arXiv subjects

Ina Park

Publications and source records attributed to Ina Park.

9 recordsLinked to original sources

arXiv2026

Universal Signature of Hundness and Its Quantification

Hund's coupling $J$ induces fundamentally different correlation effects from Hubbard $U$. This leads to a violation of the Brinkman--Rice scenario and the emergence of a Janus-faced phase owing to its low-energy effectiveness, in which band renormalization is confined below a characteristic energy scale. We propose a quantitative framework to capture low-energy effectiveness through two correlation factors: $z_L$ for low-energy quasiparticle renormalization and $z_H$ for high-energy charge fluctuations, newly introduced in this study. The discrepancy between $z_L$ and $z_H$ reflects the Hund character of the correlation. By establishing a one-to-one correspondence between correlation factors and the spin and charge susceptibilities, we identify the spin-degree-of-freedom effectiveness as the microscopic origin of low-energy effectiveness. Our framework, validated across multiorbital models and real materials, provides a universal and quantitative measure of Hundness.

cond-mat.str-el↗

arXiv2026

Correlation-driven tunability of altermagnetism in RuO$_2$

RuO$_2$ has been regarded as a prototypical candidate for metallic altermagnet, offering a potential platform for high-speed and high-efficiency spintronics. However, the magnetic ground state of RuO$_2$ remains a topic of active debate due to conflicting experimental reports. In this work, we investigate the effect of electron correlations in RuO$_2$ using density functional theory combined with dynamical mean-field theory (DFT+DMFT). In contrast to previous DFT-based studies, DFT+DMFT captures essential dynamical correlation effects, yielding spectral functions and optical conductivities in excellent quantitative agreement with experiments, and further reveals that RuO$_2$ resides in the close vicinity of both the paramagnetic-altermagnetic phase boundary and the itinerant-localized crossover, rendering the magnetic ground state highly susceptible to external perturbations. Indeed, even a minimal compressive strain of $\sim$0.5% is sufficient to drive the system into an altermagnetic phase. These findings elucidate the origin of the conflicting experimental observations and reveal that dynamical correlation effects are the key driving force behind the highly tunable magnetic ground state of RuO$_2$.

cond-mat.mtrl-sci↗

arXiv2026

Neural-Network Quantum Embedding Solvers for Correlated Materials

Quantum impurity solvers are the computational bottleneck of quantum embedding approaches to correlated materials, such as dynamical mean-field theory (DMFT). We show that neural networks trained on synthetic, material-agnostic data learn the impurity mapping from hybridization functions and local interactions to Green's functions with quantitative accuracy for both model systems and real materials, providing fast solvers for single- and multi-orbital models. Benchmarks against numerically controlled quantum Monte Carlo show that the method reproduces the Mott transition, multi-orbital phase diagrams of Hubbard-Kanamori models, and the electronic properties of SrVO$_3$ and SrMnO$_3$. The learned solvers achieve orders-of-magnitude speedup and can initialize controlled calculations, dramatically accelerating DMFT while preserving accuracy.

cond-mat.str-el↗

arXiv2025

Impact of strong electronic correlations on altermagnets: the case of NiS2

One of the distinguishing features of an altermagnet is that its spin-up and spin-down bands display a nodal momentum-dependent splitting even in the absence of spin-orbit coupling. While this property has been investigated in many weakly-correlated altermagnetic materials, the impact of strong electron-electron interactions on the spin-dependent electronic structure has remained little explored, particularly in metals. Here, we propose NiS2 as a prototypical strongly correlated metallic altermagnet. While at ambient pressure this compound is an altermagnetic Mott insulator, it undergoes a pressure-driven metal-insulator transition (MIT) while maintaining its altermagnetic ordered phase. By systematically comparing DFT, DFT+U, and DFT+DMFT calculations on the metallic altermagnetic phase near the MIT, we disentangle how strong static and dynamic correlations modify the electronic structure. Specifically, the spin splitting of the bands is modified not only through the enhancement of the local magnetic moment caused by static correlations, but also by the momentum-dependent bandwidth renormalization caused by dynamic correlations. Moreover, dynamic electronic correlations cause a pronounced lifetime asymmetry between the spin-up and spin-down quasiparticles, an effect that is amplified by the particle-hole asymmetry promoted by Hund's correlations. Our results not only shed light on the rich landscape of correlation effects in metallic altermagnets, but also establishes NiS2 as a platform to investigate the interplay between Mott and Hund physics and altermagnetic order.

cond-mat.str-el↗

arXiv2025

Pressure-induced thermal expansion anomalies in dhcp iron hydride associated with magnetoelastic coupling

Iron hydride with a double hexagonal close-packed structure (dhcp-FeH$_{x}$) undergoes a ferromagnetic-paramagnetic transition without changing its crystal structure. Despite its relevance to metal-hydrogen interactions and magnetically driven elasticity, the extensive investigation of this phase is almost limited to room temperature. Here, we performed XRD measurements at high pressure and high temperature, identifying the singularity in the temperature-volume relationship as the Curie temperature ($T_\text{C}$). Pressurization lowered the $T_\text{C}$ of dhcp-FeH$_{x}$, and pronounced volume anomalies, indicating that pressure enhanced magnetoelastic coupling. Density functional theory combined with dynamical mean-field theory (DFT+DMFT) reproduced the spontaneous magnetization and its negative pressure dependence of $T_\text{C}$, consistent with our experimental results. This establishes a methodology for determining magnetic transition temperatures and magnetoelastic coupling effects, and highlights dhcp-FeH$_{x}$ as a unique model system for providing new insights into itinerant-electron magnetism.

cond-mat.mtrl-sci↗

arXiv2023

Superconductivity of metastable dihydrides at ambient pressure

Hydrogen in metals is a significant research area with far-reaching implications, encompassing diverse fields such as hydrogen storage, metal-insulator transitions, and the recently emerging phenomenon of room-temperature ($\textit{$T_C$}$) superconductivity under high pressure. Hydrogen atoms pose challenges in experiments as they are nearly invisible, and they are considered within ideal crystalline structures in theoretical predictions, which hampers research on the formation of meta-stable hydrides. Here, we propose pressure-induced hydrogen migration from tetrahedral site ($\textit{T}$) to octahedral site ($\textit{O}$),forming $LaH_x^OH_{2-x}^{T}$ in cubic $LaH^2$.Under decompression, it retains $H_x^O$ occupancy, and is dynamically stable even at ambient pressure, enabling a synthesis route of metastable dihydrides via compression-decompression process. We predict that the electron phonon coupling strength of $LaH_x^OH_{2-x}^{T}$ is enhanced with increasing $\textit{x}$, and the associated $\textit{$T_C$}$ reaches up to 10.8 $\textit{K}$ at ambient pressure. Furthermore, we calculated stoichiometric hydrogen migration threshold pressure ($\textit{$P_C$}$) for various lanthanides dihydrides ($\textit{R}$$H_2$, where $\textit{R}$=Y, Sc, Nd, and Lu), and found an inversely linear relation between $\textit{$P_C$}$ and ionic radii of $\textit{R}$. We propose that the highest $\textit{$T_C$}$ in the face-centered-cubic dihydride system can be realized by optimizing the $\textit{O}$/$\textit{T}$-site occupancies.

cond-mat.supr-con↗

arXiv2023

Clean realization of the Hund physics near the Mott transition: $\mathrm{NiS_2}$ under pressure

Strong correlation effects caused by Hund's coupling have been actively studied during the past decade. Hund's metal, strongly correlated while far from the Mott insulating limit, was studied as a representative example. However, recently, it was revealed that a typical Mott system also exhibits a sign of Hund physics by investigating the kink structure in the spectral function of $\mathrm{NiS_{2-x}Se_x}$. Therefore, to understand the Hund physics in a half-filled multi-orbital system near the metal-insulator transition, we studied pressure-induced metallic states of $\mathrm{NiS_2}$ by using density functional theory plus dynamical mean-field theory. Hund physics, responsible for suppressing local spin fluctuation, gives low-energy effective correlations, separated from Mott physics, which suppresses charge fluctuation at higher energy. This effect is prominent when $J$ becomes comparable to the quasiparticle kinetic energy, showing apparent scaling behavior of the kink position $E_{kink} \sim J \cdot Z$. We suggest that the Hund effect can also be observed in the optical conductivity as a non-Drude-like tail with $1/\omega$ frequency dependence and non-monotonic temperature evolution of the integrated optical spectral weight at a fixed frequency. Our study demonstrates the important role of Hund's coupling for electronic correlations even in a half-filled system.

cond-mat.str-el↗

arXiv2023

Origin of superionic state in Earth's inner core

Earth's inner core (IC) serves as a reservoir for volatile elements, which significantly affects its behavior and properties. Recent studies suggest that superionicity can be observed in ice and iron hydrides under high-pressure and temperature conditions, providing an alternative understanding of the planet's interior. In this study, we demonstrated that electride formation drives the superionic state in iron hydride under IC pressure conditions. The electride stabilizes the iron lattice and provides a pathway for volatile diffusion. The coupling between lattice stability and superionicity is triggered near 100 GPa and enhanced at higher pressures. The electride-driven superionicity can also be generalized for volatiles in other rocky planetary cores. These findings provide new insights into the mechanisms of core formation and evolution of rocky planets.

physics.geo-ph↗

arXiv2021

Extraordinary Photostability and Davydov Splitting in BN-Sandwiched Single-Layer Tetracene Molecular Crystals

Two-dimensional molecular crystals have been beyond the reach of systematic investigation because of the lack or instability of their well-defined forms. Here, we demonstrate drastically enhanced photostability and Davydov splitting in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D crystals of graphene or hexagonal BN. Molecular orientation and long-range order mapped with polarized wide-field photoluminescence imaging and optical second-harmonic generation revealed high crystallinity of the 2D Tc and its distinctive orientational registry with the 2D inorganic crystals, which were also verified with first-principles calculations. The reduced dielectric screening in 2D space was manifested by enlarged Davydov splitting and attenuated vibronic sidebands in the excitonic absorption and emission of monolayer Tc crystals. Photostable 2D molecular crystals and their size effects will lead to novel photophysical principles and photonic applications.

cond-mat.mtrl-sci↗