arXiv subjects

A. A. Aczel

Publications and source records attributed to A. A. Aczel.

At least 55 records · Page 3Linked to original sources

arXiv2017

Hidden Order Signatures in the Antiferromagnetic Phase of U(Ru$_{1-x}$Fe$_x$)$_2$Si$_2$

We present a comprehensive set of elastic and inelastic neutron scattering measurements on a range of Fe-doped samples of U(Ru$_{1-x}$Fe$_x$)$_2$Si$_2$ with 0.01~$\le~x~\le$~0.15. All of the samples measured exhibit long-range antiferromagnetic order, with the size of the magnetic moment quickly increasing to 0.51~$μ_B$ at 2.5\% doping and continuing to increase monotonically with doping, reaching 0.69~$μ_B$ at 15\% doping. Time-of-flight and inelastic triple-axis measurements show the existence of excitations at (1~0~0) and (1.4~0~0) in all samples, which are also observed in the parent compound. While the excitations in the 1\% doping are quantitatively identical to the parent material, the gap and width of the excitations change rapidly at 2.5\% Fe doping and above. The 1\% doped sample shows evidence for a separation in temperature between the hidden order and antiferromagnetic transitions, suggesting that the antiferromagnetic state emerges at very low Fe dopings. The combined neutron scattering data suggests not only discontinuous changes in the magnetic moment and excitations between the hidden order and antiferromagnetic phases, but that these changes continue to evolve up to at least $x$~=~0.15.

cond-mat.str-el↗

arXiv2017

Neutron Diffraction and $μ$SR Studies of Two Polymorphs of Nickel Niobate (NiNb$_2$O$_6$)

Neutron diffraction and muon spin relaxation ($μ$SR) studies are presented for the newly characterized polymorph of NiNb$_2$O$_6$ ($β$-NiNb$_2$O$_6$) with space group P4$_2$/n and $μ$SR data only for the previously known columbite structure polymorph with space group Pbcn. The magnetic structure of the P4$_2$/n form was determined from neutron diffraction using both powder and single crystal data. Powder neutron diffraction determined an ordering wave vector $\vec{k}$ = ($\frac{1}{2},\frac{1}{2},\frac{1}{2}$). Single crystal data confirmed the same $\vec{k}$-vector and showed that the correct magnetic structure consists of antiferromagnetically-coupled chains running along the a or b-axes in adjacent Ni$^{2+}$ layers perpendicular to the c-axis, which is consistent with the expected exchange interaction hierarchy in this system. The refined magnetic structure is compared with the known magnetic structures of the closely related tri-rutile phases, NiSb$_2$O$_6$ and NiTa$_2$O$_6$. $μ$SR data finds a transition temperature of $T_N \sim$ 15 K for this system, while the columbite polymorph exhibits a lower $T_N =$ 5.7(3) K. Our $μ$SR measurements also allowed us to estimate the critical exponent of the order parameter $β$ for each polymorph. We found $β=$ 0.25(3) and 0.16(2) for the $β$ and columbite polymorphs respectively. The single crystal neutron scattering data gives a value for the critical exponent $β=$~0.28(3) for $β$-NiNb$_2$O$_6$, in agreement with the $μ$SR value. While both systems have $β$ values less than 0.3, which is indicative of reduced dimensionality, this effect appears to be much stronger for the columbite system. In other words, although both systems appear to well-described by $S = 1$ spin chains, the interchain interactions in the $β$-polymorph are likely much larger.

cond-mat.other↗

arXiv2017

Frustrated spin-1/2 molecular magnetism in the mixed-valence antiferromagnets Ba3MRu2O9 (M = In, Y, Lu)

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron scattering measurements on three members of the family Ba3MRu2O9, where M = In, Y and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocks form a frustrated, quasi-two-dimensional triangular lattice. Our zero and longitudinal field muSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero field muon spin polarization indicative of long-range magnetic order in the Lu sample. The static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.

cond-mat.str-el↗

arXiv2016

Revisiting the ground state of CoAl$_2$O$_4$: comparison to the conventional antiferromagnet MnAl$_2$O$_4$

The A-site spinel material, CoAl2O4, is a physical realization of the frustrated diamond-lattice antiferromagnet, a model in which is predicted to contain unique incommensurate or `spin-spiral liquid' ground states. Our previous single-crystal neutron scattering study instead classified it as a `kinetically-inhibited' antiferromagnet, where the long ranged correlations of a collinear Neel ground state are blocked by the freezing of domain wall motion below a first-order phase transition at T* = 6.5 K. The current paper expands on our original results in several important ways. New elastic and inelastic neutron measurements are presented that show our initial conclusions are affected by neither the sample measured nor the instrument resolution, while measurements to temperatures as low as T = 250 mK limit the possible role being played by low-lying thermal excitations. Polarized diffuse neutron measurements confirm reports of short-range antiferromagnetic correlations and diffuse streaks of scattering, but major diffuse features are explained as signatures of overlapping critical correlations between neighboring Brillouin zones. Finally, and critically, this paper presents detailed elastic and inelastic measurements of magnetic correlations in a single-crystal of MnAl2O4, which acts as an unfrustrated analogue to CoAl2O4. The unfrustrated material is shown to have a classical continuous phase transition to Neel order at T_N = 39 K, with collective spinwave excitations and Lorentzian-like critical correlations which diverge at the transition. Direct comparison between the two compounds indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse correlations, but rather in the nature of the frozen state below T*. The higher level of cation inversion in the MnAl2O4 sample indicates that this novel behavior is primarily an effect of greater next-nearest-neighbor exchange.

cond-mat.str-el↗

arXiv2016

Dominant Kitaev interactions on the fcc lattice in iridate double perovskites La2BIrO6 (B=Mg, Zn)

We have performed inelastic neutron scattering experiments to investigate the magnetic excitations in the quasi-face-centered-cubic (fcc) iridate double perovskites La2ZnIrO6 and La2MgIrO6, which are characterized by A-type antiferromagnetic ground states. The neutron scattering data, which reveal gapped spin wave excitations with a very weak dispersion, are shown to be well-described by theoretical calculations on a model Hamiltonian with dominant Kitaev interactions. Our surprising finding shows that in contrast to honeycomb materials, where the Kitaev interaction manifests itself via complex magnetic order, even conventional magnetic orders in certain geometrically frustrated systems might owe their existence to unconventional Kitaev exchange.

cond-mat.str-el↗

arXiv2016

Highly-anisotropic exchange interactions of $j_{\rm eff}=1/2$ iridium moments on the fcc lattice in La$_2B$IrO$_6$ ($B$ $=$ Mg, Zn)

We have performed inelastic neutron scattering (INS) experiments to investigate the magnetic excitations in the weakly distorted face-centered-cubic (fcc) iridate double perovskites La$_2$ZnIrO$_6$ and La$_2$MgIrO$_6$, which are characterized by A-type antiferromagnetic ground states. The powder inelastic neutron scattering data on these geometrically frustrated $j_{\rm eff}=1/2$ Mott insulators provide clear evidence for gapped spin wave excitations with very weak dispersion. The INS results and thermodynamic data on these materials can be reproduced by conventional Heisenberg-Ising models with significant uniaxial Ising anisotropy and sizeable second-neighbor ferromagnetic interactions. Such a uniaxial Ising exchange interaction is symmetry-forbidden on the ideal fcc lattice, so that it can only arise from the weak crystal distortions away from the ideal fcc limit. This may suggest that even weak distortions in $j_{\rm eff}=1/2$ Mott insulators might lead to strong exchange anisotropies. More tantalizingly, however, we find an alternative viable explanation of the INS results in terms of spin models with a dominant Kitaev interaction. In contrast to the uniaxial Ising exchange, the highly-directional Kitaev interaction is a type of exchange anisotropy which is symmetry-allowed even on the ideal fcc lattice. The Kitaev model has a magnon gap induced by quantum order-by-disorder, while weak anisotropies of the Kitaev couplings generated by the symmetry-lowering due to lattice distortions, can pin the order and enhance the magnon gap. Our findings highlight how even conventional magnetic orders in heavy transition metal oxides may be driven by highly-directional exchange interactions rooted in strong spin-orbit coupling.

cond-mat.str-el↗

arXiv2016

Fragile singlet ground state magnetism in pyrochlore osmates R2Os2O7 (R=Y and Ho)

The singlet ground state magnetism in pyrochlore osmates Y$_2$Os$_2$O$_7$ and Ho$_2$Os$_2$O$_7$ is studied by DC and AC susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected non-magnetic singlet in the strong spin-orbit coupling (SOC) limit for Os$^{4+}$ ($5d^4$), Y$_2$Os$_2$O$_7$ exhibits a spin-glass (SG) ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the non-magnetic state in the strong SOC limit and the magnetic state in the strong superexchange limit. Ho$_2$Os$_2$O$_7$ has the same structural distortion as occurs in Y$_2$Os$_2$O$_7$. However, the Os sublattice in Ho$_2$Os$_2$O$_7$ shows long-range magnetic ordering below 36\,K. The sharp difference of the magnetic ground state between Y$_2$Os$_2$O$_7$ and Ho$_2$Os$_2$O$_7$ signals the singlet ground state magnetism in $R_2$Os$_2$O$_7$ is fragile and can be disturbed by the weak $4f-5d$ interactions.

cond-mat.str-el↗

arXiv2016

XY Antiferromagnetic Ground State in the Effective S=1/2 Pyrochlore Yb2Ge2O7

We report neutron scattering and muon spin relaxation measurements (muSR) on the pyrochlore antiferromagnet Yb2Ge2O7. Inelastic neutron scattering was used to probe the transitions between crystal electric field levels, allowing us to determine the eigenvalues and eigenvectors appropriate to the J=7/2 Yb3+ ion in this environment. The crystal electric field ground state doublet in Yb2Ge2O7 corresponds primarily to m_J = +/- 1/2 with local XY anisotropy, consistent with an S_eff = 1/2 description for the Yb moments. muSR measurements reveal the presence of an ordering transition at T_N = 0.57 K with persistent weak dynamics in the ordered state. Finally, we present neutron diffraction measurements that reveal a clear phase transition to the k = (000) Gamma_5 ground state with an ordered magnetic moment of 0.3(1) mu_B per Yb ion. We compare and contrast this phenomenology with the low temperature behavior of Yb2Ti2O7 and Er2Ti2O7, the prototypical S_eff = 1/2 XY pyrochlore magnets.

cond-mat.str-el↗

arXiv2016

Structural and magnetic properties of the 5$d^2$ double perovskites Sr$_2 B$ReO$_6$ ($B$ $=$ Y, In)

We have performed magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation experiments to investigate the magnetic ground states of the 5$d^2$ double perovskites Sr$_2$YReO$_6$ and Sr$_2$InReO$_6$. We find that Sr$_2$YReO$_6$ is a spin glass, while Sr$_2$InReO$_6$ hosts a non-magnetic singlet state. By making detailed comparisons with other 5$d^2$ double perovskites, we argue that a delicate interplay between spin-orbit coupling, non-cubic crystal fields, and exchange interactions plays a key role in the great variation of magnetic ground states observed for this family of materials.

cond-mat.str-el↗

arXiv2016

Structural transition and orbital glass physics in near itinerant CoV2O4

The ferrimagnetic spinel $\mathrm{CoV_2O_4}$ has been a topic of intense recent interest, both as a frustrated insulator with unquenched orbital degeneracy and as a near-itinerant magnet which can be driven metallic with moderate applied pressure. Here, we report on our recent neutron diffraction and inelastic scattering measurements on powders with minimal cation site disorder. Our main new result is the identification of a weak ($\frac{Δa}{a} \sim 10^{-4}$), first order structural phase transition at $T^*$ = 90 K, the same temperature where spin canting was seen in recent single crystal measurements. This transition is characterized by a short-range distortion of oxygen octahedral positions, and inelastic data further establish a weak $Δ\sim 1.25 meV$ spin gap at low temperature. Together, these findings provide strong support for the local orbital picture and the existence of an orbital glass state at temperatures below $T^*$.

cond-mat.str-el↗

arXiv2015

Spin dynamics near a putative antiferromagnetic quantum critical point in Cu substituted BaFe$_2$As$_2$ and its relation to high-temperature superconductivity

We present the results of elastic and inelastic neutron scattering measurements on non-superconducting Ba(Fe${_{0.957}}$Cu${_{0.043}}$)${_2}$As${_2}$, a composition close to a quantum critical point between AFM ordered and paramagnetic phases. By comparing these results with the spin fluctuations in the low Cu composition as well as the parent compound BaFe$_2$As$_2$ and superconducting Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ compounds, we demonstrate that paramagnon-like spin fluctuations are evident in the antiferromagnetically ordered state of Ba(Fe$_{0.957}$Cu$_{0.043}$)$_2$As$_2$, which is distinct from the AFM-like spin fluctuations in the superconducting compounds. Our observations suggest that Cu substitution decouples the interaction between quasiparticles and the spin fluctuations. We also show that the spin-spin correlation length, ${ξ(T)}$, increases rapidly as the temperature is lowered and find ${ω/T}$ scaling behavior, the hallmark of quantum criticality, at an antiferromagnetic quantum critical point.

cond-mat.str-el↗

arXiv2015

Evolution of competing magnetic order in the Jeff=1/2 insulating state of Sr2Ir1-xRuxO4

We investigate the magnetic properties of the series Sr2Ir1-xRuxO4 with neutron, resonant x-ray and magnetization measurements. The results indicate an evolution and coexistence of magnetic structures via a spin flop transition from ab-plane to c-axis collinear order as the 5d Ir4+ ions are replaced with an increasing concentration of 4d Ru4+ ions. The magnetic structures within the ordered regime of the phase diagram (x<0.3) are reported. Despite the changes in magnetic structure no alteration of the Jeff=1/2 ground state is observed. The behavior of Sr2Ir1-xRuxO4 is consistent with electronic phase separation and diverges from a standard scenario of hole doping. The role of lattice alterations with doping on the magnetic and insulating behavior is considered. The results presented here provide insight into the magnetic insulating states in strong spin-orbit coupled materials and the role perturbations play in altering the behavior.

cond-mat.str-el↗

arXiv2015

Magnetic Correlations in the Quasi-2D Semiconducting Ferromagnet CrSiTe$_3$

Intrinsic, two-dimensional ferromagnetic semiconductors are an important class of materials for overcoming the limitations of dilute magnetic semiconductors for spintronics applications. CrSiTe$_3$ is a particularly interesting member of this class, since it can likely be exfoliated down to single layers, where T$_C$ is predicted to increase dramatically. Establishing the nature of the magnetism in the bulk is a necessary precursor to understanding the magnetic behavior in thin film samples and the possible applications of this material. In this work, we use elastic and inelastic neutron scattering to measure the magnetic properties of single crystalline CrSiTe$_3$. We find that there is a very small single ion anisotropy favoring magnetic ordering along the $c$-axis and that the measured spin waves fit well to a model where the moments are only weakly coupled along that direction. Finally, we find that both static and dynamic correlations persist within the $ab$-plane up to at least 300 K, strong evidence of this material's two-dimensional characteristics that are relevant for future studies on thin film and monolayer samples.

cond-mat.str-el↗

arXiv2015

Light atom quantum oscillations in UC and US

High energy vibrational scattering in the binary systems UC and US is measured using time-of-flight inelastic neutron scattering. A clear set of well-defined peaks equally separated in energy is observed in UC, corresponding to harmonic oscillations of the light C atoms in a cage of heavy U atoms. The scattering is much weaker in US and only a few oscillator peaks are visible. We show how the difference between the materials can be understood by considering the neutron scattering lengths and masses of the lighter atoms. Monte Carlo ray tracing is used to simulate the scattering, with near quantitative agreement with the data in UC, and some differences with US. The possibility of observing anharmonicity and anisotropy in the potentials of the light atoms is investigated in UC. Overall the observed data is well accounted for by considering each light atom as a single atom isotropic quantum harmonic oscillator.

cond-mat.mtrl-sci↗

arXiv2015

Strong competition between orbital-ordering and itinerancy in a frustrated spinel vanadate

The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions.

cond-mat.str-el↗

arXiv2015

Spin-orbit coupled j=1/2 iridium moments on the geometrically frustrated fcc lattice

Motivated by experiments on the double perovskites La2ZnIrO6 and La2MgIrO6, we study the magnetism of spin-orbit coupled j=1/2 iridium moments on the three-dimensional, geometrically frustrated, face-centered cubic lattice. The symmetry-allowed nearest-neighbor interaction includes Heisenberg, Kitaev, and symmetric off-diagonal exchange. A Luttinger-Tisza analysis shows a rich variety of orders, including collinear A-type antiferromagnetism, stripe order with moments along the [111]-direction, and incommensurate non-coplanar spirals, and we use Monte Carlo simulations to determine their magnetic ordering temperatures. We argue that existing thermodynamic data on these iridates underscores the presence of a dominant Kitaev exchange, and also suggest a resolution to the puzzle of why La2ZnIrO6 exhibits `weak' ferromagnetism, but La2MgIrO6 does not.

cond-mat.str-el↗

arXiv2015

Influence of interstitial Mn on magnetism in room-temperature ferromagnet Mn(1+delta)Sb

We report elastic and inelastic neutron scattering measurements of the high-TC ferromagnet Mn(1+delta)Sb. Measurements were performed on a large, TC=434 K, single crystal with interstitial Mn content of delta~0.13. The neutron diffraction results reveal that the interstitial Mn has a magnetic moment, and that it is aligned antiparallel to the main Mn moment. We perform density functional theory calculations including the interstitial Mn, and find the interstitial to be magnetic in agreement with the diffraction data. The inelastic neutron scattering measurements reveal two features in the magnetic dynamics: i) a spin-wave-like dispersion emanating from ferromagnetic Bragg positions (H K 2n), and ii) a broad, non-dispersive signal centered at forbidden Bragg positions (H$\,$K$\,$2$n$+1). The inelastic spectrum cannot be modeled by simple linear spin-wave theory calculations, and appears to be significantly altered by the presence of the interstitial Mn ions. The results show that the influence of the interstitial Mn on the magnetic state in this system is more important than previously understood.

cond-mat.str-el↗

arXiv2015

Proximate Kitaev Quantum Spin Liquid Behaviour in α-RuCl$_3$

Topological states of matter such as quantum spin liquids (QSLs) are of great interest because of their remarkable predicted properties including protection of quantum information and the emergence of Majorana fermions. Such QSLs, however, have proven difficult to identify experimentally. The most promising approach is to study their exotic nature via the wave-vector and intensity dependence of their dynamical response in neutron scattering. A major search has centered on iridate materials which are proposed to realize the celebrated Kitaev model on a honeycomb lattice - a prototypical topological QSL system in two dimensions (2D). The difficulties of iridium for neutron measurements have, however, impeded progress significantly. Here we provide experimental evidence that a material based on ruthenium, α-RuCl$_3$ realizes the same Kitaev physics but is highly amenable to neutron investigation. Our measurements confirm the requisite strong spin-orbit coupling, and a low temperature magnetic order that matches the predicted phase proximate to the QSL. We also show that stacking faults, inherent to the highly 2D nature of the material, readily explain some puzzling results to date. Measurements of the dynamical response functions, especially at energies and temperatures above that where interlayer effects are manifest, are naturally accounted for in terms of deconfinement physics expected for QSLs. Via a comparison to the recently calculated dynamics from gauge flux excitations and Majorana fermions of the pure Kitaev model we propose α-RuCl$_3$ as the prime candidate for experimental realization of fractionalized Kitaev physics.

cond-mat.mtrl-sci↗