Session Y6: Magnetic Complex Oxides II

The extreme quantum limit in lightly-doped SrTiO$_{3}$

When a three dimensional electron gas is placed in a sufficiently strong magnetic field, it is said to be in the quantum limit when the cyclotron energy $\hbar \omega_{c}$ \textgreater $\varepsilon_{F}$ \textgreater \textgreater kT, and all of the electrons occupy the lowest Landau level. Achieving this limit in a material requires a small Fermi energy relative to the applied magnetic field, and a weak disorder potential such that magnetic freeze-out is avoided. We present an experimental study of lightly-doped single crystals of SrTiO$_{3}$, which remain good bulk conductors in temperatures down to 25 mK and magnetic fields up to 45 T. Our measurements probe deep into the quantum limit, where $\hbar \omega _{c}$ \textgreater \textgreater $\varepsilon_{F}$ and theory has long predicted that electron-electron interactions can drive the system into a charge density wave or Wigner crystal like state. A number of interesting features arise in electrical transport in this regime, including a striking re-entrant nonlinearity in the current-voltage characteristics. We discuss these features in the context of possible correlated electron states, and present a picture based on magnetic field induced puddling of electrons in a disorder potential landscape.   

Phonon-induced ultrafast band gap control in LaTiO{\$}\textunderscore 3{\$}

We propose a route for ultrafast band gap engineering in correlated transition metal oxides by using optically driven phonons. We show that the {\$}$\backslash $Gamma{\$}-point electron band energies can be deterministically tuned in the nonequilibrium state. Taking the Mott insulator LaTiO$_{\mathrm{3}}$ as an example, we show that such phonon-assisted processes dynamically induce an indirect-to-direct band gap transition or even a metal-to-insulator transition, depending on the electron correlation strength. We explain the origin of the dynamical band structure control and also establish its generality by examining related oxides. Lastly, we describe experimental routes to realize the band structure control with impulsive stimulated Raman scattering.   

Tunning the magnetic ordering in EuTiO$_3$ through doping

EuTiO$_3$ (ETO) is a complex oxide that displays strong spin-lattice coupling, large magnetoelectric effects, and undergoes a series of structural and magnetic phase transitions when subjected to pressure or epitaxial strain. ETO adopts a cubic structure and is paramagnetic at high temperatures, while at very low temperatures it transforms to an antiferrodistortive tetragonal structure with a G-type antiferromagnetic (AFM) ordering. Several approaches have been presented to tune the magnetic ordering from the G-type antiferromagnetism to the F-type ferromagnetism, often relying on external pressure or epitaxial strain. Doping through substitution of trivalent species on the europium sites or creation of oxygen vacancies have also been proposed to lead to ferromagnetism. However, the fundamental mechanism by which excess electrons from impurities or defects lead to ferromagnetic ordering is unclear. In this study, we explore the effects of doping on the magnetic ordering in EuTiO$_3$ through first-principles calculations. We show how itinerant carriers in the Ti-$d$-derived conduction-band states interact with europium $f$ states, inducing an alignment of the large moments on the europium ions. The effects of doping of different types of magnetic ordering are considered, a   

Point defects, impurities, and small hole polarons in GdTiO$_3$

GdTiO$_3$(GTO) has become the focus of great interest because of its use in complex-oxide heterostructures that display two-dimensional electron gases with unprecedented high densities. GTO is a Mott insulator, with a band gap arising within the partially filled Ti 3$d$ band due to strong electron-electron interactions. GTO often displays hole conductivity, likely attributed to defects or impurities, yet the cause of this unintentional conductivity has not yet been explored. We therefore used density functional theory with a hybrid functional to study their electronic structure. Among native defects, the cation vacancies have the lowest formation energies in oxygen-rich conditions, and oxygen vacancies have the lowest formation energy in oxygen-poor conditions. Among the impurities, r$_\mathrm{Gd}$, H$_i$ and C$_\mathrm{O}$ have the lowest formation energies. The defects and impurities are intrinsically stable only in a single ``natural'' charge state, to which various numbers of hole polarons can be bound, which explains the frequent observation of $p$-type hopping conductivity in the rare-earth titanates. These small hole polarons also lead to optical absorption and act as electron traps in devices. \\ \\ Work supported by NSF and by the LEAST Center.   

Low-energy dispersion of dynamic charge stripes in La$_{1.75}$Sr$_{0.25}$NiO$_{4}$ observed with inelastic neutron scattering

The dynamic stripe correlations have been the subject of intense research, owing to the possible links with high-T$_{c}$ superconductivity. In light of a recently published, direct observation of charge-stripe fluctuations in La$_{2-x}$Sr$_{x}$NiO$_{4}$ using inelastic neutron scattering\footnote{S. Anissimova, $et$ $al.$, Nat. Commun. 5, 3467 (2014)}, we did a follow-up neutron experiment on a $x$=0.25 sample to characterize the low-energy dispersion of these dynamic charge stripes using the HYSPEC instrument at the Spallation Neutron Source. The scattering signals are collected in the vicinity of a charge-order peak with a large wave vector (4.4, 3, 0), where dynamic spin-stripe correlations are negligible. Mapping the low-energy charge-stripe fluctuations in a wide temperature range, we observe a finite dispersion along the stripe-modulation direction at T$\geq$160K where the charge stripes become disordered, while the steep dispersion in the orthogonal direction is not resolved.   

Ultrafast Dynamics of the Symmetry Breaking in Charge-ordered La$_{\mathrm{1.75}}$Sr$_{\mathrm{0.25}}$NiO$_{\mathrm{4}}$ Single Crystals.

We report equilibrium and ultrafast optical pump-THz probe spectroscopy of the stripe-phase rare-earth nickelate compound La$_{\mathrm{1.75}}$Sr$_{\mathrm{0.25}}$NiO$_{\mathrm{4}}$, unveiling the ultrafast dynamics of the crystal symmetry breaking and of local electronic arrangements. At low temperatures the folding of finite momenta vibrations due to symmetry breaking lead to the appearance of new IR-active resonances, particularly around the phonon bending mode frequency ($\approx $11 THz). Ultrafast experiments in the multi-THz spectral range show sharp THz reflectivity modulations associated with the phonon zone-folding dynamics, while the background conductivity is reminiscent of the opening of the mid-IR pseudogap. We combine experimental data with DFT calculations of the phonon dispersion to reveal the distinct dynamics of the LO and TO phonon modes at finite momenta. This work provides new insight in the role of polar electron-phonon coupling and symmetry breaking in charge-ordered systems.   

Single crystal preparation and long-range charge fluctuations in the square-planar nickelate La4Ni3O8.

Since the discovery of high-Tc superconductivity in cuprates, intensive effort has been focused on a search for superconductivity in related materials, with particular attention on nickelates. Bulk nickelates containing square-planar coordinated Ni$+$ are of interest because Ni1$+$ is isoelectronic with Cu2$+$, the building block of high-Tc cuprates. Here we report the first single crystal synthesis of La4Ni3O8, a layered nickelate containing square-planar coordinated Ni$^{\mathrm{+}}$ with crystallographic and electronic structure related to that of cuprates. Magnetic susceptibility, resistivity, and heat capacity measurements confirm the reported phase transition at \textasciitilde 105 K[1]. Long-range charge fluctuations with q\textasciitilde (1/3, 1/3, L) was observed for the first time through synchrotron X-ray single crystal diffraction. Our results challenge the current understanding of the origin of the phase transition. Availability of bulk La4Ni3O8 single crystals is also of significant importance for unraveling its ambiguous ground-state magnetic structure, the spin state of the Ni ion, and potential for superconductivity in nickelates involving Ni$+$ in a square-planar coordination. [1] Poltavets, V. V. et al. PRL 2010, 104, 206403.   

Cooperative phonon effects in the metal-insulator transitions of manganite and nickelate perovskites

Metal-insulator transitions in manganite and nickelate perovskites depend on the competition between the electron kinetic energy, which favors the metallic phase, and the electron-phonon coupling and Coulomb interaction, which favor localization. The size of the A-site cation controls the relative rotation of the octahedral structural units of the perovskite in the range of $0$ - $15^\circ$. This is accompanied by changes in the metal-insulator transition temperature from $0$ - $600\text{K}$. This effect is commonly attributed to modification in the electron bandwidth from changes in orbital overlap.\\\\ Although previous theoretical studies of these materials include the electron-phonon interaction, they typically do not consider cooperative phonon effects. Using a phenomenological model of the perovskite structure, we show that the long-range anisotropic forces arising from inter-site phonon interactions are modulated by changes in the octahedral rotation. We demonstrate using statistical mechanical calculations that these changes in the strain interaction can capture the variation in transition temperature with tolerance factor observed in both the manganites and nickelates.   

Direct proof of static charge stripe correlations in La$_{2-x}$Ba$_x$CuO$_4$

The nature of charge stripe order in the cuprates, and in particular whether the stripes are static or dynamic, is a key issue in understanding the relationship between stripes and superconductivity. In La$_{2-x}$Ba$_x$CuO$_4$ (LBCO) a low temperature structural distortion is widely believed to pin stripes into fixed, static domains, but such an assertion has never been directly verified. We performed resonant soft x-ray photon correlation spectroscopy (XPCS) to probe the charge order Bragg peak of 1/8 doped LBCO. At low temperatures, we observe time-independent x-ray speckle patterns persisting for more than three hours, proving the static nature of the stripes and we go on to discuss how stripe order melts with increasing temperature. Our results demonstrate that the combination of XPCS with diffraction limited light sources such as the National Synchrotron Light Source II can probe the dynamics of even subtle order parameters such as stripes in the cuprates.   

Quasi-static magnetoelectric quadrupoles as the order parameter for the pseudo-gap phase in cuprate superconductors

A characteristic of ferroic materials is the emergence of a temporally static finite expectation value of an order parameter. Here, we introduce a new mechanism [1] for ferroic order, in which a non-zero quasi-static magnetoelectric quadrupolar order appears due the coupling of fluctuating spin magnetic dipole moments and polar optical phonons. Using first-principles calculations within the LSDA$+U$ method of density functional theory, we calculate the magnitude of the effect for the prototypical cuprate superconductor, HgBa$_2$CuO$_{4+\delta}$. We show that our proposed mechanism is consistent, to our knowledge, with many experimental observations for the onset of the pseudo-gap phase and therefore propose the quasi-static magnetoelectric quadrupole as a possible pseudo-gap order parameter. Finally, we show that our mechanism embraces some key aspects of previous theoretical models, in particular the description of the pseudo-gap phase in terms of orbital currents. [1] M. Fechner, M. J. A. Fierz, F. Th\"ole, U. Staub, and N. A. Spaldin, arXiv 1510.04844, (2015).   

Quantum oscillations in a bilayer with broken mirror symmetry: a minimal model for YBa$_2$Cu$_3$O$_{6 + \delta}$

Using an exact numerical solution and semiclassical analysis, we investigate quantum oscillations (QOs) in a model of a bilayer system with an anisotropic (elliptical) electron pocket in each plane. Key features of QO experiments in the high temperature superconducting cuprate YBCO can be reproduced by such a model, in particular the pattern of oscillation frequencies (which reflect ``magnetic breakdown'' between the two pockets) and the polar and azimuthal angular dependence of the oscillation amplitudes. However, the requisite magnetic breakdown is possible only under the assumption that the horizontal mirror plane symmetry is spontaneously broken and that the bilayer tunneling, $t_\perp$, is substantially renormalized from its `bare' value. Under the assumption that $t_{\perp} = \tilde{Z}t_\perp^{(0)}$, where $\tilde{Z}$ is a measure of the quasiparticle weight, this suggests that $\tilde{Z} \lesssim 1/20$. Detailed comparisons with new YBa$_2$Cu$_3$O$_{6.58}$ QO data, taken over a very broad range of magnetic field, confirm specific predictions made by the breakdown scenario.   

Controlling Spin Ordering in Rare-Earth Perovskite Vanadates

We investigate the role and influence of local structure distortions on the antiferromagnetic spin ordering temperatures for large $A$-site radii $R$VO3 perovskites ($R$=Yb-La) using a combination of data analytics (DA) and density functional theory (DFT). First, mode crystallography is used to parameterize the structural phase space. Next, we identify the important local structural features that correlate strongly with the N\'eel temperatures ($T_N$) using Pearson correlation coefficients. From this data, we then formulate a regression model using gradient boosted decision trees (GBDT) that returns the relative importance of each feature in predicting $T_N$. Our analysis indicates that the amplitude of the subtle Jahn-Teller active mode, which leads to variations in the V-O bond lengths and angles, could be used as an effective structural control parameter to modify the spin ordering temperature. We then validate this data-driven structure-property relationship in artificial vanadate structures using $T_N$ based on both our GBDT model and a model Hamiltonian using DFT energies. This combined approach allows us to gauge the accuracy of existing physical models for the antiferromagnetic ordering in vanadates and opens possible strategies to design materials with targeted $T_N$.   

Giant Magnetocaloric Effect in the Double-perovskite Gd2NiMnO6

We have synthesized single crystal of Gd$_{\mathrm{2}}$NiMnO$_{\mathrm{6}}$(GNMO) by the Bi-flux method and investigated magnetocaloric effect in them by magnetic measurements. Magnetic susceptibility of GNMO increases smoothly as temperature decrease and ferromagnetic order occurs below 135 K, and additional anomaly show at low temperature, indicative of the onset of Gd$^{\mathrm{3+}}$ spin arrangement. At the temperature, magnetic entropy change, -$\Delta $S$_{\mathrm{M}}$, with the field changes of 0-9 T, calculated from isothermal M(H) data using Maxwell relation, exhibits sharp peak. This peak is gigantic and cryogenic, these make GNMO promising cryogenic magnetic refrigerant materials.