Overview

To reconcile complex properties of quantum materials, I approach the challenge by performing integrated and collaborative experimental work to understand astonishing physical phenomena and apply the knowledge to further design for the potential applications. Quantum materials host the platform where multiple degrees of freedom, including spin, lattice, and orbital, intertwine and play roles in stabilizing distinct quantum phases. By material synthesis and fundamental physical characterizations, the system exhibiting the low dimensional magnetism and behaviors involving correlated electrons are particular interests to me, where quantum physics lies at the core of interpreting the system and has potentials for the functional quantum device. My research mainly utilizes neutrons, synchrotron X-ray, and muons to probe these degrees of freedom collaboratively, peeking into material properties. Then I explore the chance of tuning different phases by external perturbations. These will provide insights on the material properties from interdisciplinary measurements and enlighten a pathway for designing functional quantum materials. This page contains some brief descriptions of my current work. Please feel free to reach out if you are interested!

Two dimensional van der Waals materials

I am growing tremendous interest in the layered magnetic van der Waals (vdW) materials. The cleavable and highly flexible physical properties offer a unique platform for efficiently manipulating magnetic states via external perturbations in a two-dimensional (2D) magnet. One system I am working on is the intercalated transition metal dichalcogenide (TMDC), FexNbS2 system. This system belongs to a large family of magnetic intercalated TMDCs TxMA2 (T = 3d transition metal, M = Nb, Ta and A = Se, S), where magnetic ions are introduced into the 2H-MA2 structure [1, 2]. The revival of studying this system comes from the recent finding of intriguing spintronic properties, including resistance switching and magnetic memory effects coupled to the antiferromagnetic ordering in the bulk crystal [3]. We performed comprehensive single-crystal neutron diffraction measurements on the samples close to the stoichiometry ratio of x=1/3 samples. Magnetic defects are usually considered to suppress magnetic correlations. Surprisingly, we found a highly tunable magnetic state controlled by a small number of magnetic defects (dx ~ +/-0.01) in this 2D vdW magnet crystal [4]. More work including photoemission, synchrotron X-ray scattering, muon spin relaxation and inelastic neutron work are in progress, unveiling rich fundamental physics in this fascinating system that is an ideal model for the basis of the future spintronic device.

Reference

[1] S. S. P. Parkin et. al. Philosophical Magazine B 41, 65 (1980).

[2] R. H. Friend et. al. The Philosophical Magazine: A Journal of Theoretical Experimental and Applied Physics 35, 1269 (1977).

[3] E. Maniv et. al. Science Advances 7, 10.1126/sciadv.abd8452 (2021).

[4] S. Wu et. al. arXiv:2106.01341 (2021)

(a) Crystal structure of transition metal dichalcogenide Fe1/3+xNbS2. (b) Intriguing x dependent switching behaviors accompanied by antiferromagnetic order [3]. (c) Highly tunable magnetic ground states discovered by single crystal neutron diffraction experiments, (d) showing Fe defects tuned different superlattice peaks by varying small x value (from -0.01 to 0.01) [4]. Collaborative (e) photoemission and (f) muon spin rotation works in progress exhibit more rich physics in this system.

Unconventional High Temperature Superconductors

Unconventional superconductivity has attracted many interests in the community of material science and condensed matter physics for decades. As one of the archetypes, iron-based superconductors (FeSCs) host unconventional superconductivity near a putative quantum critical point where collective excitations could play an essential role in the pairing mechanism. These fluctuations and corresponding orders emerge from the interplay between multi-degrees of freedom. In the phase diagram of carrier-doped iron-pnictides, the under-doped phase space is widely inhabited by intertwined nematic and magnetic orders. However, an asymmetry in the phase diagrams is notable, and a unique reentrant tetragonal magnetic phase is involved on the hole-doped side [1]. Magnetic and nematic susceptibilities, which describe the strength of magnetic and nematic fluctuations, have been extensively studied.

On the other hand, their length scale - namely the magnetic and nematic correlation lengths, have not been systematically studied. To address this question, we measured in-plane transverse acoustic phonons via inelastic X-ray scattering. By a self-contained method of analysis, we extract the nematic susceptibility and nematic correlation length. The results show a short-range nematic fluctuation that may favor superconductivity, emphasizing the nematic correlation length for understanding the iron-based superconductors [2]. This work provides a direct comparison with the electron-doped case [3], opening up more future work on the studies of nematic and magnetic correlation lengths. The analysis method introduced in this work can also be generalized to be used in other coupled nematic-soft acoustic phonons phase transitions.

Reference

[1] S. Avci et. al. Nature Comm. 5, 3845 (2014)

[2] S. Wu et. al. Phys. Rev. Lett. 126, 107001 (2021)

[3] A. M. Merritt et. al. Phys. Rev. Lett. 124, 157001 (2020)

(a) Phase diagram for the hole-doped Fe-pnictide, Sr1-xNaxFe2As2, with an unique C4 phase. (b) Mean-field approach to analyze in-plane acoustic transverse phonon via inelastic X-ray scattering measurements. (c) Representative phonon softening upon decreasing the temperature in the sample. (d) Extracted nematic correlation length, bare nematic susceptibility, and renormalized shear modulus. [2] These measurements provide insights on short-range nematic fluctuations in hole-doped pnictides, in contrast with the electron-doped cases [3].

My other interest in FeSCs lies in the studies on other variants beyond a two-dimensional square lattice in most FeSCs. One of my works is to study the quasi-one-dimensional (1D) Fe spin ladder system BaFe2X3 (X = Se, S). This system introduces a 1D prototype and an insulating parent compound to the FeSCs. More interestingly, recent reports of pressure-induced superconductivity introduce hydrostatic pressure as an external tuning parameter to vary electronic ground states [1, 2]. The lack of information on the magnetic properties across the pressure-temperature (P-T) phase diagram is notable. That is a result of the experimental challenges of performing neutron diffraction experiments at simultaneous conditions of high pressure (beyond 2 GPa) and cryogenic temperatures, leaving a crucial gap in the experimental characterization of this system. We explored a comprehensive characterization of the magnetostructural properties across a large region of the P-T phase diagram by using complementary experimental techniques, including neutron powder diffraction, muon spin relaxation and X-ray diffraction under hydrostatic pressure [3]. Our results provide important insights into the origin of the unusual block-type magnetism, the role of electronic correlations and orbital selectivity, and potentially the mechanism of superconductivity in these systems, greatly enriching the discussion of magnetism and superconductivity in iron-based materials. To a broader context, I am interested in exploring the hydrostatic pressure-tuned unconventional superconductivity, which brings in a disorder-free tuning knob beyond the chemical substitution.

Reference

[1] H. Takahashi et. al. Nat. Mater. 14, 1008 (2015)

[2] J. Ying et. al. Phys. Rev. B 95, 241109 (2017)

[3] S Wu et. al. Phys. Rev. B 100, 214511 (2019)

(a) Quasi-1D Fe-ladder structure of BaFe2X3 (X=Se,S) and its block-type, stripe type spin ordered states. Both sister compounds exhibit insulating properties at ambient pressure and unveil a high-pressure superconducting phase adjacent to the insulator-metal phase transition [1, 2] (Also see P-T phase diagram ). Measurements under the hydrostatic pressure by the (b) Synchrotron X-ray, (c) Muon spin rotation, and (d) Powder neutron diffraction techniques. [3, 4]

Kondo Lattice system

CeNiAsO is a metallic Kondo lattice system that exhibits pressure-induced quantum critical point (QCP) with the indication of Fermi-surface (FS) reconstruction [1]. Metallic quantum criticality presents profound intellectual challenges, and CeNiAsO is a promising model system in this area about which precious little is presently known. Through neutron scattering and muon spin rotation measurements, we report incommensurate (IC) magnetism with the small FS near quantum criticality in CeNiAsO. Specifically, we unveil two successive magnetic phases, an IC longitudinal spin density wave (SDW) for the upper and a coplanar commensurate state for the lower phase [2]. This work exposes spin correlations near metallic quantum critical in CeNiAsO, breaking experimental ground to distinguish commensurate and IC order. The itinerant nature near QCP provides essential information for classifying and theoretically modeling the system, more widely the metallic QCP. The studies under hydrostatic pressure are in progress to investigate the putative metallic quantum critical point in CeNiAsO.

Reference

[1] Y. Luo et. al. Nat. Mater. 13, 777 (2014)

[2] S Wu et. al. Phys. Rev. Lett. 122, 197203 (2019)

(a) Crystal structure of CeNiAsO. (b) Muon spin relaxation and (c) neutron diffraction measurements exhibit incommensurate and commensurate magnetism in CeNiAsO near metallic quantum critical point [2].

Other projects during my PhD (Before 2018). See my old sites:

shanwu jhu