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UID:69d255c8bbe02
DTSTART:20230524T080000Z
SEQUENCE:0
TRANSP:OPAQUE
LOCATION:ICFO Auditorium and Online (Teams)
SUMMARY:ICFO | IPSITA DAS
CLASS:PUBLIC
DESCRIPTION:The discovery of two dimensional materials opened a unique path
 way to study the electronic properties of quantum materials which are othe
 rwise absent in bulk systems. Soon after the discovery of graphene (2004)\
 , a plethora of 2D materials were found with various diverse properties su
 ch as\, metals\, insulators\, semiconductors\, superconductors\, magnets e
 tc. These materials can be assembled by using the van der Waals (vdW) forc
 e\, which greatly extends the possibilities of studying new phenomena. Ini
 tially vdW heterostructures have been made by vertically stacking differen
 t layers. However\, twist angle plays an interesting tuning knob to engine
 er the electronic properties of the 2D heterostructures. Following long st
 anding theoretical predictions\, people have observed exotic quantum pheno
 mena in twisted bilayer graphene in 2018.\nIn this thesis\, we have studie
 d the electronic properties of magic angle twisted bilayer graphene (MATBG
 )\, which consists of two graphene layers rotated by an angle &theta\; = 1
 .1&deg\;. It has been experimentally shown that MATBG possesses flat elect
 ronic bands in the low energy scale\, which hosts multiple correlated phen
 omena such as correlated insulators\, superconductivity\, magnetism etc.\n
 We studied different phases of MATBG in the presence of a magnetic field t
 o reveal the zero-field ground state of the system. In the presence of a s
 mall magnetic field (B &lt\; 3 T)\, the Hall conductance of MATBG shows qu
 antisation with the Chern numbers C = &plusmn\;1\, &plusmn\;2\, &plusmn\;3
  and &plusmn\;4 which nucleate from different integer fillings of the moir
 &eacute\; bands\, &nu\; = &plusmn\;3\, &plusmn\;2\, &plusmn\;1 and 0 respe
 ctively. These phases can be interpreted as spin and valley polarized many
  body Chern insulators. The exact sequence and correspondence of the Chern
  numbers and filling factors suggest that these states are directly driven
  by electronic interaction\, which specifically break the time-reversal sy
 mmetry in the system.\nWe have also studied the evolution of the phase spa
 ce of MATBG in high magnetic field and explored the Hofstadter spectrum. D
 ue to the large moir&eacute\; unit cell area\, MATBG reaches one full flux
  quantum (&Phi\;0) per moir&eacute\; unit cell close to 30 T. We studied a
  detailed magneto-transport behaviour of MATBG upto B = 31 T. At &Phi\;0\,
  reentrant correlated insulators are observed at &nu\; = +2\, +3. Interact
 ion driven Fermi surface reconstruction is also observed at other fillings
  of the band which are identified by the emergence of a new set of Landau 
 levels (LLs).\nWe further studied the higher energy dispersive bands in th
 e presence of a magnetic field. The analysis of the LL crossings in the Ra
 shba-like dispersive bands enables a parameter free comparison to a newly 
 derived magic series of level crossings in a magnetic field and provides c
 onstraints on the parameters of the Bistritzer-MacDonald Hamiltonian. For 
 the first time\, this allows us to experimentally verify the band structur
 e of MATBG.\nIn the next section of this thesis\, we have studied the effe
 ct of Coulomb screening on the ground state of the quantum phases such as 
 correlated insulator and superconductor. The coexistence of these two stat
 es prompts intriguing questions about their relationships. We have directl
 y tuned the electronic correlations by changing the separation between the
  graphene and a metallic screening layer. Correlated insulators are suppre
 ssed when the separations are smaller than the typical Wannier orbital siz
 e ( ~ 15 nm) and also in devices with twist angles slightly away from magi
 c angle (&theta\; = 1.1&deg\; &plusmn\; 0.05&deg\;). Upon extinction of th
 e insulating orders\, the vacated phase space is taken over by the superco
 nductors. Finally\, we study the temperature depe0ndence of the resistance
  and unveil a strange metal phase upto a very low temperature T = 40 mK.\n
 We thus have experimentally demonstrated the effect of several external pa
 rameters (magnetic field\, screening\, temperature etc.) on the ground sta
 te of MATBG and how they alter the microscopic mechanism of different corr
 elated phenomena in the system.\nThesis Director: Prof Dr. Dmitri Efetov 
DTSTAMP:20260405T123000Z
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