arXiv subjects

A. M. Hallas

Publications and source records attributed to A. M. Hallas.

At least 19 recordsLinked to original sources

arXiv2021

Neutron scattering study of the kagome metal Sc3Mn3Al7Si5

Sc3Mn3Al7Si5 is a rare example of a correlated metal in which the Mn moments form a kagome lattice. The absence of magnetic ordering to the lowest temperatures suggests that geometrical frustration of magnetic interactions may lead to strong magnetic fluctuations. We have performed inelastic neutron scattering measurements on Sc3Mn3Al7Si5, finding that phonon scattering dominates for energies from ~20 - 50 meV. These results are in good agreement with ab initio calculations of the phonon dispersions and densities of states, and as well reproduce the measured specific heat. A weak magnetic signal was detected at energies less than ~10 meV, present only at the lowest temperatures. The magnetic signal is broad and quasielastic, as expected for metallic paramagnets.

cond-mat.str-el↗

arXiv2021

Absence of moment fragmentation in the mixed $B$-site pyrochlore Nd$_{2}$GaSbO$_{7}$

Nd-based pyrochlore oxides of the form Nd$_{2}B_{2}$O$_{7}$ have garnered a significant amount of interest owing to the moment fragmentation physics observed in Nd$_{2}$Zr$_{2}$O$_{7}$ and speculated in Nd$_{2}$Hf$_{2}$O$_{7}$. Notably this phenomenon is not ubiquitous in this family, as it is absent in Nd$_{2}$Sn$_{2}$O$_{7}$, which features a smaller ionic radius on the $B$-site. Here, we explore the necessary conditions for moment fragmentation in the Nd pyrochlore family through a detailed study of the mixed $B$-site pyrochlore Nd$_{2}$GaSbO$_{7}$. The $B$-site of this system is characterized by significant disorder and an extremely small average ionic radius. Similarly to Nd$_{2}$Sn$_{2}$O$_{7}$, we find no evidence for moment fragmentation through our bulk characterization and neutron scattering experiments, indicating that chemical pressure (and not necessarily the $B$-site disorder) plays a key role in the presence or absence of this phenomenon in this material family. Surprisingly, the presence of significant $B$-site disorder in Nd$_{2}$GaSbO$_{7}$ does not generate a spin glass ground state and instead the same all-in-all-out magnetic order identified in other Nd pyrochlores is found here.

cond-mat.str-el↗

arXiv2020

Intertwined Magnetic Dipolar and Electric Quadrupolar Correlations in the Pyrochlore Tb$_2$Ge$_2$O$_7$

We present a comprehensive experimental and theoretical study of the pyrochlore Tb$_2$Ge$_2$O$_7$, an exemplary realization of a material whose properties are dominated by competition between magnetic dipolar and electric quadrupolar correlations. The dipolar and quadrupolar correlations evolve over three distinct regimes that we characterize via heat capacity, elastic and inelastic neutron scattering. In the first regime, above $T^*=1.1$ K, significant quadrupolar correlations lead to an intense inelastic mode that cannot be accounted for within a scenario with solely magnetic dipole-dipole correlations. The onset of extended dipole correlations occurs in the intermediate regime, between $T^*=1.1$ K and $T_c = 0.25$ K, with the formation of a collective paramagnetic state characterized by extended ferromagnetic canted spin ice domains. Here, long-range order is impeded not only by the usual frustration operating in classical spin ice systems, but also by a competition between dipolar and quadrupolar correlations. Finally, in the lowest temperature regime, below $T_c=0.25$ K, there is an abrupt and significant increase in the dipole ordered moment. The majority of the ordered moment remains tied up in the ferromagnetic spin ice-like state, but an additional $\mathbf{k}=(0,0,1)$ antiferromagnetic order parameter also develops. Simultaneously, the spectral weight of the inelastic mode, which is a proxy for the quadrupolar correlations, is observed to drop, indicating that dipole order ultimately wins out. Tb$_2$Ge$_2$O$_7$ is therefore a remarkable platform to study intertwined dipolar and quadrupolar correlations in a magnetically frustrated system and provides important insights into the physics of the whole family of terbium pyrochlores.

cond-mat.str-el↗

arXiv2020

Quantum critical point in the itinerant ferromagnet Ni$_{1-x}$Rh$_x$

We report a chemical substitution-induced ferromagnetic quantum critical point in polycrystalline Ni$_{1-x}$Rh$_x$ alloys. Through magnetization and muon spin relaxation measurements, we show that the ferromagnetic ordering temperature is suppressed continuously to zero at $x_{crit} = 0.375$ while the magnetic volume fraction remains 100% up to $x_{crit}$, pointing to a second order transition. Non-Fermi liquid behavior is observed close to $x_{crit}$, where the electronic specific heat $C_{el}/T$ diverges logarithmically, while immediately above $x_{crit}$ the volume thermal expansion coefficient $α_{V}/T$ and the Grüneisen ratio $Γ= α_{V}/C_{el}$ both diverge logarithmically in the low temperature limit, further indication of a ferromagnetic quantum critical point in Ni$_{1-x}$Rh$_x$.

cond-mat.str-el↗

arXiv2019

Complex Transport and Magnetism in Inhomogeneous Mixed Valence Ce$_3$Ir$_4$Ge$_{13}$

We report the discovery of Ce$_3$Ir$_4$Ge$_{13}$, a new Remeika phase compound with a complex array of structural, electronic, and magnetic properties. Our single crystal x-ray diffraction measurements show that Ce$_3$Ir$_4$Ge$_{13}$ forms in the tetragonally distorted $I4_1/amd$ space group. The electrical resistivity is almost temperature independent over three decades in temperature, from 0.4 K to 400 K, while the Hall coefficient measurements are consistent with a low-carrier semimetal. Magnetic susceptibility measurements reveal an effective moment of $μ^{\text{exp}}_{\text{eff}} = 1.87 μ_B$/Ce, suggesting that this material has a mixture of magnetic Ce$^{3+}$ and non-magnetic Ce$^{4+}$. Upon cooling, Ce$_3$Ir$_4$Ge$_{13}$ first enters a short range magnetically ordered state below $T_{\text{SRO}}=10$ K, marked by a deviation from Curie-Weiss behavior in susceptibility and a broad field-independent heat capacity anomaly. At lower temperatures, we observe a second, sharper peak in the heat capacity at $T^* = 1.7$ K, concurrent with a splitting of the field-cooled and zero-field-cooled susceptibilities. A small resistivity drop at $T^*$ suggests a loss of spin disorder scattering consistent with a magnetic ordering or spin freezing transition. Ce$_3$Ir$_4$Ge$_{13}$ is therefore a rare example of an inhomogeneous mixed valence compound with a complex array of thermodynamic and transport properties.

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arXiv2019

Probing the quantum phase transition in Mott insulator BaCoS_2 tuned by pressure and Ni-substitution

We present a muon spin relaxation study of the Mott transition in BaCoS_2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni-substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS_2 under pressure displays minimal change in the ordered magnetic moment S_ord until it collapses abruptly upon entering the antiferromagnetic metallic state at p_cr ~ 1.3 GPa. In contrast, S_ord in the Ni-doped system Ba(Co_{1-x}Ni_{x})S_{2} steadily decreases with increasing x until the antiferromagnetic metallic region is reached at x_cr ~ 0.22. In both cases, significant phase separation between magnetic and nonmagnetic regions develops when approaching p_cr or x_cr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RENiO_3 and V_2O_3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.

cond-mat.str-el↗

arXiv2018

Low frequency spin dynamics in XY quantum spin ice Yb$_2$Pt$_2$O$_7$

The XY pyrochlore Yb$_2$Ti$_2$O$_7$, with pseudo spin 1/2 at the Yb$^{3+}$ site, has been celebrated as potential host for the quantum spin ice state. The substitution of non-magnetic Ti with Pt gives Yb$_2$Pt$_2$O$_7$, a system with remarkably similar magnetic properties. The large nuclear gyromagnetic ratio ($γ_{n}/2 π= 9.15$~MHz/T) of $^{195}$Pt makes Yb$_2$Pt$_2$O$_7$ an ideal material for NMR investigation of its unconventional magnetic properties. Based on the $^{195}$Pt nuclear spin-lattice relaxation rate $1/T_1$ and the magnetic specific heat $C_{p}$ measured in a broad range of magnetic field $B_{ext}$, we demonstrate that the field-induced magnon gap linearly decreases with $B_{ext}$ but additional low energy mode of spin excitations emerge below $\sim 0.5$~T.

cond-mat.str-el↗

arXiv2018

Low-carrier density and fragile magnetism in a Kondo lattice system

Kondo-based semimetals and semiconductors are of extensive current interest as a viable platform for strongly correlated states. It is thus important to understand the routes towards such dilute-carrier correlated states. One established pathway is through Kondo effect in metallic non-magnetic analogues. Here we advance a new mechanism, through which Kondo-based semimetals develop out of conduction electrons with a low carrier-density in the presence of an even number of rare-earth sites. We demonstrate this effect by studying the Kondo material Yb3Ir4Ge13 along with its closed-f-shell counterpart, Lu3Ir4Ge13. Through magnetotransport, optical conductivity and thermodynamic measurements, we establish that the correlated semimetallic state of Yb3Ir4Ge13 below its Kondo temperature originates from the Kondo effect of a low carrier conduction-electron background. In addition, it displays fragile magnetism at very low temperatures, which, in turn, can be tuned to a non Fermi liquid regime through Lu-for-Yb substitution. These findings are connected with recent theoretical studies in simplified models. Our results open an entirely new venue to explore the strong correlation physics in a semimetallic environment.

cond-mat.str-el↗

arXiv2018

$μ$SR study of spin freezing and persistent spin dynamics in NaCaNi$_2$F$_7$

A new pyrochlore compound, NaCaNi$_2$F$_7$, was recently synthesized and has a single magnetic site with spin-1 Ni$^{2+}$. We present zero field (ZF) and longitudinal field (LF) muon spin rotation ($μ$SR) measurements on this pyrochlore. Density functional theory (DFT) calculations show that the most likely muon site is located between two fluorine ions, but off-centre. A characteristic F-$μ$-F muon spin polarization function is observed at high temperatures where Ni spin fluctuations are sufficiently rapid. The Ni$^{2+}$ spins undergo spin freezing into a disordered ground state below 4~K, with a characteristic internal field strength of 140~G. Persistent Ni spin dynamics are present to our lowest temperatures (75~mK), a feature characteristic of many geometrically frustrated magnetic systems.

cond-mat.str-el↗

arXiv2018

Dipolar-Octupolar Ising Antiferromagnetism in Sm$_2$Ti$_2$O$_7$: A Moment Fragmentation Candidate

Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm$_2$Ti$_2$O$_7$. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm$_2$Ti$_2$O$_7$. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm$^{3+}$ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm$_2$Ti$_2$O$_7$ orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) $μ_B$ below $T_N=0.35$ K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm$^{3+}$ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.

cond-mat.str-el↗

arXiv2018

Magneto-elastic induced vibronic bound state in the spin ice pyrochlore Ho$_2$Ti$_2$O$_7$

The single ion physics of Ho$_2$Ti$_2$O$_7$ is well-understood to produce strong Ising anisotropy, which is an essential ingredient to its low-temperature spin ice state. We present inelastic neutron scattering measurements on Ho$_2$Ti$_2$O$_7$ that reveal a clear inconsistency with its established single ion Hamiltonian. Specifically, we show that a crystal field doublet near 60~meV is split by approximately 3~meV. Furthermore, this crystal field splitting is not isolated to Ho$_2$Ti$_2$O$_7$ but can also be found in its chemical pressure analogs, Ho$_2$Ge$_2$O$_7$ and Ho$_2$Sn$_2$O$_7$. We demonstrate that the origin of this effect is a vibronic bound state, resulting from the entanglement of a phonon and crystal field excitation. We derive the microscopic Hamiltonian that describes the magneto-elastic coupling and provides a quantitative description of the inelastic neutron spectra.

cond-mat.str-el↗

arXiv2018

Anomalous metamagnetism in the low carrier density Kondo lattice YbRh3Si7

We report complex metamagnetic transitions in single crystals of the new low carrier Kondo antiferromagnet YbRh3Si7. Electrical transport, magnetization, and specific heat measurements reveal antiferromagnetic order at T_N = 7.5 K. Neutron diffraction measurements show that the magnetic ground state of YbRh3Si7 is a collinear antiferromagnet where the moments are aligned in the ab plane. With such an ordered state, no metamagnetic transitions are expected when a magnetic field is applied along the c axis. It is therefore surprising that high field magnetization, torque, and resistivity measurements with H||c reveal two metamagnetic transitions at mu_0H_1 = 6.7 T and mu_0H_2 = 21 T. When the field is tilted away from the c axis, towards the ab plane, both metamagnetic transitions are shifted to higher fields. The first metamagnetic transition leads to an abrupt increase in the electrical resistivity, while the second transition is accompanied by a dramatic reduction in the electrical resistivity. Thus, the magnetic and electronic degrees of freedom in YbRh3Si7 are strongly coupled. We discuss the origin of the anomalous metamagnetism and conclude that it is related to competition between crystal electric field anisotropy and anisotropic exchange interactions.

cond-mat.str-el↗

arXiv2017

Phase Competition in the Palmer-Chalker XY Pyrochlore Er$_2$Pt$_2$O$_7$

We report neutron scattering measurements on Er$_2$Pt$_2$O$_7$, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er$_2$Pt$_2$O$_7$ is the first XY pyrochlore to order into the $k=0$, $Γ_7$ magnetic structure (the Palmer-Chalker state), at $T_N = 0.38$ K. This contrasts with its sister XY pyrochlore antiferromagnets Er$_2$Ti$_2$O$_7$ and Er$_2$Ge$_2$O$_7$, both of which order into $Γ_5$ magnetic structures at much higher temperatures, $T_N=1.2$ K and $1.4$ K, respectively. In this temperature range, the magnetic heat capacity of Er$_2$Pt$_2$O$_7$ contains a broad anomaly centered at $T^*=1.5$ K. Our inelastic neutron scattering measurements reveal that this broad heat capacity anomaly sets the temperature scale for strong short-range spin fluctuations. Below $T_N = 0.38$ K, Er$_2$Pt$_2$O$_7$ displays a gapped spin wave spectrum with an intense, flat band of excitations at lower energy and a weak, diffusive band of excitations at higher energy. The flat band is well-described by classical spin wave calculations, but these calculations also predict sharp dispersive branches at higher energy, a striking discrepancy with the experimental data. This, in concert with the strong suppression of $T_N$, is attributable to enhanced quantum fluctuations due to phase competition between the $Γ_7$ and $Γ_5$ states that border each other within a classically predicted phase diagram.

cond-mat.str-el↗

arXiv2017

Effect of Chemical Pressure on the Crystal Electric Field States of Erbium Pyrochlore Magnets

We have carried out a systematic study of the crystal electric field excitations in the family of cubic pyrochlores Er$_2B_2$O$_7$, with $B=$~Ti, Ge, Pt, and Sn, using neutron spectroscopy. All members of this family are magnetic insulators based on 4$f^{11}$ Er$^{3+}$ and non-magnetic $B^{4+}$. At sufficiently low temperatures, long-range antiferromagnetic order is observed in each of these Er$_2B_2$O$_7$ pyrochlores. Our inelastic neutron scattering measurements probe the transitions from the ground state doublet to excited crystal electric field states belonging to the $J=15/2$ Hund's rules manifold. This allows us to quantitatively determine the energy eigenvalues and eigenfunctions of these $(2J+1)=16$ states across the Er$_2B_2$O$_7$ series. The different ionic sizes associated with different non-magnetic $B^{4+}$ cations correspond to positive or negative chemical pressure, depending on the relative contraction or expansion of the crystal lattice, which gives rise to slightly different local environments at the Er$^{3+}$ site. Our results show that the $g$-tensor components, which characterize the anisotropy of the ground state doublet eigenfunctions, are XY-like for all four members of the Er$_2B_2$O$_7$ series. However, the XY anisotropy is much stronger for Er$_2$Pt$_2$O$_7$ and Er$_2$Sn$_2$O$_7$ ($\frac{g_{\perp}}{g_z} > 25$), than for Er$_2$Ge$_2$O$_7$ and Er$_2$Ti$_2$O$_7$ ($\frac{g_{\perp}}{g_z} < 4$). The variation in the nature of the XY-anisotropy in these systems correlates strongly with their ground states, as Er$_2$Ge$_2$O$_7$ and Er$_2$Ti$_2$O$_7$ order into $Γ_5$ magnetic structures, while Er$_2$Pt$_2$O$_7$ and Er$_2$Sn$_2$O$_7$ order in the $Γ_7$ Palmer-Chalker structure.

cond-mat.str-el↗

arXiv2017

$μ$SR study of noncentrosymmetric superconductor PbTaSe$_2$

We present muon spin rotation ($μ$SR) measurements on the noncentrosymmetric superconductor PbTaSe$_2$. From measurements in an applied transverse field between $H_{c1}$ and $H_{c2}$, we extract the superfluid density as a function of temperature in the vortex state. This data can be fit with a fully gapped two-band model, consistent with previous evidence from ARPES, thermal conductivity, and resistivity. Furthermore, zero field measurements show no evidence for a time reversal symmetry breaking field greater than 0.05~G in the superconducting state. This makes exotic fully gapped spin-triplet states unlikely, and hence we contend that PbTaSe$_2$ is characterized by conventional BCS s-wave superconductivity in multiple bands.

cond-mat.supr-con↗

arXiv2017

Experimental evidence for field induced emergent clock anisotropies in the XY pyrochlore Er$_2$Ti$_2$O$_7$

The XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$ exhibits a rare case of $Z_6$ discrete symmetry breaking in its $ψ_2$ magnetic ground state. Despite being well-studied theoretically, systems with high discrete symmetry breakings are uncommon in nature and, thus, Er$_2$Ti$_2$O$_7$ provides an experimental playground for the study of broken $Z_n$ symmetry, for $n>2$. A recent theoretical work examined the effect of a magnetic field on a pyrochlore lattice with broken $Z_6$ symmetry and applied it to Er$_2$Ti$_2$O$_7$. This study predicted multiple domain transitions depending on the crystallographic orientation of the magnetic field, inducing rich and controllable magnetothermodynamic behavior. In this work, we present neutron scattering measurements on Er$_2$Ti$_2$O$_7$ with a magnetic field applied along the [001] and [111] directions, and provide the first experimental observation of these exotic domain transitions. In a [001] field, we observe a $ψ_2$ to $ψ_3$ transition at a critical field of 0.18$\pm$0.05T. We are thus able to extend the concept of the spin-flop transition, which has long been observed in Ising systems, to higher discrete $Z_n$ symmetries. In a [111] field, we observe a series of domain-based phase transitions for fields of 0.15$\pm$0.03T and 0.40$\pm$0.03T. We show that these field-induced transitions are consistent with the emergence of two-fold, three-fold and possibly six-fold Zeeman terms. Considering all the possible $ψ_2$ and $ψ_3$ domains, these Zeeman terms can be mapped onto an analog clock - exemplifying a literal clock anisotropy. Lastly, our quantitative analysis of the [001] domain transition in Er$_2$Ti$_2$O$_7$ is consistent with order-by-disorder as the dominant ground state selection mechanism.

cond-mat.str-el↗

arXiv2016

Magnetic dilution and domain selection in the XY pyrochlore antiferromagnet Er$_{2}$Ti$_2$O$_7$

Below $T_N = 1.1$K, the XY pyrochlore Er$_2$Ti$_2$O$_7$ orders into a $k=0$ non-collinear, antiferromagnetic structure referred to as the $ψ_2$ state. The magnetic order in Er$_2$Ti$_2$O$_7$ is known to obey conventional three dimensional (3D) percolation in the presence of magnetic dilution, and in that sense is robust to disorder. Recently, however, two theoretical studies have predicted that the $ψ_2$ structure should be unstable to the formation of a related $ψ_3$ magnetic structure in the presence of magnetic vacancies. To investigate these theories, we have carried out systematic elastic and inelastic neutron scattering studies of three single crystals of Er$_{2-x}$Y$_x$Ti$_2$O$_7$ with $x=0$ (pure), 0.2 (10$\%$-Y) and 0.4 (20$\%$-Y), where magnetic Er$^{3+}$ is substituted by non-magnetic Y$^{3+}$. We find that the $ψ_2$ ground state of pure Er$_2$Ti$_2$O$_7$ is significantly affected by magnetic dilution. The characteristic domain selection associated with the $ψ_2$ state, and the corresponding energy gap separating $ψ_2$ from $ψ_3$, vanish for Y$^{3+}$ substitutions between 10$\%$-Y and 20$\%$-Y, far removed from the 3D percolation threshold of $\sim$60$\%$-Y. The resulting ground state for Er$_2$Ti$_2$O$_7$ with magnetic dilutions from 20$\%$-Y up to the percolation threshold is naturally interpreted as a frozen mosaic of $ψ_2$ and $ψ_3$ domains.

cond-mat.str-el↗

arXiv2016

Relief of Frustration in the Heisenberg Pyrochlore Antiferromagnet Gd$_2$Pt$_2$O$_7$

The gadolinium pyrochlores, Gd$_2B_2$O$_7$, are amongst the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd$_2$Pt$_2$O$_7$, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd$_2$Pt$_2$O$_7$ undergoes an antiferromagnetic ordering transition at $T_N = 1.6$ K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a non-divergent relaxation rate in $μ$SR. The form of the heat capacity below $T_N$ suggests that the ground state is an anisotropic collinear antiferromagnet with an excitation spectrum that is gapped by 0.245(1) meV. The ordering temperature in Gd$_2$Pt$_2$O$_7$, $T_N = 1.6$ K, is a substantial 160% increase from other gadolinium pyrochlores, which have been found to order at 1 K or lower. We attribute this enhancement in $T_N$ to the $B$-site cation, platinum, which, despite being non-magnetic, has a filled $5d$ $t_{2g}$ orbital and an empty $5d$ $e_g$ orbital that can facilitate superexchange. Thus, the magnetic frustration in Gd$_2$Pt$_2$O$_7$ is partially "relieved", thereby promoting magnetic order.

cond-mat.str-el↗