arXiv subjects

Mercouri G. Kanatzidis

Publications and source records attributed to Mercouri G. Kanatzidis.

At least 37 records · Page 2Linked to original sources

arXiv2022

Deterministic synthesis of phase pure 2D perovskites via progressive transformation of layer thickness

Two-dimensional (2D) halide perovskites have emerged as semiconductor platforms for realizing efficient and durable optoelectronic devices. However, the reproducible synthesis of 2D perovskite crystals with desired layer thickness (or n value) greater than 2, has been an enduring challenge due to the lack of kinetic control (temperature, time, stoichiometry) for each layer thickness. Here, we demonstrate a novel method term as the kinetically controlled space confinement (KCSC) for the deterministic growth of phase pure Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) 2D perovskites. The phase-pure growth was achieved by progressively increasing the temperature (fixed time) or the crystallization time (fixed temperature), which allowed for an acute control of the crystallization kinetics. We also observe a systematic transformation from a lower n-value to n=3, 4, 5, 6 in 2D perovskites. In-situ photoluminescence spectroscopy and imaging suggest that the progressive transformation from lower to higher n-value occurs via intercalation of excess precursor ions. These experiments enabled the development of a machine learning assisted multi-parameter phase diagram, which predicts the growth of 2D phase with a specific n-value.

cond-mat.mtrl-sci↗

arXiv2022

Direct observation of multiple conduction-band minima in high-performance thermoelectric SnSe

We report time- and angle-resolved photoemission spectroscopy on SnSe which currently attracts great interest due to its extremely high thermoelectric performance. Laser-assisted photoemission signals are observed within $\pm$20 fs of the pump pulse arrival. Around 30-50 fs after the photoexcitation, the conduction band minima are not populated by the photoexcited electrons while the valence bands are considerably broadened. In going from 90 fs to 550 fs after the photoexcitation, the photoexcited carriers are decayed into the multiple conduction band minima. The observed conduction bands are consistent with the band structure calculations. The multiple conduction minima suggest possibility of high and anisotropic thermoelectric performance of n-type SnSe single crystal if it is realized.

cond-mat.mtrl-sci↗

arXiv2022

Spin and charge density waves in the quasi-one-dimensional KMn6Bi5

AMn6Bi5 materials (A = Na, K, Rb and Cs) consisting of unique Mn-cluster chains emerge as a new family of superconductors with the suppression of their antiferromagnetic (AFM) order under high pressures. Here, we report transverse incommensurate spin density waves (SDWs) for the Mn atoms with a propagating direction along the chain axes as a ground state for KMn6Bi5 by single crystal neutron diffraction. The SDWs have a refined amplitude of ~2.46 Bohr magnetons for the Mn atoms in the pentagons and ~0.29 Bohr magnetons with a large standard deviation for Mn atoms in the center between the pentagons. AFM dominate both the nearest-neighbor Mn-Mn interactions within the pentagon and next-nearest-neighbor Mn-Mn interactions out of the pentagon (along the propagating wave). The SDWs exhibit both local and itinerant characteristics probably formed by a cooperative interaction between local magnetic exchange and conduction electrons. A significant magnetoelastic effect during the AFM transition, especially along the chain direction, has been demonstrated by temperature-dependent x-ray powder diffraction. Single crystal x-ray diffraction below the AFM transition revealed satellite peaks originating from charge density waves along the chain direction with a q-vector twice as large as the SDW one, pointing to a strong real space coupling between them. Our work not only manifests a fascinating interplay among spin, charge, lattice and one dimensionality to trigger intertwined orders in KMn6Bi5 but also provides important piece of information for the magnetic structure of the parent compound to understand the mechanism of superconductivity in this new family.

cond-mat.str-el↗

arXiv2022

Direct visualization of ultrafast lattice ordering triggered by an electron-hole plasma in 2D perovskites

Direct visualization of ultrafast coupling between charge carriers and lattice degrees of freedom in photo-excited semiconductors has remained a long-standing challenge and is critical for understanding the light-induced physical behavior of materials under extreme non-equilibrium conditions. Here, by monitoring the evolution of the wave-vector resolved ultrafast electron diffraction intensity following above-bandgap photo-excitation, we obtain a direct visual of the structural dynamics in monocrystalline 2D perovskites. Analysis reveals a surprising, light-induced ultrafast lattice ordering resulting from a strong interaction between hot-carriers and the perovskite lattice, which induces an in-plane octahedra rotation, towards a more symmetric phase. Correlated ultrafast spectroscopy performed at the same carrier density as ultrafast electron diffraction reveals that the creation of a hot and dense electron-hole plasma triggers lattice ordering at short timescales by modulating the crystal cohesive energy. Finally, we show that the interaction between the carrier gas and the lattice can be altered by tailoring the rigidity of the 2D perovskite by choosing the appropriate organic spacer layer.

cond-mat.mtrl-sci↗

arXiv2022

Field-induced quantum critical point in the new itinerant antiferromagnet Ti$_3$Cu$_4$

New phases of matter emerge at the edge of magnetic instabilities. In local moment systems, such as heavy fermions, the magnetism can be destabilized by pressure, chemical doping, and, rarely, by magnetic field, towards a zero-temperature transition at a quantum critical point (QCP). Even more rare are instances of QCPs induced by pressure or doping in itinerant moment systems, with no known examples of analogous field-induced \textit{T} = 0 transitions. Here we report the discovery of a new itinerant antiferromagnet with no magnetic constituents, in single crystals of Ti$_3$Cu$_4$ with $T_N$ = 11.3 K. Band structure calculations point to an orbital-selective, spin density wave ground state, a consequence of the square net structural motif in Ti$_3$Cu$_4$. A small magnetic field, $H_C$ = 4.87 T, suppresses the long-range order via a continuous second-order transition, resulting in a field-induced QCP. The magnetic Grüneisen ratio diverges as $H \rightarrow H_C$ and $T\rightarrow0$, with a sign change at $H_C$ and $T^{-1}$ scaling at $H~=~H_C$, providing evidence from thermodynamic measurements for quantum criticality for $H \parallel c$. Non-Fermi liquid (NFL) to Fermi liquid (FL) crossover is observed close to the QCP, as revealed by the power law behavior of the electrical resistivity.

cond-mat.str-el↗

arXiv2021

Giant Non-resonant Infrared Second Order Nonlinearity in $γ$-NaAsSe$_2$

Infrared laser systems are vital for applications in spectroscopy, communications, and biomedical devices, where infrared nonlinear optical (NLO) crystals are required for broadband frequency down-conversion. Such crystals need to have high non-resonant NLO coefficients, a large bandgap, low absorption coefficient, phase-matchability among other competing demands, e.g., a larger bandgap leads to smaller NLO coefficients. Here, we report the successful growth of single crystals of $γ$-NaAsSe$_2$ that exhibit a giant second harmonic generation (SHG) susceptibility of d$_{11}$=590 pm V$^{-1}$ at 2$μ$m wavelength; this is ~ eighteen times larger than that of commercial AgGaSe$_2$ while retaining a similar bandgap of ~1.87eV, making it an outstanding candidate for quasi-phase-matched devices utilizing d$_{11}$. In addition, $γ$-NaAsSe$_2$ is both Type I and Type II phase-matchable, and has a transparency range up to 16$μ$m wavelength. Thus $γ$-NaAsSe2 is a promising bulk NLO crystal for infrared laser applications.

physics.optics↗

arXiv2021

Direct visualization of polaron formation in the thermoelectric SnSe

SnSe is a layered material that currently holds the record for bulk thermoelectric efficiency. The primary determinant of this high efficiency is thought to be the anomalously low thermal conductivity resulting from strong anharmonic coupling within the phonon system. Here we show that the nature of the carrier system in SnSe is also determined by strong coupling to phonons by directly visualizing polaron formation in the material. We employ ultrafast electron diffraction and diffuse scattering to track the response of phonons in both momentum and time to the photodoping of free carriers across the bandgap, observing the bimodal and anisotropic lattice distortions that drive carrier localization. Relatively large (\SI{18.7}{\angstrom}), quasi-1D polarons are formed on the \SI{300}{\femto\second} timescale with smaller (\SI{4.2}{\angstrom}) 3D polarons taking an order of magnitude longer (\SI{4}{\pico\second}) to form. This difference appears to be a consequence of the profoundly anisotropic electron-phonon coupling in SnSe, with strong Fröhlich coupling only to zone center polar optical phonons. These results demonstrate that carriers in SnSe at optimal doping levels results in a high polaron density and that strong electron-phonon coupling is also critical to the thermoelectric performance of this benchmark material and potentially high-performance thermoelectrics more generally.

cond-mat.mtrl-sci↗

arXiv2021

Extended Kohler$^,$s Rule of Magnetoresistance

A notable phenomenon in topological semimetals is the violation of Kohler$^,$s rule, which dictates that the magnetoresistance $MR$ obeys a scaling behavior of $MR = f(H/ρ_0$), where $MR = [ρ_H-ρ_0]/ρ_0$ and $H$ is the magnetic field, with $ρ_H$ and $ρ_0$ being the resistivity at $H$ and zero field, respectively. Here we report a violation originating from thermally-induced change in the carrier density. We find that the magnetoresistance of the Weyl semimetal, TaP, follows an extended Kohler$^,$s rule $MR = f[H/(n_Tρ_0)]$, with $n_T$ describing the temperature dependence of the carrier density. We show that $n_T$ is associated with the Fermi level and the dispersion relation of the semimetal, providing a new way to reveal information on the electronic bandstructure. We offer a fundamental understanding of the violation and validity of Kohler$^,$s rule in terms of different temperature-responses of $n_T$. We apply our extended Kohler$^,$s rule to BaFe$_2$(As$_{1-x}$P$_x$)$_2$ to settle a long-standing debate on the scaling behavior of the normal-state magnetoresistance of a superconductor, namely, $MR$ ~ $tan^2θ_H$, where $θ_H$ is the Hall angle. We further validate the extended Kohler$^,$s rule and demonstrate its generality in a semiconductor, InSb, where the temperature-dependent carrier density can be reliably determined both theoretically and experimentally.

cond-mat.mtrl-sci↗

arXiv2021

Exploring low lattice thermal conductivity materials using chemical bonding principles

Semiconductors with very low lattice thermal conductivities are highly desired for applications relevant to thermal energy conversion and management, such as thermoelectrics and thermal barrier coatings. Although the crystal structure and chemical bonding are known to play vital roles in shaping heat transfer behavior, material design approaches of lowering lattice thermal conductivity using chemical bonding principles are uncommon. In this work, we present an effective strategy of weakening interatomic interactions and therefore suppressing lattice thermal conductivity based on chemical bonding principles and develop a high-efficiency approach of discovering low $κ_{\rm L}$ materials by screening the local coordination environments of crystalline compounds. The followed first-principles calculations uncover 30 hitherto unexplored compounds with (ultra)low lattice thermal conductivities from thirteen prototype crystal structures contained in the inorganic crystal structure database. Furthermore, we demonstrate an approach of rationally designing high-performance thermoelectrics by additionally incorporating cations with stereochemically active lone-pair electrons. Our results not only provide fundamental insights into the physical origin of the low lattice thermal conductivity in a large family of copper-based compounds but also offer an efficient approach to discovery and design materials with targeted thermal transport properties.

cond-mat.mtrl-sci↗

arXiv2021

Quasi-two-dimensional heterostructures (K$M_{1-x}$Te)(LaTe$_{3}$) ($M$ = Mn, Zn) with charge density waves

Layered heterostructure materials with two different functional building blocks can teach us about emergent physical properties and phenomena arising from interactions between the layers. We report the intergrowth compounds KLa$M$$_{1-x}$Te$_{4}$ ($M$ = Mn, Zn; $x\approx$ 0.35) featuring two chemically distinct alternating layers [LaTe$_3$] and [K$M$$_{1-x}$Te]. Their crystal structures are incommensurate, determined by single X-ray diffraction for the Mn compound and transmission electron microscope (TEM) study for the Zn compound. KLaMn$_{1-x}$Te$_{4}$ crystallizes in the orthorhombic superspace group $Pmnm$(01/2$γ$)$s$00 with lattice parameters $a$ = 4.4815(3) Å, $b$ = 21.6649(16) Å and $c$ = 4.5220(3) Å. It exhibits charge density wave (CDW) order at room temperature with a modulation wave vector $\mathbf{q}$ = 1/2$\mathbf{b}$* + 0.3478$\mathbf{c}$* originating from electronic instability of Te-square nets in [LaTe$_{3}$] layers. The Mn analog exhibits a cluster spin glass behavior with spin freezing temperature $T_{\mathrm{f}}$ $\approx$ 5 K attributed to disordered Mn vacancies and competing magnetic interactions in the [Mn$_{1-x}$Te] layers. The Zn analog also has charge density wave order at room temperature with a similar $\mathbf{q}$-vector having the $\mathbf{c}$* component ~ 0.346 confirmed by selected-area electron diffraction (SAED). Electron transfer from [K$M_{1-x}$Te] to [LaTe$_{3}$] layers exists in KLa$M_{1-x}$Te$_{4}$, leading to an enhanced electronic specific heat coefficient. The resistivities of KLa$M_{1-x}$Te$_{4}$ ($M$ = Mn, Zn) exhibit metallic behavior at high temperatures and an upturn at low temperatures, suggesting partial localization of carriers in the [LaTe$_{3}$] layers with some degree of disorder associated with the $M$ atom vacancies in the [$M_{1-x}$Te] layers.

cond-mat.mtrl-sci↗

arXiv2020

Accelerated Discovery of a Large Family of Quaternary Chalcogenides with very Low Lattice Thermal Conductivity

The development of efficient thermal energy management devices such as thermoelectrics, barrier coatings, and thermal data-storage disks often relies on compounds that possess very low lattice thermal conductivity ($κ_l$). Here, we present the computational prediction of a large family of 628 thermodynamically stable quaternary chalcogenides, AMM'Q$_3$ (A = alkali/alkaline earth/post-transition metals; M/M' = transition metals, lanthanides; Q = chalcogens) using high-throughput density functional theory (DFT) calculations. We validate the presence of low-$κ_l$ in this family of materials by calculating $κ_l$ of several predicted stable compounds using the Peierls-Boltzmann transport equation within a first-principles DFT framework. Our analysis reveals that the low-$κ_l$ in the AMM'Q$_3$ compounds originates from the presence of either a strong lattice anharmonicity that enhances the phonon scatterings or rattlers cations that lead to multiple scattering channels in their crystal structures. Our predictions suggest new experimental research opportunities in the synthesis and characterization of these stable, low-$κ_l$ compounds.

cond-mat.mtrl-sci↗

arXiv2020

Static Rashba Effect by Surface Reconstruction and Photon Recycling in the Dynamic Indirect Gap of APbBr3 (A = Cs, CH3NH3) Single Crystals

Recently, halide perovskites have gained significant attention from the perspective of efficient spintronics owing to Rashba effect. This effect occurs as a consequence of strong spin-orbit coupling under noncentrosymmetric environment, which can be dynamic and/or static. However, there exist intense debates on the origin of broken inversion symmetry since the halide perovskites typically crystallize into a centrosymmetric structure. In order to clarify the issue, we examine both dynamic and static effects in the all-inorganic CsPbBr3 and organic-inorganic CH3NH3PbBr3 (MAPbBr3) perovskite single crystals by employing temperature- and polarization-dependent photoluminescence excitation spectroscopy. The perovskite single crystals manifest the dynamic effect by photon recycling in the indirect Rashba gap, causing dual peaks in the photoluminescence. But the effect vanishes in CsPbBr3 at low temperatures (< 50 K), accompanied by a striking color change of the crystal, arising presumably from lower degrees of freedom for inversion symmetry breaking associated with the thermal motion of the spherical Cs cation, compared with the polar MA cation in MAPbBr3. We also show that static Rashba effect occurs only in MAPbBr3 below 90 K due to surface reconstruction via MA-cation ordering, which likely extends across a few layers from the crystal surface to the interior. We further demonstrate that this static Rashba effect can be completely suppressed upon surface treatment with poly methyl methacrylate (PMMA) coating. We believe that our results provide a rationale for the Rashba effects in halide perovskites.

physics.app-ph↗

arXiv2020

Dirac surface states in superconductors: a dual topological proximity effect

In this paper we present scanning tunneling microscopy of Bi$_2$Se$_3$ with superconducting Nb deposited on the surface. We find that the topologically protected surface states of the Bi$_2$Se$_3$ leak into the superconducting over-layer, suggesting a dual topological proximity effect. Coupling between theses states and the Nb states leads to an effective pairing mechanism for the surface states, leading to a modified model for a topological superconductor in these systems. This model is consistent with fits between the experimental data and the theory.

cond-mat.supr-con↗

arXiv2020

An integral method for the calculation of the reduction in interfacial free energy due to interfacial segregation

A method based on the Gibbs' adsorption isotherm is developed to calculate the decrease in interfacial free energy resulting from solute segregation at an internal interface, built on measured concentration profiles. Utilizing atom-probe tomography (APT), we first measure a concentration profile of the relative interfacial excess of solute atoms across an interface. To accomplish this we utilize a new method based on J. W. Cahn's formalism for the calculation of the Gibbs interfacial excess. We also introduce a method to calculate the decrease in interfacial free energy that is caused by the segregating solute atoms. This method yields a discrete profile of the decrease in interfacial free energies, which takes into account the measured concentration profile and calculated Gibbsian excess profile. We demonstrate that this method can be used for both homo- and hetero-phase interfaces and takes into account the actual distribution of solute atoms across an interface as determined by APT. It is applied to the case of the semiconducting system PbTe-PbS 12 mol.%-Na 1 mol.%, where Na segregation at the PbS/PbTe interface is anticipated to reduce the interfacial free energy of the {100} facets. We also consider the case of the nickel-based Alloy 600, where B and Si segregation are suspected to impede inter-granular stress corrosion cracking (IGSCC) at homo- (GB) and hetero-phase metal carbide (M7C3) interfaces. The concentration profiles associated with internal interfaces are measured by APT using an ultraviolet (wavelength = 355 nm) laser to dissect nanotips on an atom-by-atom and atomic plane-by-plane basis.

cond-mat.mtrl-sci↗

arXiv2019

Nonequilibrium dynamics of spontaneous symmetry breaking into a hidden state of charge-density wave

Nonequilibrium phase transition plays a pivotal role in a broad physical context from condensed matter to cosmology. Tracking the formation of non-equilibrium phases in condensed matter is challenging and requires a resolution of the long-range cooperativity on ultra-short timescales. Here, we study the spontaneous symmetry breaking transformation of a charge-density wave system from a stripe phase into a checkerboard state. Such a state is thermodynamically forbidden, but is introduced through a system quench using ultrashort, intense laser pulses. The dynamics is mediated by the soft modes that unfold spontaneously and order the field on a timescale ~1 ps. Using the coherent electron diffraction with ~100 fs resolution, we capture the entire course and demonstrate nonergodic behavior proximal to symmetry breaking that is crucial for stabilizing the hidden states. Remarkably, the thermalization due to carriers cooling arrests the remnants of the transient orders into the topological defects in the eventual state with distinct new properties that last for more than 1 ns. The fundamental dynamics observed here opens an intriguing perspective of controlling phase transitions in quantum materials far from equilibrium.

cond-mat.mes-hall↗

arXiv2019

Magnetization governed magnetoresistance anisotropy in topological semimetal CeBi

Magnetic topological semimetals, the latest member of topological quantum materials, are attracting extensive attention as they may lead to topologically-driven spintronics. Currently, magnetotransport investigations on these materials are focused on anomalous Hall effect. Here, we report on the magnetoresistance anisotropy of topological semimetal CeBi, which has tunable magnetic structures arising from localized Ce 4f electrons and exhibits both negative and positive magnetoresistances, depending on the temperature. We found that the angle dependence of the negative magnetoresistance, regardless of its large variation with the magnitude of the magnetic field and with temperature, is solely dictated by the field-induced magnetization that is orientated along a primary crystalline axis and flops under the influence of a rotating magnetic field. The results reveal the strong interaction between conduction electrons and magnetization in CeBi. They also indicate that magnetoresistance anisotropy can be used to uncover the magnetic behavior and the correlation between transport phenomena and magnetism in magnetic topological semimetals.

cond-mat.mtrl-sci↗

arXiv2019

Orbital-flop Induced Magnetoresistance Anisotropy in Rare Earth Monopnictide CeSb

The charge and spin of the electrons in solids have been extensively exploited in electronic devices and in the development of spintronics. Another attribute of electrons - their orbital nature - is attracting growing interest for understanding exotic phenomena and in creating the next-generation of quantum devices such as orbital qubits. Here, we report on orbital-flop induced magnetoresistance anisotropy in CeSb. In the low temperature high magnetic-field driven ferromagnetic state, a series of additional minima appear in the angle-dependent magnetoresistance. These minima arise from the anisotropic magnetization originating from orbital-flops and from the enhanced electron scattering from magnetic multidomains formed around the first-order orbital-flop transition. The measured magnetization anisotropy can be accounted for with a phenomenological model involving orbital-flops and a spin-valve-like structure is used to demonstrate the viable utilization of orbital-flop phenomenon. Our results showcase a contribution of orbital behavior in the emergence of intriguing phenomena.

cond-mat.mtrl-sci↗

arXiv2019

Low Frequency Carrier Kinetics in Perovskite Solar Cells

Hybrid organic-inorganic halide perovskite solar cells have emerged as leading candidates for third-generation photovoltaic technology. Despite the rapid improvement in power conversion efficiency (PCE) for perovskite solar cells in recent years, the low-frequency carrier kinetics that underlie practical roadblocks such as hysteresis and degradation remain relatively poorly understood. In an effort to bridge this knowledge gap, we perform here correlated low-frequency noise (LFN) and impedance spectroscopy (IS) characterization that elucidates carrier kinetics in operating perovskite solar cells. Specifically, we focus on planar cell geometries with a SnO2 electron transport layer and two different hole transport layers, namely, poly(triarylamine) (PTAA) and Spiro-OMeTAD. PTAA and Sprio-OMeTAD cells with moderate PCEs of 5 to 12 percent possess a Lorentzian feature at 200 Hz in LFN measurements that corresponds to a crossover from electrode to dielectric polarization. In comparison, Spiro-OMeTAD cells with high PCEs (15 percent) show four orders of magnitude lower LFN amplitude and are accompanied by a cyclostationary process. Through a systematic study of more than a dozen solar cells, we establish a correlation with noise amplitude, power conversion efficiency, and fill factor. Overall, this work establishes correlated LFN and IS as an effective methodology for quantifying low frequency carrier kinetics in perovskite solar cells, thereby providing new physical insights that can rationally guide ongoing efforts to improve device performance, reproducibility, and stability.

physics.app-ph↗