arXiv subjects

S. W. Cheong

Publications and source records attributed to S. W. Cheong.

At least 19 recordsLinked to original sources

arXiv2022

Vibrational fingerprints of ferroelectric hafnia

Hafnia (HfO2) is a promising material for emerging chip applications due to its high-k dielectric behaviour, suitability for negative capacitance heterostructures, scalable ferroelectricity, and silicon compatibility. The lattice dynamics along with phononic properties such as thermal conductivity, contraction, and heat capacity are under-explored, primarily due to the absence of high quality single crystals. Herein, we report the vibrational properties of a series of HfO2 crystals stabilized with yttrium (chemical formula HfO2:xY, where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. We untangle the effects of Y by testing our calculations against the measured Raman and infrared spectra of the cubic, antipolar orthorhombic, and monoclinic phases and then proceed to reveal the signature modes of polar orthorhombic hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and paves the way for an analysis of mode contributions to high-k dielectric and ferroelectric properties for chip technologies.

cond-mat.mtrl-sci↗

arXiv2021

Local mechanism of valence bond formation in IrTe$_2$

Doped IrTe$_2$ is considered a platform for topological superconductivity and therefore receives currently a lot of interest. In addition, the superconductivity in these materials exists in close vicinity of electronic valence bond crystals, which we explore here by means of high-pressure single crystal x-ray diffraction in combination with density functional theory. Our crystallographic refinements provide unprecedented information about the structural evolution as a function of applied pressure up to 42 GPa. Using this structural information for density functional theory calculations, we show that the valence bond formation in IrTe$_2$ is driven by changes in the Ir-Te-Ir bond angle. When a valence bond is formed, this bond angle decreases drastically, leading to a stabilization of local valence bonds large enough to push them out of a broad band continuum. This unusual local mechanism of valence bond formation in an itinerant material provides a natural explanation for the different valence bond orders in IrTe$_2$, implies a very strong electron-phonon coupling and is most likely relevant for the superconductivity as well.

cond-mat.str-el↗

arXiv2021

Pressure-Dependent Phase Transitions in Hybrid Improper Ferroelectric Ruddlesden-Popper Oxides

The temperature-dependent phase transitions in Ruddlesden-Popper oxides with perovskite bilayers have been under increased scrutiny in recent years due to the so-called hybrid improper ferroelectricity that some chemical compositions exhibit. However, little is currently understood about the hydrostatic pressure dependence of these phase transitions. Herein we present the results of a high-pressure powder synchrotron X-ray diffraction experiment and $ab~initio$ calculations on the bilayered Ruddlesden-Popper phases Ca$_{3}$Mn$_{2}$O$_{7}$ and Ca$_{3}$Ti$_{2}$O$_{7}$. In both compounds we observe a first-order phase transition between polar $A2_{1}am$ and non-polar $Acaa$ structures. Interestingly, we show that while the application of pressure ultimately favours a non-polar phase -- as is commonly observed for proper ferroelectrics -- regions of response exist where pressure actually acts to increase the polar mode amplitudes. The reason for this can be untangled by considering the varied response of octahedral tilts and rotations to hydrostatic pressure and their trilinear coupling with the polar instability.

cond-mat.mtrl-sci↗

arXiv2018

The role of the rare earth in the lattice and magnetic coupling in multiferroic h-HoMnO3

We used Raman scattering to study the lattice and magnetic excitations in the hexagonal HoMnO3 single crystals. The E2 phonon mode at 237 cm-1 is affected by the magnetic order. This mode is related to the displacement of Mn and O ions in a-b plane and modulates the Mn-O-Mn bond angles in a-b plane and the in-plane Mn-Mn superexchange interaction. The mode at 269 cm-1 associated to the displacement of the apical Ho3+ ions along the c direction presents an abrupt change of slope at TN showing that the role of the rare earth ions can not be neglected in the magnetic transition. We have identified magnon and crystal field excitations. The temperature dependence of the magnetic excitations has been compared to the Mn and Ho moment and indicates that the exchange interaction pattern between Mn and Ho atoms drives the uniaxial anisotropy gap above the Mn-spin-rotation transition.

cond-mat.str-el↗

arXiv2017

Asymmetric splitting of an antiferromagnetic resonance via quartic exchange interactions in multiferroic hexagonal HoMnO$_3$

The symmetric splitting of two spin-wave branches in an antiferromagnetic resonance (AFR) experiment has been an essential measurement of antiferromagnets for over half a century. In this work, circularly polarized time-domain THz spectroscopy experiments performed on the low symmetry multiferroic h-HoMnO$_3$ reveal an AFR of the Mn sublattice to split asymmetrically in applied magnetic field, with an $\approx$ 50\% difference in $g$-factors between the high and low energy branches of this excitation. The temperature dependence of the $g$-factors, including a drastic renormalization at the Ho spin ordering temperature, reveals this asymmetry to unambiguously stem from Ho-Mn interactions. Theoretical calculations demonstrate the AFR asymmetry is not explained by conventional Ho-Mn exchange mechanisms alone and are only reproduced if quartic spin interactions are also included in the spin Hamiltonian. Our results provide a paradigm for the optical study of such novel interactions in hexagonal manganites and low symmetry antiferromagnets in general.

cond-mat.str-el↗

arXiv2016

Structural and Magnetic Characterization of Large Area, Free-Standing Thin Films of Magnetic Ion Intercalated Dichalcogenides Mn0.25TaS2 and Fe0.25TaS2

Free-standing thin films of magnetic ion intercalated transition metal dichalcogenides are produced using ultramicrotoming techniques. Films of thicknesses ranging from 30nm to 250nm were achieved and characterized using transmission electron diffraction and X-ray magnetic circular dichroism. Diffraction measurements visualize the long range crystallographic ordering of the intercalated ions, while the dichroism measurements directly assess the orbital contributions to the total magnetic moment. We thus verify the unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital contribution in Mn0.25TaS2. Such films can be used in a wide variety of ultrafast X-ray and electron techniques that benefit from transmission geometries, and allow measurements of ultrafast structural, electronic, and magnetization dynamics in space and time.

cond-mat.mtrl-sci↗

arXiv2014

Ferroelectric and Magnetic Domains in LuFe2O4 Observed by Scanning Probe Microscopy

LuFe2O4 is a multiferroic system which exhibits ferroelectricity, charge order, and ferrimagnetic order simultaneously below ~230 K. The ferroelectric domains of LuFe2O4 are imaged with both piezoresponse force microscopy (PFM) and electrostatic force microscopy (EFM), while the magnetic domains are characterized by magnetic force microscopy (MFM). Comparison of PFM and EFM results lead to a conclusion that the ferroelectricity is of electronic origin as opposed to the usual displacive one. Simultaneous characterization of ferroelectric and magnetic domains by EFM and MFM, respectively, on the same surface of LuFe2O4 reveals that both domains have irregular patterns of similar shape, but the length scales are quite different. The domain size is approximately 100 nm for the ferroelectric domains while the magnetic domain size is much larger and gets as large as 1μm. We also demonstrate that the origin of the formation of irregular domains in LuFe2O4 is not extrinsic but intrinsic.

cond-mat.str-el↗

arXiv2014

Optical evidence of surface state suppression in Bi based topological insulators

A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi$_2$Te$_2$Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon mediated coupling between bulk and surface states suppresses surface conductance as temperature rises.

cond-mat.mtrl-sci↗

arXiv2013

Octahedral tilting induced ferroelectricity in the ASnO${_3}$/BSnO${_3}$ superlattice

The effect of the octahedral tilting of ASnO3 (A = Ca, Sr, Ba) parent compound and bi-color ASnO3/BSnO3 superlattice (A, B = Ca, Sr, Ba) was predicted from density-functional theory. In the ASnO3 parent compound, the structural phase transition as a function of the A-site cation size was correlated with the magnitude of the two octahedral tilting modes (a-a-c0 tilting and a0a0c+ tilting). The magnitude of the octahedral tilting modes in the superlattices was analyzed quantitatively and found to be associated with that of the constituent parent materials. The ASnO3/BSnO3 superlattices showed hybrid improper ferroelectricity resulting from the coupling of two octahedral tilting modes (a-a-c0 tilting and a0a0c+ tilting), which are also responsible for the structural phase transition from a tetragonal to orthorhombic phase. The ferroelectricity due to A-site mirror symmetry breaking is a secondary order parameter for an orthorhombic phase transition in the bi-color superlattice and is related to the Γ5- symmetry mode. The coupling between the tilting modes and ferroelectric mode in the bi-color superlattice of ASnO3/BSnO3 was analyzed by group theory and symmetry mode analysis.

cond-mat.mtrl-sci↗

arXiv2012

Lattice and spin excitations in multiferroic h-YbMnO3

Lattice and spin excitations have been studied by Raman scattering in hexagonal YbMnO3 single crystals. The temperature dependences of the phonon modes show that the E2 mode at 256 cm-1 related to the displacement of Mn and O ions in a-b plane is coupled to the spin order. The A1 phonon mode at 678 cm-1 presents a soft mode behavior at the Neel temperature. Connected to the motion of the apical oxygen ions along the c direction, this mode controls directly the Mn-Mn interactions between adjacent Mn planes and the superexchange path. Crystal field and magnon mode excitations have been identified. The temperature investigation of the spin excitations shows that the spin structure is strongly influence by the Yb-Mn interaction. Under a magnetic field along the c axis, we have investigated the magnetic reordering and its impact on the spin excitations.

cond-mat.str-el↗

arXiv2012

Raman study of the phonon symmetries in BiFeO$_3$ single crystals

In bismuth ferrite (BiFeO3), antiferromagnetic and ferroelectric order coexist at room temperature, making it of particular interest for studying magnetoelectric coupling. The mutual control of magnetic and electric properties is very useful for a wide variety of applications. This has led to an enormous amount of research into the properties of BiFeO$_3$. Nonetheless, one of the most fundamental aspects of this material, namely the symmetries of the lattice vibrations, remains controversial.We present a comprehensive Raman study of BiFeO$_3$ single crystals with the approach of monitoring the Raman spectra while rotating the polarization direction of the excitation laser. Our method results in unambiguous assignment of the phonon symmetries and explains the origin of the controversy in the literature. Furthermore, it provides access to the Raman tensor elements enabling direct comparison with theoretical calculations. Hence, this allows the study of symmetry breaking and coupling mechanisms in a wide range of complex materials and may lead to a noninvasive, all-optical method to determine the orientation and magnitude of the ferroelectric polarization.

cond-mat.mtrl-sci↗

arXiv2011

X-ray induced electronic structure change in CuIr$_2$S$_4$

The electronic structure of CuIr$_2$S$_4$ has been investigated using various bulk-sensitive x-ray spectroscopic methods near the Ir $L_3$-edge: resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy in the partial fluorescence yield (PFY-XAS) mode, and resonant x-ray emission spectroscopy (RXES). A strong RIXS signal (0.75 eV) resulting from a charge-density-wave gap opening is observed below the metal-insulator transition temperature of 230 K. The resultant modification of electronic structure is consistent with the density functional theory prediction. In the spin- and charge- dimer disordered phase induced by x-ray irradiation below 50 K, we find that a broad peak around 0.4 eV appears in the RIXS spectrum.

cond-mat.str-el↗

arXiv2011

Fabrication and Characterization of Topological Insulator Bi$_2$Se$_3$ Nanocrystals

In the recently discovered class of materials known as topological insulators, the presence of strong spin-orbit coupling causes certain topological invariants in the bulk to differ from their values in vacuum. The sudden change of invariants at the interface results in metallic, time reversal invariant surface states whose properties are useful for applications in spintronics and quantum computation. However, a key challenge is to fabricate these materials on the nanoscale appropriate for devices and probing the surface. To this end we have produced 2 nm thick nanocrystals of the topological insulator Bi$_2$Se$_3$ via mechanical exfoliation. For crystals thinner than 10 nm we observe the emergence of an additional mode in the Raman spectrum. The emergent mode intensity together with the other results presented here provide a recipe for production and thickness characterization of Bi$_2$Se$_3$ nanocrystals.

cond-mat.mtrl-sci↗

arXiv2010

Surface Reconstruction of Hexagonal Y-doped HoMnO3 and LuMnO3 studied using low-energy electron diffraction

We have investigated the (0001) surfaces of several hexagonal manganite perovskites by low-energy electron diffraction (LEED) in order to determine if the surface periodicity is different from that of the bulk materials. These LEED studies were conducted using near-normal incidence geometry with a low energy electron microscope (LEEM)/LEED apparatus from room temperature to 1200 degrees Celsius and with an electron energy in the range of 15-50 eV. Diffraction patterns showed features of bulk-terminated periodicity as well as a 2\times2 surface reconstruction. Possible origins for this surface reconstruction structure are discussed and comparisons are made with surface studies of other complex oxides.

cond-mat.str-el↗

arXiv2010

Origin of Electric Field Induced Magnetization in Multiferroic HoMnO3

We have performed polarized and unpolarized small angle neutron scattering experiments on single crystals of HoMnO3 and have found that an increase in magnetic scattering at low momentum transfers begins upon cooling through temperatures close to the spin reorientation transition at TSR ~ 40 K. We attribute the increase to an uncompensated magnetization arising within antiferromagnetic domain walls. Polarized neutron scattering experiments performed while applying an electric field show that the field suppresses magnetic scattering below T ~ 50 K, indicating that the electric field affects the magnetization via the antiferromagnetic domain walls rather than through a change to the bulk magnetic order.

cond-mat.str-el↗

arXiv2009

Spin-Charge-Lattice Coupling through Resonant Multi-Magnon Excitations in Multiferroic BiFeO3

Spin-charge-lattice coupling mediated by multi-magnon processes is demonstrated in multiferroic BiFeO3. Experimental evidence of two and three magnons excitations as well as multimagnon coupling at electronic energy scales and high temperatures are reported. Temperature dependent Raman experiments show up to five resonant enhancements of the 2-magnon excitation below the Neel temperature. These are shown to be collective interactions between on-site Fe d-d electronic resonance, phonons and multimagnons

cond-mat.other↗

arXiv2008

3:1 magnetization plateau and suppression of ferroelectric polarization in an Ising chain multiferroic

Ferroelectric Ising chain magnet Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ ($x\simeq$0.96) was studied in magnetic fields up to 33 T. Magnetization and neutron scattering measurements reveal successive metamagnetic transitions from the zero-field $\uparrow \uparrow \downarrow \downarrow$ spin configuration to the $\uparrow \uparrow \uparrow \downarrow$ state with a broad magnetization plateau, and then to the $\uparrow \uparrow \uparrow \uparrow$ state. The absence of hysteresis in these plateaus reveals an intriguing coupling between the intra-chain state and the three-dimensional geometrically frustrated magnetic system. Inversion symmetry, broken in the $\uparrow \uparrow \downarrow \downarrow$ state, is restored in the $\uparrow \uparrow \uparrow \downarrow$ state, leading to the complete suppression of the electric polarization driven by symmetric superexchange.

cond-mat.str-el↗

arXiv2004

Probing spin correlations with phonons in the strongly frustrated magnet ZnCr2O4

The spin-lattice coupling plays an important role in strongly frustrated magnets. In ZnCr$_2$O$_4$, an excellent realization of the Heisenberg antiferromagnet on the ``pyrochlore'' network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at $T_c = 12.5$ K. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of an optical phonon triplet in ZnCr$_2$O$_4$ tracks the nearest-neighbor spin correlations above $T_c$. The splitting of the phonon triplet below $T_c$ provides a way to measure of the spin-Peierls order parameter.

cond-mat.str-el↗