Lihai Wang

Symmetry Switching of Negative Thermal Expansion by Chemical Control

The layered perovskite Ca3-xSrxMn2O7 is shown to exhibit a switching from a material exhibiting uniaxial negative to positive thermal expansion as a function of x. The switching is shown to be related to two closely competing phases with different symmetries. The negative thermal expansion (NTE) effect is maximized when the solid solution is tuned closest to this region of phase space but is switched off suddenly on passing though the transition. Our results show for the first time that, by understanding the symmetry of the competing phases alone, one may achieve unprecedented chemical control of this unusual property.

Domain topology and domain switching kinetics in a hybrid improper ferroelectric

Charged polar interfaces such as charged ferroelectric walls or heterostructured interfaces of ZnO/(Zn,Mg)O and LaAlO3/SrTiO3, across which the normal component of electric polarization changes suddenly, can host large two-dimensional conduction. Charged ferroelectric walls, which are energetically unfavourable in general, were found to be mysteriously abundant in hybrid improper ferroelectric (Ca,Sr)3Ti2O7 crystals. From the exploration of antiphase boundaries in bilayer-perovskites, here we discover that each of four polarization-direction states is degenerate with two antiphase domains, and these eight structural variants form a Z4 × Z2 domain structure with Z3 vortices and five distinct types of domain walls, whose topology is directly relevant to the presence of abundant charged walls. We also discover a zipper-like nature of antiphase boundaries, which are the reversible creation/annihilation centres of pairs of two types of ferroelectric walls (and also Z3-vortex pairs) in 90° and 180° polarization switching. Our results demonstrate the unexpectedly rich nature of hybrid improper ferroelectricity.

Interrelation between domain structures and polarization switching in hybrid improper ferroelectric Ca3(Mn,Ti)2O7

Ca3Mn2O7 and Ca3Ti2O7 have been proposed as the prototypical hybrid improper ferroelectrics (HIFs), and a significant magnetoelectric (ME) coupling in magnetic Ca3Mn2O7 is, in fact, reported theoretically and experimentally. Although the switchability of polarization is confirmed in Ca3Ti2O7 and other non-magnetic HIFs, there is no report of switchable polarization in the isostructural Ca3Mn2O7. We constructed the phase diagram of Ca3Mn2-xTixO7 through our systematic study of a series of single crystalline Ca3Mn2-xTixO7 (x = 0, 0.1, 1, 1.5, and 2). Using transmission electron microscopy, we have unveiled the unique domain structure of Ca3Mn2O7: the high-density 90° stacking of a- and b-domains along the c-axis due to the phase transition through an intermediate Acca phase and the in-plane irregular wavy ferroelastic twin domains. The interrelation between domain structures and physical properties is unprecedented: the stacking along the c-axis prevents the switching of polarization and causes the irregula...

Nanoscale Superconducting Honeycomb Charge Order in IrTe2

Entanglement of charge orderings and other electronic orders such as superconductivity is in the core of challenging physics issues of complex materials including high temperature superconductivity. Here, we report on the observation of a unique nanometer scale honeycomb charge ordering of the cleaved IrTe2 surface, which hosts a superconducting state. IrTe2 was recently established to exhibit an intriguing cascade of stripe charge orders. The stripe phases coexist with a hexagonal phase, which is formed locally and falls into a superconducting state below 3 K. The atomic and electronic structures of the honeycomb and hexagon pattern of this phase are consistent with the charge order nature, but the superconductivity does not survive on neighboring stripe charge order domains. The present work provides an intriguing physics issue and a new direction of functionalization for two-dimensional materials.

The low-temperature highly correlated quantum phase in the charge-density-wave 1T-TaS2 compound

A prototypical quasi-2D metallic compound, 1T-TaS2 has been extensively studied due to an intricate interplay between a Mott-insulating ground state and a charge-density-wave order. In the low-temperature phase, 12 out of 13 Ta4+ 5d-electrons form molecular orbitals in hexagonal star-of-David patterns, leaving one 5d-electron with S  = ½ spin free. This orphan quantum spin with a large spin-orbit interaction is expected to form a highly correlated phase of its own. And it is most likely that they will form some kind of a short-range order out of a strongly spin-orbit coupled Hilbert space. In order to investigate the low-temperature magnetic properties, we performed a series of measurements including neutron scattering and muon experiments. The obtained data clearly indicate the presence of the short-ranged phase and put the upper bound on ~0.4 µB for the size of the magnetic moment, consistent with the orphan-spin scenario.Charge-density waves: Alone no longerEvidence for a hidden form of quantum correlation is uncovered by researchers in Korea, the UK, France and the USA. Marie Kratochvilova et al. from the Institute of Basic Science identify magnetic ordering in a material that is already well-studied for its charge-ordering properties. Charge-density waves occur when the electrons in a crystalline material create a standing-wave pattern. For example, an electron in each tantalum atom in tantalum disulfide (1T-TaS2) can strongly interact with its neighbor to form a Star of David pattern: one electron on each of the twelve vertices, and one in the middle. Kratochvilova et al. now find evidence that these thirteenth ‘orphan’ electrons exhibit their own type of ordering. The team measured the low-temperature magnetic properties of 1T-TaS2 using multiple techniques and identified short-range magnetic ordering in agreement with theoretical predictions.

Correlated electronic states at domain walls of a Mott-charge-density-wave insulator 1 T -TaS 2

Domain walls in interacting electronic systems can have distinct localized states, which often govern physical properties and may lead to unprecedented functionalities and novel devices. However, electronic states within domain walls themselves have not been clearly identified and understood for strongly correlated electron systems. Here, we resolve the electronic states localized on domain walls in a Mott-charge-density-wave insulator 1T-TaS2 using scanning tunneling spectroscopy. We establish that the domain wall state decomposes into two nonconducting states located at the center of domain walls and edges of domains. Theoretical calculations reveal their atomistic origin as the local reconstruction of domain walls under the strong influence of electron correlation. Our results introduce a concept for the domain wall electronic property, the walls own internal degrees of freedom, which is potentially related to the controllability of domain wall electronic properties.The electronic states within domain walls in an interacting electronic system remain elusive. Here, Cho et al. report that the domain wall state in a charge-density-wave insulator 1T-TaS2 decomposes into two localized but nonconducting states at the center or edges of domain walls.

Heat transport study of the spin liquid candidate 1T−TaS2

We present the ultra-low-temperature thermal conductivity measurements on single crystals of the prototypical charge-density-wave material 1

The direct observation of ferromagnetic domain of single crystal CrSiTe3

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Moiré Superstructure and Dimensional Crossover of 2D Electronic States on Nanoscale Lead Quantum Films

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Structural and magnetic characterization of large area, free-standing thin films of magnetic ion intercalated dichalcogenides Mn0.25TaS2 and Fe0.25TaS2

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