Research
Artificial Quantum Crystals > How to play "LEGO" with atoms?
Crystals are like LEGO consisting of interlocking bricks made of atomic layers. The "residents" of this LEGO world are the electrons who behave quantum mechanically. For instance, coulomb repulsion and spin-orbit coupling are two of their most important interactions. The former is key to collective electronic behaviors, such as Mott insulators, metal-insulator transition, magnetism, and superconductivity. The latter often leads to nontrivial quantum waves due to Berry curvatures, giving rise to topological phases and a variety of Hall effects. To understand the underlying physics, we build artificial crystals with atomic precision by laser molecular beam epitaxy to simulate toy-model Hamiltonian of various symmetry configurations. We analyze the emerging and often surprising properties by a suite of measurement techniques.
To realize isolated pseudospin-half square lattice, we created monolayer-thick SrIrO3 quantum wells separated by SrTiO3 spacing layers. By varying the spacer thickness, we were able to control the interlayer coupling and reach the 2D limit of the Néel temperature.
L. Hao et al.
Phys. Rev. Lett. 119, 027204 (2017)
Unlike ferromagnets, antiferromagnets, by definition, don't respond to magnetic field. We show that the response could be huge when we implement anisotropic interactions that don't induce anisotropy at 2D, i.e., a hidden SU (2) symmetry.
L. Hao et al.
Nature Physics 14, 806 (2018)
How to distinguish water ice and dry ice? Melt them! The disordered state above the transition to a spontaneous ordering is often the key. We discovered a Mott semiconducting state with significant electron-hole pairing fluctuations above the antiferromagnetic transition.
L. Hao et al.
Nature Communication 10, 5301 (2019)
Slater insulator and Mott insulator are solutions of the Hubbard Hamiltonian at opposite limits that can be adiabatically connected, in analogy to the BCS-BEC crossover. We showed that the Slater-Mott crossover can be mediated by varying epitaxial strain.
J. Yang et al.
Phys. Rev. Lett. 124, 177601 (2020)
Hydrogen is a quantum mechanic dopant since ionized hydrogen is proton. Stabilizing them a material is non-trivial and sometimes difficult such as iridates. We found that the artificial superlattice provides a great solution as the spacer layer stabilizes the protonated structure.
M. Wang et al.
Adv. Funct. Mater. 31, 2100261 (2021)
Berry phase is a key concept that shows a quantum particle is described by a wave function instead of probability density. It is considered as a property of noninteracting electrons. We found that correlated electrons can exhibit Berry phase-driven transport in a 2D Mott insulator.
J. Yang et al.
Phys. Rev. X 12, 031015 (2022)
A monolayer and a bilayer of the same material could show antagonistic orderings. We created a hybrid structure to combine them as a double quantum well. While preferring orthogonal spin axes individually, they are forced to settle with each other and behave a single magnetic system.
D. Gong et al.
Phys. Rev. Lett. 129, 187201 (2022)
No real magnet is truly isotropic. Crystalline symmetry always imposes finite anisotropy to the spins. When random vacancies by site dilution are present, we found the fluctuations of anisotropic spins is isotropic at 2D, enabling a dramatic increase of the ordering under magnetic field.
J. Yang et al.
Nano Lett. 23, 11409 (2023)
Opportunities - Students will learn laser molecular beam epitaxy for atomic layering deposition, x-ray diffraction and atomic force microscopy for structural characterization, (magneto-)transport measurements on electronic and thermal properties, and magnetometry for determining magnetic transitions. There are chances to regularly visit synchrotrons inside and outside the US to perform resonant x-ray scattering and spectroscopy for element-resolved analysis of symmetry-breaking orders and electronic structure.
Geometric Frustration > What would happen when spins are "depressed"?
To cope with frustration, one often needs to come up with creative solutions to the problem. The same is true for spins, but they have to achieve that quantum mechanically, which could provide us a source of novel quantum phenomena as well as innovative quantum technologies. Frustration of spins often occurs geometrically due to the symmetry of crystal lattice. These systems called geometrically frustrated quantum magnets are known for a variety of exotic quantum states and collective excitations, such as magnetic monopole, spinon, and magnon. We create and study materials of all sorts of forms, where these entangled and/or topological quasi-particles could interact with correlated electrons to leave their imprints on the electronic behaviors.
We designed and synthesized a pyrochlore heterostructure where the breaking of the spin ice rule and the condensation of magnetic monopole in insulating Dy2Ti2O7 can be detected by anomalous magnetoresistance in the Bi2Ir2O7 conducting layer.
H. Zhang et al.
Nature Communication 14, 1404 (2023)
We observed anomalous magnetoresistance that is indicative of significant ferromagnetic spin fluctuations above the Néel temperature of a centrosymmetric skyrmion lattice, pointing to itinerant electrons-mediated magnetic correlations at zero and finite wave vectors.
H. Zhang et al.
New J. Phys. 22, 083056 (2020)
Opportunities - Students will learn single crystal growth and laser molecular beam epitaxy for materials synthesis, x-ray diffraction and atomic force microscopy for structural characterization, ultralow-temperature high-magnetic-field transport measurements on electronic and thermal properties, ultralow-temperature magnetometry for determining magnetic transitions, synchrotron resonant x-ray scattering and spectroscopy for element-resolved analysis of symmetry-breaking orders and electronic structure. There are chances to regularly visit the National High Magnetic Field Laboratory.
Multi-Modal Platform > Outsmart nature in gauging quantum materials?
Nature is very smart in finding the energy-minimal state by utilizing all degrees of freedom. This means the charge, spin, orbital, and lattice degrees of freedom can be all involved simultaneously in quantum materials. On one hand, this could be very useful for application since the electric, magnetic, mechanic, and thermal properties are all coupled together. On the other hand, this complex entanglement is an headache when trying to understand the microscopic mechanism. We develop a multi-modal sample environment designed to bridge the gap between the atomic scale interactions and the macroscopic properties. The good news is we know symmetry is always crucial in physics. Therefore, this platform will have the capabilities of applying multiple symmetry-breaking fields, varying temperature, measuring x-ray scattering/spectroscopy/imaging, and monitoring macroscopic responses all at the same time. It could be used to obtain detailed understanding of low-dimensional magnetism, metal-insulator transition, orbital/charge ordering, superconductivity, and multiferroic criticality.
We demonstrate that an in situ anisotropic strain up to only 0.05% can modulate the metamagnetic transition field of a spin–orbit‐coupled Mott insulator by almost 300% due to the complete strain‐tuning of the transition between the spin‐flop and spin‐flip limits.
H. Zhang et al.
Advanced Materials 32, 2002451 (2020)
Our combined simultaneous measurements of spontaneous orthorhombicity and resistivity anisotropy of iron pnictide show that elasto X-ray diffraction is a powerful technique to probe and disentangle the nemato-elastic and nemato-transport couplings.
J.J. Sanchez et al.
Nature Materials 20, 1519 (2021)
We show that anisotropic strain, the conjugate field of nematicity, reduces the Tc of an iron pnictide, providing direct evidence of nematic fluctuations-mediate superconductivity and demonstrating tunable mechanical control of a high-temperature superconductor.
P. Malinowski et al.
Nature Physics 16, 1189 (2020)
Magnetic interactions are usually quadratic and either ferromagnetic or antiferromagnetic. We found a shear strain on the ab-plane of Sr2IrO4 can induce a quartic interaction that is both at the same time but for different axes. As a result, a spin modulation stabilized along the c-axis.
S. Pandey et al.
Phys. Rev. Lett. 129, 027203 (2022)
Opportunities - Students will learn characterizing both single crystal and thin film samples by a number x-ray diffraction techniques, measuring electric properties through a variety of electronics with field and temperature dependence, applying in situ strain/stress with piezo-strain cells; designing measurement setups with Labview programming and control, detecting magnetic orders by magnetometry, and probing symmetry-breaking orders by synchrotron resonant x-ray scattering and spectroscopy as well as neutron scattering. Students have chances to be positioned at national labs for long terms.