BEGIN:VCALENDAR
VERSION:2.0
METHOD:PUBLISH
CALSCALE:GREGORIAN
PRODID:-//WordPress - MECv6.4.2//EN
X-ORIGINAL-URL:https://www.lps.u-psud.fr/
X-WR-CALNAME:Laboratoire de physique des Solides
X-WR-CALDESC:
REFRESH-INTERVAL;VALUE=DURATION:PT1H
X-PUBLISHED-TTL:PT1H
X-MS-OLK-FORCEINSPECTOROPEN:TRUE
BEGIN:VEVENT
CLASS:PUBLIC
DTSTART;TZID=Europe/Paris:20261109T140000
DTEND;TZID=Europe/Paris:20261109T180000
DTSTAMP:20261106T105100
UID:MEC-94aada62f90dd50a84ca74304563d5db@lps.u-psud.fr
CREATED:20261106
LAST-MODIFIED:20261106
PRIORITY:5
TRANSP:OPAQUE
SUMMARY:Matthieu Bard : Orbital susceptibility and non-reciprocal magneto-transport as probes of topology: Graphene bilayer and Bismuth nanowires
DESCRIPTION:\nIn solid state systems, most often magnetism is due to the spin behavior of electrons. But in materials with negligible spin contribution, the orbital motion of delocalized electrons can also induce a significant amount of magnetism. This magnetism can be directly linked to the electronic properties of the system, such as topology or the geometry of the electronic bands and states. The different terms contributing to the orbital susceptibility can be decomposed into into inter-band and intra-band terms. The interband term, or Landau Peierls, is determined by the single band dispersion, namely its curvatures. The inter-band effects depend on the Bloch wavefunctions of the bands and can be written as the sum of a Berry curvature term and a quantum geometry term. Our group has previously measured the orbital diamagnetism of monolayer graphene. In a hBN/ graphene heterostructure, we also previously measured orbital paramagnetism due to saddle points in the moiré band structure. This thesis focuses on Bernal bilayer graphene, whose band structure can be modulated with an electric field. In particular, an adjustable bandgap can be induced, with interesting interband orbital magnetic features that can be tested. For instance, a non-zero diamagnetism is expected in the insulating bandgap, devoid of electronic states. Moreover, at the edges of the gap, the Berry curvature should cause a divergent paramagnetism. We measure this orbital magnetism using detectors based on the giant magnetoresistance effect. These detectors transform a small magnetic field into a measurable electric signal and allow to reach a precision of 1 nT. Bilayer graphene is encapsulated between two hBN flakes and two graphite gates. In this setup, a transverse electric field can be applied and modified and the bilayer chemical potential can be moved. Transport measurements were used to follow the opening of the gap, but they also revealed the presence in our samples of AB/BA boundary states, which tend to weaken the gap. At the charge neutrality point, we find a diamagnetic peak, which depends on the electric field. When the gap is closed, its amplitude and shape correspond to the expected theory. When the bilayer band structure is gapped by an electric field, we observe a different behavior than theoretically expected, with paramagnetic contributions that still need to be understood. Beyond orbital magnetism of 2D materials, GMR sensors can also be used to measure current through fabricated mesoscopic rings. A flux periodic orbital magnetization was achieved with an AC SQUID. However, the strong dependence of the signal observed with the GMR bias still needs to be investigated.\n\n\n\nIn the second part of my PhD, we investigate nonreciprocal magnetotransport in crystalline bismuth nanowires connected to superconducting electrodes, both in the normal state and in the proximity-induced superconducting state In the induced superconducting state, non-reciprocity takes the form of a Josephson diode effect, a different critical current for positive and negative current bias. In the normal state, non-reciprocity takes the form of a bilinear magnetoresistance, linear in both current and magnetic field. We measured these two effects in the same device and found that both nonreciprocal responses display closely matching dependence on the magnetic field orientation, including a sign change at the same in-plane field angle. This suggests a common origin for both responses: a current-induced spin polarization along a well-defined crystalline axis. The crystalline orientation of the nanowire was determined by Transmission Electron Microscopy. The field dependence of the non-reciprocal critical current is consistent with a situation in which the supercurrent is carried by 50 to 100 spin-momentum-locked, ballistic one-dimensional helical channels, consistent with the bulk higher-order crystalline topology of bismuth established in previous works.\n
URL:https://www.lps.u-psud.fr/en/events/matthieu-bard-orbital-susceptibility-and-non-reciprocal-magneto-transport-as-probes-of-topology-graphene-bilayer-and-bismuth-nanowires/
CATEGORIES:PhD Defense
END:VEVENT
END:VCALENDAR