Triangular magnetic structures have gained considerable interest due to their rich magnetic behavior and structural simplicity. These structures contain the motif of a triangle as the main structural feature, leading to geometric frustration and implicitly to degenerate magnetic ground states. Most of the previous work on triangular lattice structures was performed on simple transition metal halides or oxides. Therefore, it presents an interesting challenge for materials scientists to synthesize new class of materials that preserve the quasi-two dimensionality of the structures. This talk will feature two class of materials (1) triangular materials synthesized using high-pressure hydrothermal method (2) AREQ2 (A = Alkali metal, RE= rare earth, Q = O, S, Se) triangular magnetic materials. 

First, I will focus on synthesis, magnetism and use of neutron diffraction to characterize the magnetic phase diagram of several classes of hydrothermally synthesized oxy-anions based transition metal compound series (EOyx-, E = As, Mo, Se). These linking groups can lead to an enormous array of new structure types with great potential for exploring and characterizing new emergent phenomena. Here, I will focus the role of vanadate building blocks (VO43-) in magnetically interesting transition metal layered materials. The vanadates display a rich diversity of structural behavior including multiple bridging modes such as corner and edge sharing. In addition, the presence of vacant d-orbitals in the bridging center can have a significant effect on the magnetic coupling behavior. As the first example, SrM(VO4)(OH) (Mn, Co, Ni) possess on-dimensional magnetic lattice with totally different magnetic properties depending on the transition metal cation even though they crystallizes in same space group. Further, Na2BaM(VO4)2 (M = Mn2+, Fe2+, Co2+) series all have similar chemical structures and are members of the glaserite family, but each one displays dramatically different magnetic behavior between room temperature and 2 K. Another interesting system for discussion is the mixed vanadate carbonate material A2M3(VO4)2(CO3) where A = K, Rb and M = Mn2+, Co2+. The chemical structure is quite complex and has two unique layers, one built of corner sharing vanadates and one with the trigonal planar carbonates. The material also has a complex magnetic behavior and undergoes three magnetic phase transitions between 300-2 K. 

In the second part of my talk, I will focus on the synthesis of ARESe2 single crystal growth, magnetic properties and elastic and inelastic neutron scattering. These crystals crystallize in either trigonal (R-3m) or hexagonal (P63/mmc) crystal systems and associate with an ideal triangular RE3+ layers. The magnetic properties and heat capacity of these compounds were characterized down to 0.4 K. The Yb-compounds exhibit a broader peak in heat capacity ~10 K suggesting short range ordering. 

In one spatial dimension, enhanced thermal and quantum fluctuations should preclude the existence of any long range ordered superfluid phase of matter.  Instead, the quantum liquid should be described at low energies by an emergent hydrodynamic framework known as Tomonaga-Luttinger liquid theory.  In this talk I will present details on some complimentary experimental and theoretical searches for this behavior in helium-4 including: (1) pressure driven superflow through nanopores, and (2) the excitation spectrum of a confined superfluid inside nano-engineered porous silica-based structures. For flow experiments, we have devised a framework that is able to quantitatively describe dissipation at the nanoscale leading to predictions for the critical velocity borne out by recent superflow measurements in nanopores.  In confined porous media, with radii reduced via pre-plating with rare gases, I will discuss ab initio simulations of phase and density correlations inside the pore that are in agreement with recent neutron scattering measurements.   Taken together, these results indicate significant progress towards the experimental observation of a truly one-dimensional quantum liquid.

This work was supported by the NSF through grants DMR-1809027 and DMR-1808440.  

Materials manipulation via ion or laser beams can achieve precisely tuned atomic geometries that are necessary, e.g. to engineer interactions between defects in quantum materials and for fabricating novel electronic devices with nanoscale dimensions. In addition, such beams are also used to characterize and probe materials properties by means of electronic and optical excitations. I will discuss recent quantum- mechanical first-principles predictions for electron dynamics and the subsequent ionic motion that follows after an excitation of the electronic system. Using real-time time-dependent density functional theory we simulated the underlying ultrafast time scales of electron dynamics in semiconductors and metals. Examples include long-lived electronic excitations in proton, electron, and laser irradiated bulk semiconductors that facilitate diffusion of point defects, such as oxygen vacancies in MgO. We compare such bulk simulations to aluminum surfaces under irradiation, for which we quantify electron emission, charge capture, and pre-equilibrium effects that are unique to thin films or two-dimensional materials. Limitations and possible extensions of the theoretical description will be included in the discussion.

Spintronics-based technology, which uses spins to represent and propagate information, holds promise to realize devices that surpass the current CMOS transistor technology in power, density and speed. For example, magnetic random-access memory (MRAM) based on magnetic tunnel junctions were identified as promising non-volatile memory but its use has been limited. A second generation MRAM-based on spin transfer torque has reduced currents. However, next generation MRAM based on pure spin currents may provide even more energy efficiency. My research is focused on developing power-efficient ways to generate, propagate and manipulate spins via pure spin currents. In order to develop such pure spin current technologies, the development of new materials such as topological insulators must come hand in hand with the development of new devices. In this talk, I will discuss (i) low damping ferromagnetic insulating thin films for achieving efficient spin current generation in spintronic devices, (ii) spin current generation in these films and large spin-charge interconversion in neighboring layers, (iii) spin interactions in ferromagnetic insulator/topological insulator heterostructures. Together these results lay the foundation for new energy-efficient pure spin current-based electronics.

Reference:

1. Li, P. et al. Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator. Nano Lett. 21, 1, 84 (2021).

2. Li, P. et al. Switching magnetization utilizing topological surface state. Science Advances 5, eaaw3415 (2019).