arXiv subjects

Nicola A. Spaldin

Publications and source records attributed to Nicola A. Spaldin.

At least 91 records · Page 5Linked to original sources

arXiv2020

Phono-magnetic analogs to opto-magnetic effects

The magneto-optical and opto-magnetic effects describe the interaction of light with a magnetic medium. The most prominent examples are the Faraday and Cotton-Mouton effects that modify the transmission of light through a medium, and the inverse Faraday and inverse Cotton-Mouton effects that produce effective magnetic fields for the spin in the material. Here, we introduce the phenomenology of the analogous magneto-phononic and phono-magnetic effects, in which vibrational quanta take the place of the light quanta. We show, using a combination of first-principles calculations and phenomenological modeling, that the effective magnetic fields exerted by the phonon analogs of the inverse Faraday and inverse Cotton-Mouton effects on the spins of antiferromagnetic nickel oxide yield magnitudes comparable to and potentially larger than those of the opto-magnetic originals.

cond-mat.mtrl-sci↗

arXiv2020

Longitudinal and transverse electron paramagnetic resonance in a scanning tunneling microscope

Electron paramagnetic resonance (EPR) spectroscopy is widely employed to characterize paramagnetic complexes. Recently, EPR combined with scanning tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom spatial resolution. The excitation mechanism of EPR in STM, however, is broadly debated, raising concerns about widespread application of this technique. Here, we present an extensive experimental study and modelling of EPR-STM of Fe and hydrogenated Ti atoms on an MgO surface. Our results support a piezoelectric coupling mechanism, in which the EPR species oscillate adiabatically in the inhomogeneous magnetic field of the STM tip. An analysis based on Bloch equations combined with atomic-multiplet calculations identifies different EPR driving forces. Specifically, transverse magnetic-field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic-field gradients drive the spin-2 Fe. Additionally, our results highlight the potential of piezoelectric coupling to induce electric dipole moments, thereby broadening the scope of EPR-STM to nonpolar species and nonlinear excitation schemes.

cond-mat.mes-hall↗

arXiv2020

Atomic responses to general dark matter-electron interactions

In the leading paradigm of modern cosmology, about 80% of our Universe's matter content is in the form of hypothetical, as yet undetected particles. These do not emit or absorb radiation at any observable wavelengths, and therefore constitute the so-called Dark Matter (DM) component of the Universe. Detecting the particles forming the Milky Way DM component is one of the main challenges for astroparticle physics and basic science in general. One promising way to achieve this goal is to search for rare DM-electron interactions in low-background deep underground detectors. Key to the interpretation of this search is the response of detectors' materials to elementary DM-electron interactions defined in terms of electron wave functions' overlap integrals. In this work, we compute the response of atomic argon and xenon targets used in operating DM search experiments to general, so far unexplored DM-electron interactions. We find that the rate at which atoms can be ionized via DM-electron scattering can in general be expressed in terms of four independent atomic responses, three of which we identify here for the first time. We find our new atomic responses to be numerically important in a variety of cases, which we identify and investigate thoroughly using effective theory methods. We then use our atomic responses to set 90% confidence level (C.L.) exclusion limits on the strength of a wide range of DM-electron interactions from the null result of DM search experiments using argon and xenon targets.

hep-ph↗

arXiv2020

Theoretical investigation of twin boundaries in WO$_3$: Structure, properties and implications for superconductivity

We present a theoretical study of the structure and functionality of ferroelastic domain walls in tungsten trioxide, WO$_3$. WO$_3$ has a rich structural phase diagram, with the stability and properties of the various structural phases strongly affected both by temperature and by electron doping. The existence of superconductivity is of particular interest, with the underlying mechanism as of now not well understood. In addition, reports of enhanced superconductivity at structural domain walls are particularly intriguing. Focusing specifically on the orthorhombic $β$ phase, we calculate the structure and properties of the domain walls both with and without electron doping. We use two theoretical approaches: Landau-Ginzburg theory, with free energies constructed from symmetry considerations and parameters extracted from our first-principles density functional calculations, and direct calculation using large-scale, GPU-enabled density functional theory. We find that the structure of the $β$-phase domain walls resembles that of the bulk tetragonal $α_1$ phase, and that the electronic charge tends to accumulate at the walls. Motivated by this finding, we perform ab initio computations of electron-phonon coupling in the bulk $α_1$ structure and extract the superconducting critical temperatures , $T_c$, within Bardeen-Cooper-Schrieffer theory. Our results provide insight into the experimentally observed unusual trend of decreasing Tc with increasing electronic charge carrier concentration.

cond-mat.mtrl-sci↗

arXiv2019

DFT+DMFT study of oxygen vacancies in a Mott Insulator

Oxygen vacancies are a common source of excess electrons in complex oxides. In Mott insulators these additional electrons can induce a metal-insulator transition (MIT), fundamentally altering the electronic properties of the system. Here we study the effect of oxygen vacancies in LaTiO3, a prototypical Mott insulator close to the MIT. We show that the introduction of oxygen vacancies creates a vacancy-related band immediately below the partially filled Ti-t2g bands. We study the effect of this additional band on the Mott MIT using a combination of density functional theory and dynamical mean-field theory (DFT+DMFT), employing a minimal correlated subspace consisting of effective Ti-t2g orbitals plus an additional Wannier function centered on the vacancy site. We find that the Mott insulating state in LaTiO3 is robust to the presence of the vacancy band, which remains fully occupied even in the presence of a local Coulomb repulsion.

cond-mat.str-el↗

arXiv2019

A dynamical magnetic field accompanying the motion of ferroelectric domain walls

The recently proposed dynamical multiferroic effect describes the generation of magnetization from temporally varying electric polarization. Here, we show that the effect can lead to a magnetic field at moving ferroelectric domain walls, where the rearrangement of ions corresponds to a rotation of ferroelectric polarization in time. We develop an expression for the dynamical magnetic field, and calculate the relevant parameters for the example of 90$^\circ$ and 180$^\circ$ domain walls in BaTiO$_3$ using a combination of density functional theory and phenomenological modeling. We find that the magnetic field reaches the order of several $μ$T at the center of the wall, and we propose two experiments to measure the effect with nitrogen-vacancy center magnetometry.

cond-mat.mtrl-sci↗

arXiv2019

Origin and evolution of ferroelectricity in the layered rare-earth-titanate, $R_2$Ti$_2$O$_{7}$, Lichtenberg phases

We report a systematic first-principles study based on density functional theory (DFT) of the structural and ferroelectric properties of the $R_2$Ti$_2$O$_{7}$ perovskite-related oxides with $R=$ La, Ce, Pr, and Nd. We show that, in all cases, the ferroelectric distortion to the ground-state polar $P2_1$ structure from its parent centrosymmetric $P2_1/m$ phase is driven by a single polar soft mode consisting of rotations and tilts of the TiO$_6$ octahedra combined with displacements of the $R$ ions. A secondary centrosymmetric distortion, which is stable in the parent structure, contributes substantially to the energy lowering of the ground state. We evaluate the trends in structure and polarization across the series as a function of $R$ and reconcile discrepancies in reported values of polarization in the literature. Our results confirm that the family of $R_2$Ti$_2$O$_{7}$ materials belong to the class of proper geometric ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2018

A high-energy density antiferroelectric made by interfacial electrostatic engineering

Dielectric capacitors hold a tremendous advantage for energy storage due to their fast charge/discharge times and stability in comparison to batteries and supercapacitors. A key limitation to today's dielectric capacitors, however, is the low storage capacity of conventional dielectric materials. To mitigate this issue, antiferroelectric materials have been proposed, but relatively few families of antiferroelectric materials have been identified to date. Here, we propose a new design strategy for the construction of lead-free antiferroelectric materials using interfacial electrostatic engineering. We begin with a ferroelectric material with one of the highest known bulk polarizations, BiFeO3. We show that by confining atomically-precise thin layers of BiFeO3 in a dielectric matrix that we can induce a metastable antiferroelectric structure. Application of an electric field reversibly switches between this new phase and a ferroelectric state, in addition, tuning of the dielectric layer causes coexistence of the ferroelectric and antiferroelectric states. Precise engineering of the structure generates an antiferroelectric phase with energy storage comparable to that of the best lead-based materials. The use of electrostatic confinement provides a new pathway for the design of engineered antiferroelectric materials with large and potentially coupled responses.

cond-mat.mtrl-sci↗

arXiv2018

Orbital magnetic moments of phonons

In ionic materials, circularly polarized phonons carry orbital magnetic moments that arise from circular motions of the ions, and which interact with other magnetic moments or fields. Here, we calculate the orbital magnetic moments of phonons in 35 different materials using density functional theory, and we identify the factors that lead to, and materials that show, large responses. We compute the resulting macroscopic orbital magnetic moments that can be induced by the excitation of coherent phonons using mid-infrared laser pulses, and we evaluate the magnitudes of the phonon Zeeman effect in a strong magnetic field. Finally, we apply our formalism to chiral phonons, in which the motions of the ions are intrinsically circular. The zoology presented here may serve as a guide to finding materials for phonon and spin-phonon driven phenomena.

cond-mat.mtrl-sci↗

arXiv2018

Strain-induced heteronuclear charge disproportionation in EuMnO$_3$

Charge disproportionation transitions in complex oxides most commonly link high-temperature phases containing identical cations with the same oxidation state and crystallographic site, to low-temperature phases in which charge transfer between these ions results in unequal oxidation states. Here we propose, based on density functional calculations, a related concept that we term \textit{heteronuclear} charge disproportionation, in which charge transfer occurs between different elements on different crystallographic sites. We show for the case of EuMnO$_3$ that such a transition from the experimentally observed Eu$^{3+}$Mn$^{3+}$O$_3$ phase to the so far unknown Eu$^{2+}$Mn$^{4+}$O$_3$ phase can be triggered by pressure and epitaxial strain. We then identify measurable signatures to aid in an experimental exploration of the complex pressure- and biaxial strain-dependent phase stability of EuMnO$_3$ that we hope to motivate with our predictions. We suggest other candidate crystal chemistries for heteronuclear charge disproportionation, in which novel physics could emerge from the coexistence of instabilities.

cond-mat.mtrl-sci↗

arXiv2018

Multiferroic Quantum Criticality

The zero-temperature limit of a continuous phase transition is marked by a quantum critical point, which can generate exotic physics that extends to elevated temperatures. Magnetic quantum criticality is now well known, and has been explored in systems ranging from heavy fermion metals to quantum Ising materials. Ferroelectric quantum critical behaviour has also been recently established, motivating a flurry of research investigating its consequences. Here, we introduce the concept of multiferroic quantum criticality, in which both magnetic and ferroelectric quantum criticality occur in the same system. We develop the phenomenology of multiferroic quantum critical behaviour, describe the associated experimental signatures, and propose material systems and schemes to realize it.

cond-mat.mtrl-sci↗

arXiv2018

Oxygen vacancies in bulk and at neutral domain walls in hexagonal YMnO$_3$

We use density functional calculations to investigate the accommodation and migration of oxygen vacancies in bulk hexagonal YMnO$_3$, and to study interactions between neutral ferroelectric domain walls and oxygen vacancies. Our calculations show that oxygen vacancies in bulk YMnO$_3$ are more stable in the Mn-O layers than in the Y-O layers. Migration barriers of the planar oxygen vacancies are high compared to oxygen vacancies in perovskites, and to previously reported values for oxygen interstitials in h-YMnO$_3$. The calculated polarization decreases linearly with vacancy concentration, while the out-of-plane lattice parameter expands in agreement with previous experiments. In contrast with ferroelectric perovskites, oxygen vacancies are found to be more stable in bulk than at domain walls. The tendency of oxygen vacancies to segregate away from neutral domain walls is explained by unfavorable Y-O bond lengths caused by the local strain field at the domain walls.

cond-mat.mtrl-sci↗

arXiv2018

Relationship between crystal structure and multiferroic orders in orthorhombic perovskite manganites

We use resonant and non-resonant X-ray diffraction measurements in combination with first-principles electronic structure calculations and Monte Carlo simulations to study the relationship between crystal structure and multiferroic orders in the orthorhombic perovskite manganites, o-$R$MnO$_3$ ($R$ is a rare-earth cation or Y). In particular, we focus on how the internal lattice parameters (Mn-O bond lengths and Mn-O-Mn bond angles) evolve under chemical pressure and epitaxial strain, and the effect of these structural variations on the microscopic exchange interactions and long-range magnetic order. We show that chemical pressure and epitaxial strain are accommodated differently by the crystal lattice of o-$R$MnO$_3$, which is key for understanding the difference in magnetic properties between bulk samples and strained films. Finally, we discuss the effects of these differences in the magnetism on the electric polarization in o-$R$MnO$_3$.

cond-mat.mtrl-sci↗

arXiv2018

Emphanitic anharmonicity in PbSe at high temperature and the anomalous electronic properties in the PbQ (Q=S, Se, Te) system

The temperature dependence of the local structure of PbSe has been investigated using pair distribution function (PDF) analysis of x-ray and neutron powder diffraction data and density functional theory (DFT) calculations. Observation of non-Gaussian PDF peaks at high temperature indicates the presence of significant anharmonicity, which can be modeled as Pb off-centering along [100] directions that grows on warming similar to the behavior seen in PbTe and PbS and sometimes called emphanisis. Interestingly, the emphanitic response is smaller in PbSe than in both PbS and PbTe indicating a non-monotonic response with chalcogen atomic number in the PbQ (Q=S, Se, Te) series. The DFT calculations indicate a correlation between band gap and the amplitude of [100] dipolar distortion, suggesting that emphanisis may be behind the anomalous composition and temperature dependencies of the band gaps in this series.

cond-mat.mtrl-sci↗

arXiv2018

Magnetoelectric multipoles in metals

We demonstrate computationally the existence of magnetoelectric multipoles, arising from the second order term in the multipole expansion of a magnetization density in a magnetic field, in noncentrosymmetric magnetic metals. While magnetoelectric multipoles have long been discussed in the context of the magnetoelectric effect in noncentrosymmetric magnetic insulators, they have not previosuly been identified in metallic systems, in which the mobile carriers screen any electrical polarization. Using first-principles density functional calculations we explore three specific systems: First, a conventional centrosymmetric magnetic metal, Fe, in which we break inversion symmetry by introducing a surface, which both generates magnetoelectric monopoles and allows a perpendicular magnetoelectric response. Next, the hypothetical cation-ordered perovskite, SrCaRu$_2$O$_6$, in which we study the interplay between the magnitude of the polar symmetry breaking and that of the magnetic dipoles and multipoles, finding that both scale proportionally to the structural distortion. Finally, we identify a hidden antiferromultipolar order in the noncentrosymmetric, antiferromagnetic metal Ca$_3$Ru$_2$O$_7$, and show that, while its competing magnetic phases have similar magnetic dipolar structures, their magnetoelectric multipolar structures are distinctly different, reflecting the strong differences in transport properties.

cond-mat.mtrl-sci↗

arXiv2018

Four-spin ring interaction as a source of unconventional magnetic orders in orthorhombic perovskite manganites

We use ab initio electronic structure calculations in combination with Monte Carlo simulations to investigate the magnetic and ferroelectric properties of bulk orthorhombic HoMnO$_3$ and ErMnO$_3$. Our goals are to explain the inconsistencies in the measured magnetic properties of the orthorhombic perovskite manganites (o-$R$MnO$_3$) with small rare-earth ($R$) cations or Y, as well as the contradictions between the directions and amplitudes of the electric polarizations reported by different experimental groups. Our computations stabilize several exotic magnetic orders (so-called w-spiral, H-AFM and I-AFM), whose presence resolve the contradictions in the measured magnetic and ferroelectric properties of o-$R$MnO$_3$. We show that these orders emerge due to strong four-spin ring exchange interactions.

cond-mat.mtrl-sci↗

arXiv2018

Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites

The improper ferroelectricity in YMnO$_3$ and other related multiferroic hexagonal manganites are known to cause topologically protected ferroelectric domains that give rise to rich and diverse physical phenomena. The local structure and structural coherence across the ferroelectric transition, however, were previously not well understood. Here we reveal the evolution of the local structure with temperature in YMnO$_3$ using neutron total scattering techniques, and interpret them with the help of first-principles calculations. The results show that, at room temperature, the local and average structures are consistent with the established ferroelectric $P6_3cm$ symmetry. On heating, both local and average structural analyses show striking anomalies from $\sim 800$ K up to the Curie temperature consistent with increasing fluctuations of the order parameter angle. These fluctuations result in an unusual local symmetry lowering into a \textit{continuum of structures} on heating. This local symmetry breaking persists into the high-symmetry non-polar phase, constituting an unconventional type of order-disorder transition.

cond-mat.mtrl-sci↗

arXiv2017

Correlated local dipoles in PbTe

We present a combined single-crystal x-ray diffuse scattering and ab-initio molecular dynamics study of lead telluride, PbTe. Well-known for its thermoelectric and narrow-gap semiconducting properties, PbTe recently achieved further notoriety following the report of an unusual off-centering of the lead atoms, accompanied by a local symmetry breaking, on heating. This observation, which was named emphanisis, ignited considerable controversy regarding the details of the underlying local structure and the appropriate interpretation of the total scattering experiments. In this study, we demonstrate close agreement between our diffuse scattering measurements and our calculations, which allows us to analyze features such as higher-order correlations that are accessible in the simulations but not experimentally. This allowed us to discover an unusual correlated local dipole formation extending over several unit cells with an associated local reduction of the cubic symmetry in both our x-ray diffuse scattering measurements and our molecular dynamics simulations. Importantly, when averaged spatially or temporally, the most probable positions for the ions are at the centers of their coordination polyhedra. Our results therefore clarify the nature of the local symmetry breaking, and reveal the source of the earlier controversy regarding the existence or absence of off-centering. Finally, we provide an interpretation of the behavior in terms of coupled soft optical and acoustic modes, which is linked also to the high thermoelectric performance of PbTe.

cond-mat.mtrl-sci↗