arXiv subjects

J. A. Fernandez-Baca

Publications and source records attributed to J. A. Fernandez-Baca.

At least 19 recordsLinked to original sources

arXiv2024

Incident Beamline Design for a Modern Cold Triple Axis Spectrometer at the High Flux Isotope Reactor

A modern cold triple axis spectrometer is being planned for the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Here, we describe the design of an incident beamline that will put a flux of $\sim 10^8\mathrm{\frac{n}{cm^2 s}}$ on a sample with an area of 2 cm x 2 cm. It takes current physical constraints at HFIR into account and it can accommodate both single and multiplexed analyzer-detector secondary spectrometers and large superconducting magnets. The proposed incident beamline includes a multi-channel guide with horizontal focusing, a neutron velocity selector, components to facilitate an incident beam polarization option, and a double-focusing pyrolytic graphite monochromator. This work describes the process of optimizing the guide system and monochromator and summarizes the expected performance of the incident beamline for non-polarized operation.

physics.ins-det↗

arXiv2022

Magnetic structure and spin dynamics of the quasi-2D antiferromagnet Zn-doped copper pyrovanadate

Magnetic properties of the antiferromagnet Zn$_{0.15}$Cu$_{1.85}$V$_2$O$_7$ (ZnCVO) have been thoroughly investigated on powder and single-crystal samples. The crystal structure determination using powder x-ray and neutron diffraction confirms that ZnCVO with Zn = 0.15 is isostructural with $β$-Cu$_{2}$V$_2$O$_7$ ($β$-CVO) with small deviation in the lattice parameters. Macroscopic magnetic properties measurements also confirm the similarity between the two compounds. The Cu$^{2+}$ spins were found to align along the crystallographic $c$-axis, antiparallel to their nearest neighbors connected by the leading exchange interaction $J_1$. Spin dynamics reveals a typical symmetric spin-wave dispersion with strong interactions in the $bc$-plane and weak interplane coupling. The exchange interaction analysis indicates that the spin network of ZnCVO is topologically consistent with the previous DFT prediction but the values of leading exchange interactions are contradictory. Furthermore, rather than the predicted 2D honeycomb structure, the spin network in ZnCVO could be better described by the anisotropic 2D spin network composing of $J_1$, $J_5$, and $J_6$ interactions, four bonds per one spin site, coupled by weak interplane interactions.

cond-mat.str-el↗

arXiv2019

Low-energy phonons in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ and their possible interaction with electrons measured by inelastic neutron scattering

Electron-phonon interaction in copper oxide superconductors is still enigmatic. Strong coupling for certain optic phonons is now well established experimentally, but theoretical understanding is challenging. Scattering of electrons near the Fermi surface by the longitudinal acoustic (LA) phonons is expected from basic theory because these phonons modulate electron density. We used inelastic neutron scattering on a large single crystal sample of optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ to show that low-energy LA phonons could couple to electronic density fluctuations only at small phonon wavevectors, which naturally limits any interaction to forward scattering. Such scattering should not be pairbreaking in the case of the d-wave gap. We also found that previously the reported low energy phonon spectral weight half-way to the zone boundary is consistent with conventional lattice dynamics and does not reflect an incipient charge density wave.

cond-mat.supr-con↗

arXiv2018

Neutron diffraction study on magnetic structures and transitions in Sr2Cr3As2O2

Sr2Cr3As2O2 is composed of alternating square-lattice CrO2 and Cr2As2 stacking layers, where CrO2 is isostructural to the CuO2 building-block of cuprate high-Tc superconductors and Cr2As2 to Fe2As2 of Fe-based superconductors. Current interest in this material is raised by theoretic prediction of possible superconductivity. In this neutron powder diffraction study, we discovered that magnetic moments of Cr(II) ions in the Cr2As2 sublattice develop a C-type antiferromagnetic structure below 590 K, and the moments of Cr(I) in the CrO2 sublattice form the La2CuO4 -like antiferromagnetic order below 291 K. The staggered magnetic moment 2.19(4)μ B /Cr(II) in the more itinerant Cr2As2 layer is smaller than 3.10(6)μ_B/Cr(I) in the more localized CrO2 layer. Different from previous expectation, a spin-flop transition of the Cr(II) magnetic order observed at 291 K indicates a strong coupling between the CrO2 and Cr2As2 magnetic subsystems.

cond-mat.str-el↗

arXiv2018

Soft antiphase tilt of oxygen octahedra in the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7

We report a single crystal neutron and x-ray diffraction study of the hybrid improper multiferroic Ca3Mn1.9Ti0.1O7 (CMTO), a prototypical system where the electric polarization arises from the condensation of two lattice distortion modes.With increasing temperature (T ), the out-of-plane, antiphase tilt of MnO6 decreases in amplitude while the in-plane, in-phase rotation remains robust and experiences abrupt changes across the first-order structural transition. Application of hydrostatic pressure (P) to CMTO at room temperature shows a similar effect. The consistent behavior under both T and P reveals the softness of antiphase tilt and highlights the role of the partially occupied d orbital of the transition-metal ions in determining the stability of the octahedral distortion. Polarized neutron analysis indicates the symmetry-allowed canted ferromagnetic moment is less than the 0.04 μB/Mn site, despite a substantial out-of-plane tilt of the MnO6 octahedra.

cond-mat.str-el↗

arXiv2016

Electron doping evolution of the neutron spin resonance in NaFe$_{1-x}$Co$_{x}$As

Neutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this work, we study the doping evolution of the resonances in NaFe$_{1-x}$Co$_x$As covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum, but retains approximately the same peak position even when $T_c$ drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe$_{1-x}$Co$_x$As is neither simply proportional to $T_c$ nor the superconducting gap, but is controlled by the multi-orbital character of the system and doped impurity scattering effect.

cond-mat.supr-con↗

arXiv2016

Spin-lattice coupling mediated multiferroicity in (ND4)2FeCl5D2O

We report a neutron diffraction study of the multiferroic mechanism in (ND4)2FeCl5D2O, a molecular compound that exhibits magnetically induced ferroelectricity. This material exhibits two successive magnetic transitions on cooling: a long-range order transition to an incommensurate (IC) collinear sinusoidal spin state at TN=7.3 K, followed by a second transition to an IC cycloidal spin state at TFE=6.8 K, the later of which is accompanied by spontaneous ferroelectric polarization. The cycloid structure is strongly distorted by spin-lattice coupling as evidenced by the observations of both odd and even higher-order harmonics associated with the cycloid wave vector, and a weak commensurate phase that coexists with the IC phase. The appearance of the 2nd-order harmonic coincides with the onset of the electric polarization, thereby providing unambiguous evidence that the induced electric polarization is mediated by the spin-lattice interaction. Our results for this system, in which the orbital angular momentum is expected to be quenched, are remarkably similar to those of the prototypical TbMnO3, in which the magnetoelectric effect is attributed to spin-orbit coupling.

cond-mat.str-el↗

arXiv2014

The effects of Co3O4 on the Structure and Unusual Magnetism of LaCoO3

Bulk La_wCoO3 particles with w=1.1, 1.0, 0.9, 0.8, and 0.7 were synthesized using starting materials with varying molar ratios of La2O3 and Co3O4. The resulting particles are characterized as LaCoO3 crystals interfaced with a crystalline Co3O4 phase. X-ray and neutron scattering data show little effect on the average structure and lattice parameters of the LaCoO3 phase resulting from the Co3O4 content, but magnetization data indicate that the amount of Co3O4 strongly affects the ferromagnetic ordering at the interfaces below T_C ~89K. In addition to ferromagnetic long-range order, LaCoO3 exhibits antiferromagnetic behavior with an unusual temperature dependence. The magnetization for fields 20 Oe < H < 5 kOe is fit to a combination of a power law ((T-T_C)/T_C)^beta behavior representing the ferromagnetic long-range order and sigmoid-convoluted Curie-Weiss-like behavior representing the antiferromagnetic behavior. The critical exponent beta=0.63 +- 0.02 is consistent with 2D (surface) ordering. Increased Co3O4 correlates well to increased ferromagnetism. The weakening of the antiferromagnetism below T ~ 40K is a consequence of the lattice reaching a critical rhombahedral distortion as T is decreased for core regions far from the Co3O4 interfaces. We introduce a model that describes the ferromagnetic behavior of the interface regions and the unusual antiferromagnetism of the core regions.

cond-mat.str-el↗

arXiv2014

The unusual magnetism of nanoparticle LaCoO3

Bulk and nanoparticle powders of LaCoO3 (LCO) were synthesized, and their magnetic and structural properties were studied using SQUID magnetometry and neutron diffraction. The bulk and large nanoparticles exhibit weak ferromagnetism (FM) below T ~ 85K and a crossover from strong to weak antiferromagnetic (AFM) correlations near a transition expressed in the lattice parameters, To ~ 40K. This crossover does not occur in the smallest nanoparticles; instead, the magnetic behavior is predominantly ferromagnetic. The amount of FM in the nanoparticles depends on the amount of Co3O4 impurity phase, which induces tensile strain on the LCO lattice. A core-interface model is introduced, with the core region exhibiting the AFM crossover and with FM in the interface region near surfaces and impurity phases.

cond-mat.str-el↗

arXiv2014

Direct observation of dynamic charge stripes in La2-xSrxNiO4

The insulator-to-metal transition continues to be a challenging subject, especially when electronic correlations are strong. In layered compounds, such as La2-xSrxNiO4 and La2-xBaxCuO4, the doped charge carriers can segregate into periodically-spaced charge stripes separating narrow domains of antiferromagnetic order. Although there have been theoretical proposals of dynamically fluctuating stripes, direct spectroscopic evidence of charge-stripe fluctuations has been lacking. Here we report the detection of critical lattice fluctuations, driven by charge-stripe correlations, in La2-xSrxNiO4 using inelastic neutron scattering. This scattering is detected at large momentum transfers where the magnetic form factor suppresses the spin fluctuation signal. The lattice fluctuations associated with the dynamic charge stripes are narrow in q and broad in energy. They are strongest near the charge stripe melting temperature. Our results open the way towards the quantitative theory of dynamic stripes and for directly detecting dynamical charge stripes in other strongly-correlated systems, including high-temperature superconductors such as La2-xSrxCuO4.

cond-mat.str-el↗

arXiv2014

Magnetic field-induced spontaneous polarization reversal in multiferroic Mn$_{0.85}$Co$_{0.15}$WO$_4$

The magnetic and ferroelectric properties of the multiferroic system Mn$_{1-x}$Co$_x$WO$_4$ (x=0.135, 0.15, and 0.17) are studied in magnetic fields $H_c$ oriented along the monoclinic $c$-axis. Mn$_{0.85}$Co$_{0.15}$WO$_4$, which is right at the phase boundary between two helical spin structures, exhibits a spontaneous sign change of the ferroelectric polarization when cooled in fields $H_c>$ 25 kOe. The origin of the ferroelectric polarization is studied and two magnetic exchange interactions contributing to the polarization are identified. In Mn$_{0.85}$Co$_{0.15}$WO$_4$ domains of the characteristic helical spin structures, known for x$<$0.15 and x$>$0.15, coexist and form domain boundaries. The contributions of the different domains to the global polarization are determined. The polarization reversal in Mn$_{0.85}$Co$_{0.15}$WO$_4$ can be explained by a combination of various contributions to the polarization and a strong correlation between magnetic domains of different helical spin orders resulting in a smooth transition across the domain walls which preserves the chirality of the spin spiral.

cond-mat.str-el↗

arXiv2013

Magnetism in nanoparticle LaCoO3

LaCoO3 (LCO) nanoparticles were synthesized and their magnetic and structural properties were examined using SQUID magnetometery and neutron diffraction. The nanoparticles exhibit ferromagnetic long-range order beginning at T_C approximately 87K that persists to low temperatures. This behavior is contrasted with the ferromagnetism of bulk LCO, which also starts at T_C approximately 87K but is suppressed below a second transition at T_o approximately 37K, due to a structural phase transition. The ferromagnetism in both systems is attributed to the tensile stress from particle surfaces and impurity phase interfaces. This stress locally increases the Co-O-Co bond angle gamma, and competes with the thermal contraction of the lattice. It has recently been shown that LCO loses long-range ferromagnetic order when gamma decreases below the critical value gamma_c = 162.8 degrees. Consistent with this model, we show that gamma in nanoparticles remains larger than gamma_c at low temperatures, likely a consequence of all spins being in close proximity to surfaces or interfaces.

cond-mat.str-el↗

arXiv2013

Double neutron spin resonances and gap anisotropy in underdoped superconducting NaFe0.985Co0.015As

We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe0.985Co0.015As induces a dispersive sharp resonance near Er1 = 3:25 meV and a broad dis- persionless mode at Er2 = 6 meV. However, similar measurements on overdoped superconducting NaFe0:955Co0:045As find only a single sharp resonance at Er = 7 meV. We connect these results with the observations of angle-resolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnic- tides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides.

cond-mat.supr-con↗

arXiv2013

Doping influence of spin dynamics and magnetoelectric effect in hexagonal Y$_{0.7}$Lu$_{0.3}$MnO$_{3}$

We use inelastic neutron scattering to study spin waves and their correlation with the magnetoelectric effect in Y$_{0.7}$Lu$_{0.3}$MnO$_3$. In the undoped YMnO$_3$ and LuMnO$_3$, the Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect. In Y$_{0.7}$Lu$_{0.3}$MnO$_3$, we find a much smaller in-plane (hexagonal $ab$-plane) single ion anisotropy gap that coincides with a weaker in-plane dielectric anomaly at $T_N$. Since both the smaller in-plane anisotropy gap and the weaker in-plane dielectric anomaly are coupled to a weaker Mn trimerization distortion in Y$_{0.7}$Lu$_{0.3}$MnO$_3$ comparing to YMnO$_3$ and LuMnO$_3$, we conclude that the Mn trimerization is responsible for the magnetoelectric effect and multiferroic phenomenon in Y$_{1-y}$Lu$_{y}$MnO$_{3}$.

cond-mat.str-el↗

arXiv2013

Origins of large enhancement in electromechanical coupling for nonpolar directions in ferroelectric BaTiO3

The origins of enhanced piezoelectric coupling along nonpolar crystallographic directions in ferroelectric BaTiO3 are investigated using in situ neutron spectroscopy. It is observed that an electric field applied away from the equilibrium polarization direction causes changes in the phonon spectra that lead to an increase in the interaction between the transverse acoustic and transverse optic branches (TA-TO) near the Brillouin zone center. This provides a direct lattice dynamics mechanism for enhanced electromechanical coupling, and could act as a guide for designing improved piezoelectric materials.

cond-mat.mtrl-sci↗

arXiv2012

The complex multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$

The complete magnetic and multiferroic phase diagram of Mn$_{1-x}$Co$_{x}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, and polarization experiments. We show that the ferroelectric polarization $\overrightarrow{P}$ in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At x$_{c1}$=0.075, $\overrightarrow{P}$ rotates from the $b-$axis into the $a-c$ plane and at x$_{c2}$=0.15 it flips back to the $b-$axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the ferroelectric polarization is derived by taking into account the intrachain ($c-$axis) as well as the interchain ($a-$axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1$\leq$x$\leq$0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO$_4$.

cond-mat.str-el↗

arXiv2012

Zn-induced spin dynamics in overdoped La$_{2-x}$Sr$_x$Cu$_{1-y}$Zn$_y$O$_4$

Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_y$O$_4$ ($x=0.25$, $y\approx0.01$) are measured via inelastic neutron scattering techniques. As Zn$^{2+}$ is substituted onto the Cu$^{2+}$-sites, an anomalous enhancement of the local spin susceptibility $χ^{\prime\prime}(ω)$ appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector \textbf{Q}$=(π, π, 0)$ that coexists with the known incommensurate SDW excitations at \textbf{Q}$_{HK}=(π\pmδ,π), (π,π\pmδ)$. Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu$^{2+}$-spins, and the simultaneous suppression of T$_c$ as AF fluctuations are slowed in proximity to Zn-impurities suggests the continued importance of high energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.

cond-mat.supr-con↗

arXiv2011

Robust Ferroelectric State in Multiferroic Mn$_{1-x}$Zn$_x$WO$_4$

We report the remarkably robust ferroelectric state in the multiferroic compound Mn$_{1-x}$Zn$_x$WO$_4$. The substitution of the magnetic Mn$^{2+}$ with nonmagnetic Zn$^{2+}$ reduces the magnetic exchange and provides control of the various magnetic and multiferroic states of MnWO$_4$. Only 5 % of Zn substitution results in a complete suppression of the frustrated collinear (paraelectric) low temperature phase. The helical magnetic and ferroelectric phase develops as the ground state. The multiferroic state is stable up to a high level of substitution of more than 50 %. The magnetic, thermodynamic, and dielectric properties as well as the ferroelectric polarization of single crystals of Mn$_{1-x}$Zn$_x$WO$_4$ are studied for different substitutions up to x=0.5. The magnetic phases have been identified in single crystal neutron scattering experiments. The ferroelectric polarization scales with the neutron intensity of the incommensurate peak of the helical phase.

cond-mat.str-el↗