[Show abstract][Hide abstract] ABSTRACT: A central question in the high temperature cuprate superconductors is the
fate of the parent Mott insulator upon charge doping. Here we use scanning
tunneling microscopy to investigate the local electronic structure of lightly
doped cuprate in the antiferromagnetic insulating regime. We show that the
doped charge induces a spectral weight transfer from the high energy Hubbard
bands to the low energy in-gap states. With increasing doping, a V-shaped
density of state suppression occurs at the Fermi level, which is accompanied by
the emergence of checkerboard charge order. The new STM perspective revealed
here is the cuprates first become a charge ordered insulator upon doping.
Subsequently, with further doping, Fermi surface and high temperature
superconductivity grow out of it.
[Show abstract][Hide abstract] ABSTRACT: The IrTe2 transition metal dichalcogenide undergoes a series of structural
and electronic phase transitions when doped with Pt. The nature of each phase
and the mechanism of the phase transitions have attracted much attention. In
this paper, we report scanning tunneling microscopy and spectroscopy studies of
Pt doped IrTe2 with varied Pt contents. In pure IrTe2, we find that the ground
state has a 1/6 superstructure, and the electronic structure is inconsistent
with Fermi surface nesting induced charge density wave order. Upon Pt doping,
the crystal structure changes to a 1/5 superstructure and then to a
quasi-periodic hexagonal phase. First principles calculations show that the
superstructures and electronic structures are determined by the global chemical
strain and local impurity states that can be tuned systematically by Pt doping.
[Show abstract][Hide abstract] ABSTRACT: One of the major puzzles regarding unconventional superconductivity is how
some of the most interesting superconductors are related to an insulating phase
that lies in close proximity. Here we report scanning tunneling microscopy
studies of the local electronic structure of Cu doped NaFeAs across the
superconductor to insulator transition. We find that in the highly insulating
regime the electronic spectrum develops an energy gap with diminishing density
of state at the Fermi level. The overall lineshape and strong spatial
variations of the spectra are strikingly similar to that of lightly doped
cuprates close to the parent Mott insulator. We propose that the suppression of
itinerant electron state and strong impurity potential induced by Cu dopants
lead to this insulating iron pnictide.
[Show abstract][Hide abstract] ABSTRACT: We use scanning tunneling microscopy to investigate the doping dependence of quasiparticle interference (QPI) in NaFe1-xCoxAs iron-based superconductors. The goal is to study the relation between nematic fluctuations and Cooper pairing. In the parent and underdoped compounds, where fourfold rotational symmetry is broken macroscopically, the QPI patterns reveal strong rotational anisotropy. At optimal doping, however, the QPI patterns are always fourfold symmetric. We argue this implies small nematic susceptibility and, hence, insignificant nematic fluctuation in optimally doped iron pnictides. Since TC is the highest this suggests nematic fluctuation is not a prerequistite for strong Cooper pairing.
Preview · Article · Mar 2014 · Physical Review Letters
[Show abstract][Hide abstract] ABSTRACT: In Kondo insulator samarium hexaboride SmB$_6$, strong correlation and band
hybridization lead to an insulating gap and a diverging resistance at low
temperature. The resistance divergence ends at about 5 Kelvin, a behavior
recently demonstrated to arise from the surface conductance. However, questions
remain whether and where a topological surface state exists. Quantum
oscillations have not been observed to map the Fermi surface. We solve the
problem by resolving the Landau Level quantization and Fermi surface topology
using torque magnetometry. The observed Fermi surface suggests a two
dimensional surface state on the (101) plane. Furthermore, the tracking of the
Landau Levels in the infinite magnetic field limit points to -1/2, which
indicates a 2D Dirac electronic state.
[Show abstract][Hide abstract] ABSTRACT: Although the origin of high temperature superconductivity in the iron pnictides is still under debate, it is widely believed that magnetic interactions or fluctuations have a crucial role in triggering Cooper pairing. A key issue regarding the iron pnictide phase diagram is whether long-range magnetic order can coexist with superconductivity microscopically. Here we use scanning tunnelling microscopy to investigate the local electronic structure of underdoped NaFe1-xCoxAs near the spin density wave and superconducting phase boundary. Spatially resolved spectroscopy directly reveals both the spin density wave and superconducting gaps at the same atomic location, providing compelling evidence for the microscopic coexistence of the two phases. The strengths of the two orders are shown to anti-correlate with each other, indicating the competition between them. This work implies that Cooper pairing in the iron pnictides can occur when portions of the Fermi surface are already gapped by the spin density wave order.
Preview · Article · Mar 2013 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: Herbertsmithite ZnCu3(OH)6Cl2 is a
promising system to study frustrated magnetism on S=1/2 kagome lattice.
A continuum of spinon excitations has been revealed by recent neutron
scattering measurements on single crystals. Interesting questions arise
on the fate of this spinon excitation under intense external magnetic
field. We report field-driven transitions in the high field
magnetization of single crystalline
ZnCu3(OH)6Cl2. These transitions appear
below 1 K, and the transition field values are almost independent of the
magnetic field orientation. We further discuss methods to separate the
magnetic contribution from the impurity to repeal the intrinsic response
of the kagome lattice.
[Show abstract][Hide abstract] ABSTRACT: Although the mechanism of superconductivity in the cuprates remains elusive, it is generally agreed that at the heart of the problem is the physics of doped Mott insulators. A crucial step for solving the high temperature superconductivity puzzle is to elucidate the electronic structure of the parent compound and the behaviour of doped charge carriers. Here we use scanning tunnelling microscopy to investigate the atomic-scale electronic structure of the Ca(2)CuO(2)Cl(2) parent Mott insulator of the cuprates. The full electronic spectrum across the Mott-Hubbard gap is uncovered for the first time, which reveals the particle-hole symmetric and spatially uniform Hubbard bands. Defect-induced charge carriers are found to create broad in-gap electronic states that are strongly localized in space. We show that the electronic structure of pristine Mott insulator is consistent with the Zhang-Rice singlet model, but the peculiar features of the doped electronic states require further investigations.
Full-text · Article · Jan 2013 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: We report the doping, temperature, and spatial evolutions of the electronic structure of NaFe(1-x)Co(x)As studied by scanning tunneling microscopy. In the parent state we directly observe the spin density wave gap, which exhibits unconventional features that are incompatible with simple Fermi surface nesting. The optimally doped sample has a single superconducting gap, but in the overdoped regime a novel pseudogaplike feature emerges. The pseudogaplike phase coexists with superconductivity in the ground state, persists well into the normal state, and shows strong spatial variations. The characteristics of the three distinct electronic states revealed here shed important new lights on the microscopic models for the iron-based superconductors.
[Show abstract][Hide abstract] ABSTRACT: We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K0.73Fe1.67Se2 with TC = 32 K. On the atomically resolved FeSe surface, we observe a well-defined superconducting gap and the microscopic coexistence of a charge-density modulation with √2×√2 periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block-antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides.
[Show abstract][Hide abstract] ABSTRACT: We report scanning tunneling microscopy studies of the local structural
and electronic properties of the iron selenide superconductor
K0.73Fe1.67Se2 with TC =
32K. On the atomically resolved FeSe surface, we observe well-defined
superconducting gap and the microscopic coexistence of a charge density
modulation with √2 x√2 periodicity with respect to the
original Se lattice. We propose that a possible origin of the pattern is
the electronic superstructure caused by the block antiferromagnetic
ordering of the iron moments. The widely expected iron vacancy ordering
is not observed, indicating that it is not a necessary ingredient for
superconductivity in the intercalated iron selenides.
[Show abstract][Hide abstract] ABSTRACT: We present scanning tunneling microscopy studies of the LaOFeAs parent compound of iron pnictide superconductors. High resolution spectroscopic imaging reveals strong standing wave patterns induced by quasiparticle interference of two-dimensional surface states. Fourier analysis shows that the distribution of scattering wave vectors exhibits pronounced twofold (C(2)) symmetry, strongly reminiscent of the nematic electronic state found in CaFe(1.94)Co(0.06)As(2). The implications of these results to the electronic structure of the pnictide parent states will be discussed.
Preview · Article · Feb 2011 · Physical Review Letters