Communications Physics)

   Collective behaviour of electrons, frustration induced quantum fluctuations and entanglement in quantum materials underlie some of the emergent quantum phenomena with exotic quasi-particle excitations that are highly relevant for technological applications. Herein, we present our thermodynamic and muon spin relaxation measurements, complemented by ab initio density functional theory and exact diagonalization results, on the recently synthesized frustrated antiferromagnet Li₄CuTeO₆, in which Cu²⁺ions (S= 1/2) constitute disordered spin chains and ladders along the crystallographic [101] direction with weak random inter-chain couplings. Our thermodynamic experiments detect neither long-range magnetic ordering nor spin freezing down to 45 mK despite the presence of strong antiferromagnetic interaction between Cu²⁺moments leading to a large effective Curie-Weiss temperature of − 154 K. Muon spin relaxation results are consistent with thermodynamic results. The temperature and magnetic field scaling of magnetization and specific heat reveal a data collapse pointing towards the presence of random-singlets within a disorder-driven correlated and dynamic ground-state in this frustrated antiferromagnet.

    

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3. Spin filtering effect in intrinsic 2D magnetic semiconductor Cr 2 Ge 2 Te 6

   https://doi.org/10.1063/5.0102745  

   Feng, Honglei ; Shi, Gang ; Yan, Dayu ; Li, Yong ; Shi, Youguo ; Xu, Yang ; Xiong, Peng ; Li, Yongqing ( October 2022 , Applied Physics Letters)

   All van der Waals Fe 3 GeTe 2 /Cr 2 Ge 2 Te 6 /graphite magnetic heterojunctions have been fabricated via mechanical exfoliation and stacking, and their magnetotransport properties are studied in detail. At low bias voltages, large negative junction magnetoresistances have been observed and are attributed to spin-conserving tunneling transport across an insulating Cr 2 Ge 2 Te 6 layer. With increasing bias, a crossover to Fowler–Nordheim tunneling takes place. The negative sign of the tunneling magnetoresistance suggests that the bottom of a conduction band in Cr 2 Ge 2 Te 6 belongs to minority spins, opposite to the findings of some first-principles calculations. This work shows that the vdW heterostructures based on 2D magnetic insulators are a valuable platform to gain further insight into spin polarized tunneling transport, which is the basis for pursuing high performance spintronic devices and a large variety of quantum phenomena. 

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4. 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 gaps may be an inevitable consequence of disorder in doped Mott insulators.

    

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5. Intrinsic donor-bound excitons in ultraclean monolayer semiconductors

   https://doi.org/10.1038/s41467-021-21158-8  

   Rivera, Pasqual ; He, Minhao ; Kim, Bumho ; Liu, Song ; Rubio-Verdú, Carmen ; Moon, Hyowon ; Mennel, Lukas ; Rhodes, Daniel A. ; Yu, Hongyi ; Taniguchi, Takashi ; et al ( February 2021 , Nature Communications)

   The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe₂. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellitesʼ photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.

    

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