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result(s) for
"Condensed Matter Physics"
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Graphene bilayers with a twist
Near a magic twist angle, bilayer graphene transforms from a weakly correlated Fermi liquid to a strongly correlated two-dimensional electron system with properties that are extraordinarily sensitive to carrier density and to controllable environmental factors such as the proximity of nearby gates and twist-angle variation. Among other phenomena, magic-angle twisted bilayer graphene hosts superconductivity, interaction-induced insulating states, magnetism, electronic nematicity, linear-in-temperature low-temperature resistivity and quantized anomalous Hall states. We highlight some key research results in this field, point to important questions that remain open and comment on the place of magic-angle twisted bilayer graphene in the strongly correlated quantum matter world.
Magic-angle twisted bilayer graphene plays host to many interesting phenomena, including superconductivity. This Review highlights key research results in the field, points toward important open questions, and comments on the place of magic-angle twisted bilayer graphene in the strongly correlated quantum matter world.
Journal Article
Resonant tunneling driven metal-insulator transition in double quantum-well structures of strongly correlated oxide
The metal-insulator transition (MIT), a fascinating phenomenon occurring in some strongly correlated materials, is of central interest in modern condensed-matter physics. Controlling the MIT by external stimuli is a key technological goal for applications in future electronic devices. However, the standard control by means of the field effect, which works extremely well for semiconductor transistors, faces severe difficulties when applied to the MIT. Hence, a radically different approach is needed. Here, we report an MIT induced by resonant tunneling (RT) in double quantum well (QW) structures of strongly correlated oxides. In our structures, two layers of the strongly correlated conductive oxide SrVO
3
(SVO) sandwich a barrier layer of the band insulator SrTiO
3
. The top QW is a marginal Mott-insulating SVO layer, while the bottom QW is a metallic SVO layer. Angle-resolved photoemission spectroscopy experiments reveal that the top QW layer becomes metallized when the thickness of the tunneling barrier layer is reduced. An analysis based on band structure calculations indicates that RT between the quantized states of the double QW induces the MIT. Our work opens avenues for realizing the Mott-transistor based on the wave-function engineering of strongly correlated electrons.
The metal-insulator transition is typically controlled by carrier accumulation or chemical doping. Here, the authors realize an alternative method based on resonant tunnelling in a double quantum well structure of strongly correlated oxides, which offers practical advantages over conventional methods.
Journal Article
Broad spectral tuning of ultra-low-loss polaritons in a van der Waals crystal by intercalation
Phonon polaritons—light coupled to lattice vibrations—in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field confinement, anisotropic propagation and ultra-long lifetime in the picosecond range
1
–
5
. However, the lack of tunability of their narrow and material-specific spectral range—the Reststrahlen band—severely limits their technological implementation. Here, we demonstrate that intercalation of Na atoms in the van der Waals semiconductor α-V
2
O
5
enables a broad spectral shift of Reststrahlen bands, and that the phonon polaritons excited show ultra-low losses (lifetime of 4 ± 1 ps), similar to phonon polaritons in a non-intercalated crystal (lifetime of 6 ± 1 ps). We expect our intercalation method to be applicable to other van der Waals crystals, opening the door for the use of phonon polaritons in broad spectral bands in the mid-infrared domain.
The spectral range of long-lived and confined phonon polaritons in a polar van der Waals crystal is shown to be tunable by intercalation of Na atoms, expanding their potential for nanophotonic applications in the mid-infrared domain.
Journal Article
Superconductivity in a quintuple-layer square-planar nickelate
Since the discovery of high-temperature superconductivity in copper oxide materials
1
, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials
2
. One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound Nd
0.8
Sr
0.2
NiO
2
(ref.
3
). Undoped NdNiO
2
belongs to a series of layered square-planar nickelates with chemical formula Nd
n
+1
Ni
n
O
2
n
+2
and is known as the ‘infinite-layer’ (
n
=
∞
) nickelate. Here we report the synthesis of the quintuple-layer (
n
= 5) member of this series, Nd
6
Ni
5
O
12
, in which optimal cuprate-like electron filling (
d
8.8
) is achieved without chemical doping. We observe a superconducting transition beginning at ~13 K. Electronic structure calculations, in tandem with magnetoresistive and spectroscopic measurements, suggest that Nd
6
Ni
5
O
12
interpolates between cuprate-like and infinite-layer nickelate-like behaviour. In engineering a distinct superconducting nickelate, we identify the square-planar nickelates as a new family of superconductors that can be tuned via both doping and dimensionality.
The authors report a superconducting transition beginning at 13 K in films of the quintuple-layer nickelate Nd
6
Ni
5
O
12
.
Journal Article
Charge order landscape and competition with superconductivity in kagome metals
In the kagome metals AV3Sb5 (A = K, Rb, Cs), three-dimensional charge order is the primary instability that sets the stage for other collective orders to emerge, including unidirectional stripe order, orbital flux order, electronic nematicity and superconductivity. Here, we use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensional charge order in AV3Sb5 and its interplay with superconductivity. Our approach is based on identifying an unusual splitting of kagome bands induced by three-dimensional charge order, which provides a sensitive way to refine the spatial charge patterns in neighbouring kagome planes. We found a marked dependence of the three-dimensional charge order structure on composition and doping. The observed difference between CsV3Sb5 and the other compounds potentially underpins the double-dome superconductivity in CsV3(Sb,Sn)5 and the suppression of Tc in KV3Sb5 and RbV3Sb5. Our results provide fresh insights into the rich phase diagram of AV3Sb5.The authors use high-resolution angle-resolved photoemission spectroscopy to determine the microscopic structure of three-dimensional charge order in AV3Sb5 (A = K, Rb, Cs) and its interplay with superconductivity.
Journal Article
Ultrafast hot-hole injection modifies hot-electron dynamics in Au/p-GaN heterostructures
A fundamental understanding of hot-carrier dynamics in photo-excited metal nanostructures is needed to unlock their potential for photodetection and photocatalysis. Despite numerous studies on the ultrafast dynamics of hot electrons, so far, the temporal evolution of hot holes in metal–semiconductor heterostructures remains unknown. Here, we report ultrafast (
t
< 200 fs) hot-hole injection from Au nanoparticles into the valence band of p-type GaN. The removal of hot holes from below the Au Fermi level is observed to substantially alter the thermalization dynamics of hot electrons, reducing the peak electronic temperature and the electron–phonon coupling time of the Au nanoparticles. First-principles calculations reveal that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles by modulating the electronic structure of the metal on timescales commensurate with electron–electron scattering. These results advance our understanding of hot-hole dynamics in metal–semiconductor heterostructures and offer additional strategies for manipulating the dynamics of hot carriers on ultrafast timescales.
Photo-excited gold nanoparticles are shown to provide ultrafast and efficient hot-hole injection to the valence band of p-type GaN, substantially altering hot-electron dynamics in the nanoparticles and forming a basis to design hot-hole-based optoelectronics.
Journal Article
Electronic structure of the parent compound of superconducting infinite-layer nickelates
The search continues for nickel oxide-based materials with electronic properties similar to cuprate high-temperature superconductors
1
–
10
. The recent discovery of superconductivity in the doped infinite-layer nickelate NdNiO
2
(refs.
11
,
12
) has strengthened these efforts. Here, we use X-ray spectroscopy and density functional theory to show that the electronic structure of LaNiO
2
and NdNiO
2
, while similar to the cuprates, includes significant distinctions. Unlike cuprates, the rare-earth spacer layer in the infinite-layer nickelate supports a weakly interacting three-dimensional 5
d
metallic state, which hybridizes with a quasi-two-dimensional, strongly correlated state with
3
d
x
2
−
y
2
symmetry in the NiO
2
layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare-earth intermetallics
13
–
15
, which are well known for heavy fermion behaviour, where the NiO
2
correlated layers play an analogous role to the 4
f
states in rare-earth heavy fermion compounds. This Kondo- or Anderson-lattice-like ‘oxide-intermetallic’ replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.
X-ray spectroscopy and density functional theory are used to show that the electronic structure of the parent compound of superconducting infinite-layer nickelates, while similar to the copper-based high-temperature superconductors, has significant differences.
Journal Article
Optically induced ultrafast magnetization switching in ferromagnetic spin valves
The discovery of spin-transfer torque (STT) enabled the control of the magnetization direction in magnetic devices in nanoseconds using an electrical current. Ultrashort optical pulses have also been used to manipulate the magnetization of ferrimagnets at picosecond timescales by bringing the system out of equilibrium. So far, these methods of magnetization manipulation have mostly been developed independently within the fields of spintronics and ultrafast magnetism. Here we show optically induced ultrafast magnetization reversal taking place within less than a picosecond in rare-earth-free archetypal spin valves of [Pt/Co]/Cu/[Co/Pt] commonly used for current-induced STT switching. We find that the magnetization of the free layer can be switched from a parallel to an antiparallel alignment, as in STT, indicating the presence of an unexpected, intense and ultrafast source of opposite angular momentum in our structures. Our findings provide a route to ultrafast magnetization control by bridging concepts from spintronics and ultrafast magnetism.The authors demonstrate optically induced ultrafast magnetization reversal taking place within less than a picosecond in rare-earth-free spin valves of [Pt/Co]/Cu/[Co/Pt].
Journal Article
Multi-scale ordering in highly stretchable polymer semiconducting films
Stretchable semiconducting polymers have been developed as a key component to enable skin-like wearable electronics, but their electrical performance must be improved to enable more advanced functionalities. Here, we report a solution processing approach that can achieve multi-scale ordering and alignment of conjugated polymers in stretchable semiconductors to substantially improve their charge carrier mobility. Using solution shearing with a patterned microtrench coating blade, macroscale alignment of conjugated-polymer nanostructures was achieved along the charge transport direction. In conjunction, the nanoscale spatial confinement aligns chain conformation and promotes short-range π–π ordering, substantially reducing the energetic barrier for charge carrier transport. As a result, the mobilities of stretchable conjugated-polymer films have been enhanced up to threefold and maintained under a strain up to 100%. This method may also serve as the basis for large-area manufacturing of stretchable semiconducting films, as demonstrated by the roll-to-roll coating of metre-scale films.Solution shearing of semiconducting polymers with a patterned blade induces improved alignment of the polymeric chains at the nano- and macroscale. This leads to increased charge transport in stretchable, roll-to-roll deposited organic transistors.
Journal Article