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Yang Zhang
Yang Zhang
Currently, I am a postdoctoral researcher in the Department of Physics at the University of Tennessee-Knoxville, under the supervision of Prof. Elbio Dagotto.
Currently, I am a postdoctoral researcher in the Department of Physics at the University of Tennessee-Knoxville, under the supervision of Prof. Elbio Dagotto.
My main area of research is Condensed Matter Physics, focusing on studying the physical properties of quantum materials. My research mainly aims to understand the novel physical phenomena observed in experiments, including superconductivity, magnetism, charge or spin density waves, multiferroicity, and many others. By using density functional theory, classical Monte Carlo, random phase approximation, and density matrix renormalization group methods, I can provide a fundamental understanding and insights into the nature and physical mechanisms for experimental discoveries, including superconducting pairing symmetry, phase transitions, orbital physics, ferroelectric or multiferroic mechanisms, and others.
Research interest:
Research interest:
Nickelate superconductors
In 2023, a record Tc~80 K superconductivity was found in the Ruddlesden-Popper (RP) perovskite compound La3Ni2O7 (d7.5 configuration) with a NiO6 octahedron bilayer structure [Nature 621, 493 (2023)], representing the first non-IL NiO2 layered nickelate superconductor. Very recently, the superconductivity was also reported in another RP trilayer nickelate La4Ni3O10 (d7.33) under pressure [Nature 631, 531 (2024)].
Motivated by those exciting discoveries of nickelate high-Tc superconductors, in the past year, my main interest has been focused on this topic, providing my insight into the interesting puzzles from experiments. Below are several typical works that I have done with my colleagues.
1. We introduced the “two-orbital dimer” lattice picture for bilayer La3Ni2O7, while IL NdNiO2 is analogous to a uniform chain. A large interorbital hopping between eg states via in-plane O px or py orbitals was obtained in La3Ni2O7, leading to a strong in-plane competition between antiferromagnetic (AFM) and ferromagnetic (FM) tendencies. However, this hopping is nearly zero in IL nickelates with robust in-plane AFM tendency. [Phys. Rev. B 108, L180510 (2023)].
2. We classified the key differences between bilayer La3Ni2O7 and bilayer Bi2Sr2CaCu2O8: (i) two eg orbitals are needed for nickelates vs a single dx2-y2 Cu orbital for cuprates; (ii) s±-wave pairing for nickelate vs d-wave for cuprate. Furthermore, a first-order pressure-induced structural phase transition and possible structural phase coexistence were also discussed [Nat. Commun. 15, 2470 (2024)].
3. We also found a leading s±-wave pairing symmetry spin-fluctuation in La4Ni3O10 with different nesting vectors compared with La3Ni2O7. [Phys. Rev. Lett. 133, 136001 (2024)]. Furthermore, we also provide a theoretical perspective that the superconductivity in La3Ni2O7 does not originate from alternating monolayer-trilayer stacking structures [Phys. Rev. B 110, L060510 (2024)].
4. We also studied the bilayer La3Ni2O6 systematically to understand why superconductivity is absent in this system but is obtained in bilayer La3Ni2O7 under pressure: the key issue is the different role of the d3z2−r2 orbital in La3Ni2O6 [Phys. Rev. B 109, 045151 (2024)].
1D strongly correlated electronic systems
One-dimensional (1D) correlated electronic systems are also one of my main research directions. Due to strong quantum fluctuations characteristics, many physical parameters compete, leading to many unique physical properties.
Iron chains and ladders
In previous years, working with my colleagues, we worked on some multi-orbital iron chains and ladders, by focusing on interesting magnetic states, structural phase transition, and novel electronic states. Below are some examples.
1. We constructed the phase diagram of iron ladders BaFe2S3 [Phys. Rev. B 95, 115154 (2017)] and BaFe2Se3 [Phys. Rev. B 97, 045119 (2018)], suggesting that the superconductivity may be caused by spin fluctuations of the stripe state.
2. We also developed the magnetic phase diagrams for iron ladders with different electron concentrations [Phys. Rev. B 100, 184419 (2019), Phys. Rev. B 101, 144417 (2020)].
3. We also studied the possible orbital-selective Mott phase physics and other interesting magnetic states [Phys. Rev. B 104, 125122 (2021), Commun. Phys. 6, 199 (2023), Phys. Rev. Lett. 127, 077204 (2021), Phys. Rev. B 105, 075119 (2022)].
Peierls transition
I am also interested in the 1D systems with Peierl distortion. A typical example is the dimerized chain (TaSe4)2I where my work revealed two competing charge-density-wave (CDW) states with different Ta-tetramerization patterns to be associated with the low-temperature structures [Phys. Rev. B 101, 174106 (2020)]. The semimetal-to-insulator transition induced by a Fermi-surface-driven instability was also discussed, which affects the Weyl physics developed above TCDW. Furthermore, the spin-orbit coupling could generate Rashba-like band splittings in the insulating CDW phases.
Another example is that I proposed a strategy to obtain the interesting orbital-selective Peierls phase with metallic characters that can be obtained in MoOCl2 (d2), where one orbital has a large bonding-antibonding splitting that is decoupled from each other and other orbitals contribute to the metallic conductivity [Phys. Rev. B 104, L060102 (2021)].
AFM vs SOC
In $4d/5d$ transition-metal systems, many interesting physical properties arise from the interplay of bandwidth, electronic correlations, and spin-orbit interactions. In the large SOC limit, the J = 0 singlets nonmagnetic (NM) insulating state, and associated excitonic magnetism induced by spin-orbital excitons was proposed in the d4 system [Phys. Rev. Lett. 111, 197201 (2013)].
I also made some efforts on 1D d4 chain systems with strong SOC. Due to large crystal-field splitting or hopping, the J = 0 singlet NM insulating state was brokendown in OsCl4[Appl. Phys. Lett. 120, 023101 (2022)] and AOCl2/(A= Ru or Os) [Phys. Rev. B 105, 174410 (2022)], resulting in an effective S = 1 AFM state. In addition, I found that this interesting J = 0 state can be achieved in K2OsX6 (X = F, Cl, and Br) [Phys. Rev. B 106, 155148 (2022)].
Ferroelectric or multiferroelectric systems
In my Ph. D. career, mainly working with Prof. Shuai Dong, we worked on some projects on this topic, Here are some examples.
1. Working with experimental collaborators, we proposed a simplified rigid ion model to understand the giant negative piezoelectricity in the 2D layered van der Waals material CuInP2S6 [Sci. Adv. 5, eaav3780 (2019)].
2. Furthermore, we also finished several projects on the topic of magnetoelectric coupling in layered systems, independently or with experimental collaborators, such as exchange striction-driven magneto-dielectric effects in CaOFeS [Phys. Rev. Mater. 1, 034406 (2017)], magneto-electric effect in Ca3Mn2O7 [Appl. Phys. Lett. 113, 022902 (2018)].
3. Within Landau theory, magnetism and polarity are homotopic, displaying a one-to-one correspondence between most physical characteristics. We discussed the spontaneous noncollinear electric dipole order as a ground state in a dioxydihalides family, induced by competing ferroelectric and antiferroelectric soft modes. This intrinsic of dipoles generates unique physical properties, such as Z2 × Z2 topological domains, atomic-scale dipole vortices, and negative piezoelectricity [Phys. Rev. Lett. 123, 067601 (2019)].
4. We discussed the origin of the ferroelectric distortion along the a-axis in VOI2 with a 3d1 electronic configuration. Specifically, the “pseudo Jahn-Teller” effect caused by the coupling between empty V (xz/yz and 3z2−r2 ) and O 2p states is proposed as the mechanism that stabilizes the FE distortion from the paraelectric phase. We also found very short-range antiferromagnetic coupling along the V-V chain due to the formation of nearly decoupled spin singlets in the ground state [Phys. Rev. B 103, L121114 (2021)].
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