arXiv subjects

Mercouri G. Kanatzidis

Publications and source records attributed to Mercouri G. Kanatzidis.

At least 19 recordsLinked to original sources

arXiv2026

Photoinduced phase heterogeneity and charge localization in SnSe

Time-resolved multi-terahertz (THz) spectroscopy is used to observe pump fluence-dependent dynamics in the optical conductivity of photoexcited tin selenide (SnSe) over an ultrabroadband spectral range of 0.5 - 11 THz at fluences from 0.1 - 7.5 mJ/cm$^2$. A free carrier Drude spectrum is observed at pump fluences below 3 mJ/cm$^2$, with optical phonons well described by the equilibrium Pnma structural phase. With increasing fluence, a suppression of the DC photoconductivity is observed, indicating an interruption of long range transport due to phase disorder. Concomitantly, the optical phonons exhibit features that can no longer be explained by a pure Pnma phase, with a frequency shift and narrowing of the $B^2_{1u}$ mode and a new mode appearing at $\sim$3.0 THz consistent with a transition to a higher-symmetry structure. At an intermediate fluence of 3.1 mJ/cm$^2$, a high frequency Lorentzian component consistent with phase heterogeneity appears that rapidly redshifts after 2 ps and whose amplitude exponentially decays on a 90 ps time scale. Our experimental measurements and theoretical calculations provide evidence for a non-thermal, photo-induced nucleation of higher symmetry, semi-metallic phase domains in SnSe appearing within 200 fs.

cond-mat.mtrl-sci↗

arXiv2025

4D-QENS Analysis of Correlated Ionic Conduction in SrCl$_2$

Methods of elucidating the mechanisms of fast-ion conduction in solid-state materials are pivotal for advancements in energy technologies such as batteries, fuel cells, sensors, and supercapacitors. In this study, we examine the ionic conduction pathways in single crystal SrCl$_2$, which is a fast-ion conductor above 900~K, using four-dimensional Quasi-Elastic Neutron Scattering (4D-QENS). We explore both coherent and incoherent neutron scattering at temperatures above the transition temperature into the superionic phase to explore the correlated motion of hopping anions. Refinements of the incoherent QENS yield residence times and jump probabilities between lattice sites in good agreement with previous studies, confirming that ionic hopping along nearest-neighbor directions is the most probable conduction pathway. However, the coherent QENS reveals evidence of de Gennes narrowing, indicating the importance of ionic correlations in the conduction mechanism. This highlights the need for improvements both in the theory of ionic transport in fluorite compounds and the modeling of coherent 4D-QENS in single crystals.

cond-mat.mtrl-sci↗

arXiv2025

Evolution from Topological Dirac Metal to Flat-band-Induced Antiferromagnet in Layered KxNi4S2 (0<=x<=1)

Condensed matter systems with coexisting Dirac cones and flat bands, and a switchable control between them within a single system, are desirable but remarkably uncommon. Here we report a layered quantum material system, KxNi4S2 (0 <= x <= 1), that simultaneously hosts both characteristics without involving typical Kagome/honeycomb lattices. Enabled by a topochemical K-deintercalation process, the Fermi surface can be fine-tuned continuously over a wide range of energies. Consequently, a non-magnetic Dirac-metal state with a topological nontrivial Z2 index of 1;(000), supported by first-principles calculations and high mobility up to 1471 cm2V-1s-1, is observed on the K-rich x = 1 side, whereas a flat-band induced antiferromagnetic state with TN up to 10.1 K emerges as K-content approaches 0. The KxNi4S2 system offers a versatile platform for exploring emerging phenomena and underscores a viable pathway for in-situ control of quantum materials dominated by Dirac cones, flat bands, and their interplay.

cond-mat.mtrl-sci↗

arXiv2025

Origin of the Unusual Temperature Dependence of the Upper Critical Field of Kagome Superconductor CsV3Sb5: Multiple Bands or van Hove Singularities?

Van Hove singularities (vHs) located close to the Fermi level in Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have profound influence on their electronic and transport characteristics. Specifically, magneto-transport and susceptibility measurements on CsV3Sb5 reveal an anomalous temperature dependence of the upper critical field H_c2 (T), characterized by a pronounced upward curvature for both in-plane and c-axis magnetic fields, with zero-temperature H_c2 values of ~6.0 T and ~1.2 T, respectively. Our theoretical analysis, using a newly developed single-band model incorporating vHs and gap anisotropy, suggests that the observed upper critical field behavior is predominantly driven by the anisotropy of the Fermi velocity originating from vHs, instead of multi-band effects or gap anisotropy. Increased electron scattering introduced by proton irradiation defects smears out the vHs, reduces anisotropy, and recovers the conventional H_c2 (T) behavior, corroborating our proposed model.

cond-mat.supr-con↗

arXiv2025

Lattice-charge coupling in a trilayer nickelate with intertwined density wave order

Intertwined charge and spin correlations are ubiquitous in a wide range of transition metal oxides and are often perceived as intimately related to unconventional superconductivity. Theoretically envisioned as driven by strong electronic correlations, the intertwined order is usually found to be strongly coupled to the lattice as signaled by pronounced phonon softening. Recently, both charge/spin density waves (CDW/SDW) and superconductivity have been discovered in several Ruddlesden-Popper (RP) nickelates, in particular trilayer nickelates RE4Ni3O10 (RE=Pr, La). The nature of the intertwined order and the role of lattice-charge coupling are at the heart of the debate about these materials. Using inelastic X-ray scattering, we mapped the phonon dispersions in RE4Ni3O10 and found no evidence of phonon softening near the CDW wavevector over a wide temperature range. Calculations of the electronic susceptibility revealed a peak at the observed SDW ordering vector but not at the CDW wavevector. The absence of phonon softening is in sharp contrast to that in canonical oxide materials, notably cuprates. Our experimental and theoretical findings highlight the crucial role of the spin degree of freedom and establish a foundation for understanding the interplay between superconductivity and density-wave transitions in RP nickelate superconductors and beyond.

cond-mat.str-el↗

arXiv2025

Two-dimensional perovskites with maximum symmetry enable exciton diffusion length exceeding 2 micrometers

Realizing semiconductors with high symmetry of their crystallographic structures has been a virtue of inorganic materials and has resulted in novel physical behaviors. In contrast, hybrid (organic and inorganic) crystals such as two-dimensional metal halide perovskites exhibit much lower crystal symmetry due to in-plane or out of plane octahedral distortions. Despite their amazing ability for photoinduced light emission at room temperature, the Achilles' heel of this attractive class of 2D materials for optoelectronics remains the poor control and lack of performance for charge carrier transport. Inspired by the tremendous charge carrier properties of the 3D cubic perovskite phase of FAPbI3 and combining the use of the appropriate cage cation, the spacer molecule and the temperature and rate of crystallization, we report a new series of FA-based layered two-dimensional perovskites that exhibits the highest theoretically predicted symmetry with a tetragonal P4/mmm space group, resulting in no octahedral distortion in both in-plane and out-of-plane directions. These 2D perovskites present the shortest interlayer distances (4 angstrom), which results in systematically lower bandgaps (1.7 to 1.8 eV). Finally, the absence of octahedral distortions, results in an exciton diffusion length of 2.5 μm, and a diffusivity of 4.4 cm2s-1, both of which are an order of magnitude larger compared to previously reported 2D perovskites and on par with monolayer transition metal dichalcogenides.

cond-mat.mtrl-sci↗

arXiv2025

Overcoming the surface paradox: Buried perovskite quantum dots in wide-bandgap perovskite thin films

Colloidal perovskite quantum dots (PQDs) are an exciting platform for on-demand quantum, and classical optoelectronic and photonic devices. However, their potential success is limited by the extreme sensitivity and low stability arising from their weak intrinsic lattice bond energy and complex surface chemistry. Here we report a novel platform of buried perovskite quantum dots (b-PQDs) in a three-dimensional perovskite thin-film, fabricated using one-step, flash annealing, which overcomes surface related instabilities in colloidal perovskite dots. The b-PQDs demonstrate ultrabright and stable single-dot emission, with resolution-limited linewidths below 130 μeV, photon-antibunching (g^2(0)=0.1), no blinking, suppressed spectral diffusion, and high photon count rates of 10^4/s, consistent with unity quantum yield. The ultrasharp linewidth resolves exciton fine-structures (dark and triplet excitons) and their dynamics under a magnetic field. Additionally, b-PQDs can be electrically driven to emit single photons with 1 meV linewidth and photon-antibunching (g^2(0)=0.4). These results pave the way for on-chip, low-cost single-photon sources for next generation quantum optical communication and sensing.

physics.optics↗

arXiv2024

Tunable Anomalous Hall Effect in a Kagome Ferromagnetic Weyl Semimetal

Emerging from the intricate interplay of topology and magnetism, the giant anomalous Hall effect (AHE) is the most known topological property of the recently discovered kagome ferromagnetic Weyl semimetal Co_3Sn_2S_2 with the magnetic Co atoms arranged on a kagome lattice. Here we report that the AHE in Co_3Sn_2S_2 can be fine-tuned by an applied magnetic field orientated within ~2 degrees of the kagome plane, while beyond this regime, it stays unchanged. Particularly, it can vanish in magnetic fields parallel to the kagome plane and even decrease in magnetic fields collinear with the spin direction. This tunable AHE can be attributed to local spin switching enabled by the geometrical frustration of the magnetic kagome lattice, revealing that spins in a kagome ferromagnet change their switching behavior as the magnetic field approaches the kagome plane. Our results also suggest a versatile way to tune the properties of a kagome magnet.

cond-mat.mtrl-sci↗

arXiv2024

In-situ local imaging of ferromagnetism and superconductivity in RbEuFe$_4$As$_4$

The coexistence of superconductivity and ferromagnetism is an intrinsically interesting research focus in condensed matter physics but the study is limited by low superconducting ($T_c$) and magnetic ($T_m$) transition temperatures in related materials. Here, we used a scanning superconducting quantum interference device to image the in-situ diamagnetic and ferromagnetic responses of RbEuFe$_4$As$_4$ with high $T_c$ and $T_m$. We observed significant suppression of superfluid density in vicinity of the magnetic phase transition, signifying fluctuation-enhanced magnetic scatterings between Eu spins and Fe 3$d$ conduction electrons. Intriguingly, we observed multiple ferromagnetic domains which should be absent in an ideal magnetic helical phase. The formation of these domains demonstrates a weak $c$-axis ferromagnetic component probably arising from Eu spin-canting effect, indicative of possible superconductivity-driven domain Meissner and domain vortex-antivortex phases as revealed in EuFe$_2$(As$_{0.79}$P$_{0.21}$)$_2$. Our observations highlight RbEuFe$_4$As$_4$ is a unique system which includes multiple interplay channels between superconductivity and ferromagnetism.

cond-mat.supr-con↗

arXiv2024

Kramers nodal line in the charge density wave state of YTe$_3$ and the influence of twin domains

Recent studies have focused on the relationship between charge density wave (CDW) collective electronic ground states and nontrivial topological states. Using angle-resolved photoemission and density functional theory, we establish that YTe$_3$ is a CDW-induced Kramers nodal line (KNL) metal, a newly proposed topological state of matter. YTe$_3$ is a non-magnetic quasi-2D chalcogenide with a CDW wave vector ($q_{\rm cdw}$) of 0.2907c$^*$. Scanning tunneling microscopy and low energy electron diffraction revealed two orthogonal CDW domains, each with a unidirectional CDW and similar YTe$_3$. The effective band structure (EBS) computations, using DFT-calculated folded bands, show excellent agreement with ARPES because a realistic x-ray crystal structure and twin domains are considered in the calculations. The Fermi surface and ARPES intensity plots show weak shadow bands displaced by $q_{\rm cdw}$ from the main bands. These are linked to CDW modulation, as the EBS calculation confirms. Bilayer split main and shadow bands suggest the existence of crossings, according to theory and experiment. DFT bands, including spin-orbit coupling, indicate a nodal line along the $Σ$ line from multiple band crossings perpendicular to the KNL. Additionally, doubly degenerate bands are only found along the KNL at all energies, with some bands dispersing through the Fermi level.

cond-mat.mtrl-sci↗

arXiv2024

Noncollinear electric dipoles in a polar, chiral phase of CsSnBr$_3$ perovskite

Polar and chiral crystal symmetries confer a variety of potentially useful functionalities upon solids by coupling otherwise noninteracting mechanical, electronic, optical, and magnetic degrees of freedom. We describe two unstudied phases of the 3D perovskite, CsSnBr$_3$, which emerge below 85 K due to the formation of Sn(II) lone pairs and their interaction with extant octahedral tilts. Phase II (77 K<$T$<85 K, space group $P2_1/m$) exhibits ferroaxial order driven by a noncollinear pattern of lone pair-driven distortions within the plane normal to the unique octahedral tilt axis, preserving the inversion symmetry observed at higher temperatures. Phase I ($T$<77 K, space group $P2_1$) additionally exhibits ferroelectric order due to distortions along the unique tilt axis, breaking both inversion and mirror symmetries. This polar and chiral phase exhibits second harmonic generation from the bulk and a large, intrinsic polarization$-$electrostriction coefficient along the polar axis ($Q_{22}\approx$1.1 m$^4$ C$^{-2}$), resulting in acute negative thermal expansion ($α_V=-9\times10^{-5}$ K$^{-1}$) through the onset of spontaneous polarization. The unprecedented structures of phases I and II were predicted by recursively following harmonic phonon instabilities to generate a tree of candidate structures and subsequently corroborated by synchrotron X-ray powder diffraction and polarized Raman and $^{81}$Br nuclear quadrupole resonance spectroscopies. Relativistic electronic structure scenarios compatible with reported photoluminescence measurements are discussed. Together, the polar symmetry, small bandgap, large spin-orbit splitting of Sn 5$p$ orbitals, and predicted strain sensitivity of the symmetry-breaking distortions suggest bulk samples and epitaxial films of CsSnBr$_3$ or its neighboring solid solutions as strong candidates for bulk Rashba effects.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast Photo-induced Phase Change in SnSe

Time-resolved multi-terahertz (THz) spectroscopy is used to observe an ultrafast, non-thermal electronic phase change in SnSe driven by interband photoexcitation with 1.55 eV pump photons. The transient THz photoconductivity spectrum is found to be Lorentzian-like, indicating charge localization and phase segregation. The rise of photoconductivity is bimodal in nature, with both a fast and slow component due to excitation into multiple bands and subsequent intervalley scattering. The THz conductivity magnitude, dynamics, and spectra show a drastic change in character at a critical excitation fluence of approximately 6 mJ/cm^2 due to a photo-induced phase segregation and a macroscopic collapse of the band gap.

cond-mat.mtrl-sci↗

arXiv2023

Extended Kohler's Rule of Magnetoresistance in TaCo$_2$Te$_2$

TaCo$_2$Te$_2$ is recently reported to be an air-stable, high mobility Van der Waals material with probable magnetic order. Here we investigate the scaling behavior of its magnetoresistance. We measured both the longitudinal ($ρ_{xx}$) and Hall ($ρ_{xy}$) magnetoresistivities of TaCo$_2$Te$_2$ crystals in magnetic fields parallel to the c-axis and found that the magnetoresistance violates the Kohler's rule $MR \sim f[H/ρ_0]$ while obeying the extended Kohler's rule $MR \sim f[H/(n_Tρ_0)]$, where $MR \sim [ρ_{xx}(H)-ρ_0]/ρ_0$, $H$ is the magnetic field, $n_T$ is a thermal factor, $ρ_{xx}(H)$ and $ρ_0$ are the resistivities at $H$ and zero field, respectively. While deviating from those of the densities of electrons ($n_e$) and holes ($n_h$) obtained from the two-band model analysis of the magnetoconductivities, the temperature dependence of $n_T$ is close to that of the Hall carrier densities $n_H$ calculated from the slopes of $ρ_{xy}(H)$ curves at low magnetic fields, providing a new way to obtain the thermal factor in the extended Kohler's rule.

cond-mat.mtrl-sci↗

arXiv2023

A Unified Understanding of Minimum Lattice Thermal Conductivity

We propose a first-principles model of minimum lattice thermal conductivity ($κ_{\rm L}^{\rm min}$) based on a unified theoretical treatment of thermal transport in crystals and glasses. We apply this model to thousands of inorganic compounds and discover a universal behavior of $κ_{\rm L}^{\rm min}$ in crystals in the high-temperature limit: the isotropically averaged $κ_{\rm L}^{\rm min}$ is independent of structural complexity and bounded within a range from $\sim$0.1 to $\sim$2.6 W/[m$\cdot$K], in striking contrast to the conventional phonon gas model which predicts no lower bound. We unveil the underlying physics by showing that for a given parent compound $κ_{\rm L}^{\rm min}$ is bounded from below by a value that is approximately insensitive to disorder, but the relative importance of different heat transport channels (phonon gas versus diffuson) depends strongly on the degree of disorder. Moreover, we propose that the diffuson-dominated $κ_{\rm L}^{\rm min}$ in complex and disordered compounds might be effectively approximated by the phonon gas model for an ordered compound by averaging out disorder and applying phonon unfolding. With these insights, we further bridge the knowledge gap between our model and the well-known Cahill-Watson-Pohl (CWP) model, rationalizing the successes and limitations of the CWP model in the absence of heat transfer mediated by diffusons. Finally, we construct graph network and random forest machine learning models to extend our predictions to all compounds within the Inorganic Crystal Structure Database (ICSD), which were validated against thermoelectric materials possessing experimentally measured ultralow $κ_{\rm L}$. Our work offers a unified understanding of $κ_{\rm L}^{\rm min}$, which can guide the rational engineering of materials to achieve $κ_{\rm L}^{\rm min}$.

cond-mat.mtrl-sci↗

arXiv2023

Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the $<$100$>$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.

cond-mat.mtrl-sci↗

arXiv2023

Coupling photogeneration with thermodynamic modeling of light-induced alloy segregation enables the discovery of stabilizing dopants

We developed a generalized model that considers light as energy contributed through the thermalization of excited carriers to generate excitation-intensity- and temperature-dependent phase diagrams. We find that the model replicates the light-induced phase segregation behavior of the MAPb(I,Br)3 system. From there, we sought to study how best to design new, stable, mixed-halide alloys. The resultant first-principles predictions show that the pseudo halide anion BF4- suppresses phase segregation in FA0.83Cs0.17Pb(I0.6Br0.4)3, resulting in enhanced operating stability. The findings reveal that photostability is linked with the structure and electronic properties of materials and may be overcome using informed alloying strategies.

cond-mat.mtrl-sci↗

arXiv2022

Unreliability of two-band model analysis of magnetoresistivities in unveiling temperature-driven Lifshitz transition

Recently, anomalies in the temperature dependences of the carrier density and/or mobility derived from analysis of the magnetoresistivities using the conventional two-band model have been used to unveil intriguing temperature-induced Lifshitz transitions in various materials. For instance, two temperature-driven Lifshitz transitions were inferred to exist in the Dirac nodal-line semimetal ZrSiSe, based on two-band model analysis of the Hall magnetoconductivities where the second band exhibits a change in the carrier type from holes to electrons when the temperature decreases below T = 106 K and a dip is observed in the mobility versus temperature curve at T = 80 K. Here, we revisit the experiments and two-band model analysis on ZrSiSe. We show that the anomalies in the second band may be spurious, because the first band dominates the Hall magnetoconductivities at T > 80 K, making the carrier type and mobility obtained for the second band from the two-band model analysis unreliable. That is, care must be taken in interpreting these anomalies as evidences for temperature-driven Lifshitz transitions. Our skepticism on the existence of such phase transitions in ZrSiSe is further supported by the validation of the Kohler's rule for magnetoresistances at temperatures below 180 K. This work showcases potential issues in interpreting anomalies in the temperature dependence of the carrier density and mobility derived from the analysis of magnetoconductivities or magnetoresistivities using the conventional two-band model.

cond-mat.mtrl-sci↗

arXiv2022

Excitonic Spin-Coherence Lifetimes in CdSe Nanoplatelets Increase Significantly with Core/Shell Morphology

We report spin-polarized transient absorption for colloidal CdSe nanoplatelets as functions of thickness (2 to 6 monolayer thickness) and core/shell motif. Using electro-optical modulation of co- and cross-polarization pump-probe combinations, we sensitively observe spin-polarized transitions. Core-only nanoplatelets exhibit few-picosecond spin lifetimes that weakly increase with layer thickness. Spectral content of differenced spin-polarized signals indicate biexciton binding energies that decrease with increasing thickness and smaller values than previously reported. Shell growth of CdS with controlled thicknesses, which partially delocalize the electron from the hole, significantly increases the spin lifetime to ~49 picoseconds at room temperature. Implementation of ZnS shells, which do not alter delocalization but do alter surface termination, increased spin lifetimes up to ~100 ps, bolstering the interpretation that surface termination heavily influences spin coherence, likely due to passivation of dangling bonds. Spin precession in magnetic fields both confirms long coherence lifetime at room temperature and yields excitonic g-factor.

cond-mat.mes-hall↗