We report the observation of a change in Fermi surface topology of Bi2Sr2CaCu2O 8+δ with doping. By collecting high statistics ARPES data from moderately and highly overdoped samples and dividing the data by the Fermi function, we answer a long standing question about the Fermi surface shape of Bi2Sr2CaCu2O 8+δ close to the (π,0) point. For moderately overdoped samples (Tc=80K) we find that both the bonding and antibonding sheets of the Fermi surface are hole-like. However for a doping level corresponding to Tc=55K we find that the antibonding sheet becomes electron-like. This change does not directly affect the critical temperature and therefore the superconductivity. However, since similar observations of the change of the topology of the Fermi surface were observed in LSCO [1,2] and Bi2Sr2Cu2O 6+δ [3], it appears to be a generic feature of hole-doped superconductors. Because of bilayer splitting, though, this doping value is considerably lower than that for the single layer materials, which again argues that it is unrelated to Tc. The Fermi surface is a fundamental property in condensed matter physics. In recent years there has been a lively debate about the shape of the Fermi surface of one of the most frequently studied high temperature superconductors -Bi 2 Sr 2 CaCu 2 O 8+δ [4,5]. Initially, the Fermi surface of this compound was determined by angle resolved photoemission spectroscopy (ARPES) to be holelike, and consistent with Luttinger's theorem [6]. Later there were reports of an electron-like Fermi surface when measurements were performed at higher photon energies [7,8]. High momentum resolution studies utilizing new generation electron analyzers re-established the hole-like shape of the Fermi surface for optimal doping [9,10]. The observation of bilayer splitting in the overdoped regime [11] provided further evidence that at least the bonding sheet of the Fermi surface is hole-like for a wide range of dopings. However, this study did not address the question of the shape of the antibonding sheet of the Fermi surface. For a long time, the answer to this question remained elusive, as the antibonding band near (π,0) is located very close to the chemical potential. It is important to determine the exact shape of the whole Fermi surface, as it potentially affects transport and collective properties. Knowledge of the band dispersion in this region of the Brillouin zone is also of great importance for theoretical calculations. In a broader scope, it was previously shown [1,3] that a change in the topology of the Fermi surface occurs in overdoped LSCO and heavily overdoped single layer Bi 2 Sr 2 CuO 6+δ , therefore this phenomenon may be a generic property of hole-doped cuprates. Here, we present high resolution ARPES data measured to a high degree of statistical accuracy, in order to determine the evolution of the Fermi surface with doping. We find that for moderately doped samples, both sheets of the Fermi surface are hole-like, while at higher doping levels the antibonding band becomes electron-like. The ...

We have developed a versatile near-field microscopy platform that can operate at high magnetic fields and below liquid-helium temperatures. We use this platform to demonstrate an extreme terahertz (THz) nanoscope operation and to obtain the first cryogenic magneto-THz time-domain nano-spectroscopy/imaging at temperatures as low as 1.8 K, magnetic fields of up to 5 T, and with operation of 0–2 THz. Our Cryogenic Magneto-Terahertz Scattering-type Scanning Near-field Optical Microscope (or cm-THz-sSNOM) instrument is comprised of three main equipment: (i) a 5 T split pair magnetic cryostat with a custom made insert, (ii) a custom sSNOM instrument capable of accepting ultrafast THz excitation, and (iii) a MHz repetition rate, femtosecond laser amplifier for broadband THz pulse generation and sensitive detection. We apply the cm-THz-sSNOM to obtain proof of principle measurements of superconductors and topological semimetals. The new capabilities demonstrated break grounds for studying quantum materials that require an extreme environment of cryogenic operation and/or applied magnetic fields in nanometer space, femtosecond time, and THz energy scales.

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