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Jiao, Lin,Chen, Ye,Kohama, Yoshimitsu,Graf, David,Bauer, E. D.,Singleton, John,Zhu, Jian-Xin,Weng, Zongfa,Pang, Guiming,Shang, Tian,Zhang, Jinglei,Lee, Han-Oh,Park, Tuson,Jaime, Marcelo,Thompson, J. D National Academy of Sciences 2015 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.112 No.3
<P><B>Significance</B></P><P>Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of microscopic details of a specific material. An analogous description is lacking for phase transitions that are driven at absolute zero temperature by a nonthermal control parameter. Classification of quantum-driven phase transitions is a fundamental but open problem that arises in diverse contexts and multiple classes of materials. Here we report the first observation, to our knowledge, of a sharp Fermi surface reconstruction while applying a strong magnetic field to suppress an antiferromagnetic transition to zero temperature. These experiments demonstrate that direct measurements of the Fermi surface can distinguish theoretically proposed models of quantum criticality and point to a universal description of quantum phase transitions.</P><P>Conventional, thermally driven continuous phase transitions are described by universal critical behavior that is independent of the specific microscopic details of a material. However, many current studies focus on materials that exhibit quantum-driven continuous phase transitions (quantum critical points, or QCPs) at absolute zero temperature. The classification of such QCPs and the question of whether they show universal behavior remain open issues. Here we report measurements of heat capacity and de Haas–van Alphen (dHvA) oscillations at low temperatures across a field-induced antiferromagnetic QCP (<I>B</I><SUB>c0</SUB> ≈ 50 T) in the heavy-fermion metal CeRhIn<SUB>5</SUB>. A sharp, magnetic-field-induced change in Fermi surface is detected both in the dHvA effect and Hall resistivity at [Formula] ≈ 30 T, well inside the antiferromagnetic phase. Comparisons with band-structure calculations and properties of isostructural CeCoIn<SUB>5</SUB> suggest that the Fermi-surface change at [Formula] is associated with a localized-to-itinerant transition of the Ce-4<I>f</I> electrons in CeRhIn<SUB>5</SUB>. Taken in conjunction with pressure experiments, our results demonstrate that at least two distinct classes of QCP are observable in CeRhIn<SUB>5</SUB>, a significant step toward the derivation of a universal phase diagram for QCPs.</P>
Keizo Murata,Harukazu Yoshino,Tsutomu Nakanishi,Takako Konoike,James Brooks,David Graf,Charles Mielke,George C. Papavassiliou 한국물리학회 2004 Current Applied Physics Vol.4 No.5
In the two-dimensional organic conductor, τ-(EDO-S,S-DMEDT-TTF)2(AuBr2)1+ y, we have observed Shubnikov de Hass oscillations with Landau level down ton ¼ 2 in eld up to 27 T. Motivated with this result, we extended to Hall eect study in pulsed magnetic eld up to 60 T as well as in dc eld up to 45 T and found Hall resistance plateau above 40 T (n ¼ 1). Since this system consists of two two-dimensional Fermi surface pockets, and larger pocket has a large eective mass compared with the smaller one,larger Fermi pocket behaves as a reservoir for the smaller pocket to be well-separated into completely lled and completely emptyLandau levels in a certain range of temperature and magnetic eld. The interpretation of realizing the Hall plateau can beunderstood as a new mechanism for QHE, but is quite dierent neither from localization nor eld induce spin density wave state inorganic TMTSF salts.
Murata, Keizo,Fukumoto, Yuhei,Yokogawa, Keiichi,Kang, Woun,Takaoka, Ryo,Tada, Ryota,Hirayama, H.,Brooks, James S.,Graf, David,Yoshino, Harukazu,Sasaki, Takahiko,Kato, Reizo Elsevier 2015 PHYSICA B-CONDENSED MATTER - Vol.460 No.-
<P><B>Abstract</B></P> <P>We have studied the angular dependence of magnetoresistance and Hall effect of the CDW organic conductor, HMTSF–TCNQ in order to see whether a magnetic-field-induced phase exists in the charge density wave (CDW) system, similarly to the magnetic-field-induced SDW phases in (TMTSF)<SUB>2</SUB>X. The anomaly in magnetoresistance was observed only around the pressure where the CDW is almost suppressed, i.e. around 0.8–1.1GPa, but neither at low pressures (0 and 0.5GPa) nor at high pressure above 2GPa. This behavior is quite similar to that of (TMTSF)<SUB>2</SUB>X. At 1.1GPa anomalies were found at fields of 0.2T and 10T. We speculate that at 1.1GPa the field-induced phase is located between 0.2T and 10T, where 1D Fermi surface sheet and 2D Fermi-surface pocket are present. The <I>R</I> <SUB> <I>xy</I> </SUB> shows plateau structure and <I>R</I> <SUB> <I>xx</I> </SUB> was very small in the same region, suggestive of quantum Hall effect.</P>
Dynamic band-structure tuning of graphene moiré superlattices with pressure
Yankowitz, Matthew,Jung, Jeil,Laksono, Evan,Leconte, Nicolas,Chittari, Bheema L.,Watanabe, K.,Taniguchi, T.,Adam, Shaffique,Graf, David,Dean, Cory R. Nature Publishing Group UK 2018 Nature Vol.557 No.7705
<P>Heterostructures can be assembled from atomically thin materials by combining a wide range of available van der Waals crystals, providing exciting possibilities for designer electronics'. In many cases, beyond simply realizing new material combinations, interlayer interactions lead to emergent electronic properties that are fundamentally distinct from those of the constituent layers'. A critical parameter in these structures is the interlayer coupling strength, but this is often not easy to determine and is typically considered to be a fixed property of the system. Here we demonstrate that we can controllably tune the interlayer separation in van der Waals heterostructures using hydrostatic pressure, providing a dynamic way to modify their electronic properties. In devices in which graphene is encapsulated in boron nitride and aligned with one of the encapsulating layers, we observe that increasing pressure produces a superlinear increase in the moire-superlattice-induced bandgap nearly doubling within the studied range together with an increase in the capacitive gate coupling to the active channel by as much as 25 per cent. Comparison to theoretical modelling highlights the role of atomic-scale structural deformations and how this can be altered with pressure. Our results demonstrate that combining hydrostatic pressure with controlled rotational order provides opportunities for dynamic band-structure engineering in van der Waals heterostructures.</P>