arXiv subjects

A. P. Mackenzie

Publications and source records attributed to A. P. Mackenzie.

At least 37 records · Page 2Linked to original sources

arXiv2020

Electronically driven spin-reorientation transition of the correlated polar metal Ca$_3$Ru$_2$O$_7$

Polar distortions in solids give rise to the well-known functionality of switchable macroscopic polarisation in ferroelectrics and, when combined with strong spin-orbit coupling, can mediate giant spin splittings of electronic states. While typically found in insulators, ferroelectric-like distortions can remain robust against increasing itineracy, giving rise to so-called "polar metals". Here, we investigate the temperature-dependent electronic structure of Ca$_3$Ru$_2$O$_7$, a correlated oxide metal in which octahedral tilts and rotations combine to mediate pronounced polar distortions. Our angle-resolved photoemission measurements reveal the destruction of a large hole-like Fermi surface upon cooling through a coupled structural and spin-reorientation transition at 48 K, accompanied by a sudden onset of quasiparticle coherence. We demonstrate how these result from band hybridisation mediated by a hidden Rashba-type spin-orbit coupling. This is enabled by the bulk structural distortions and unlocked when the spin reorients perpendicular to the local symmetry-breaking potential at the Ru sites. We argue that the electronic energy gain associated with the band hybridisation is actually the key driver for the phase transition, reflecting a delicate interplay between spin-orbit coupling and strong electronic correlations, and revealing a new route to control magnetic ordering in solids.

cond-mat.str-el↗

arXiv2020

Controlled introduction of defects to delafossite metals by electron irradiation

The delafossite metals PdCoO$_{2}$, PtCoO$_{2}$ and PdCrO$_{2}$ are among the highest conductivity materials known, with low temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure, or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross-section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately $0.001\%$. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths, and highlights how unusual these delafossite metals are in comparison with the vast majority of other multi-component oxides and alloys. We discuss the implications of our findings for future materials research.

cond-mat.mtrl-sci↗

arXiv2019

The Fermi surface of PtCoO2 from quantum oscillations and electronic structure calculations

The delafossite series of layered oxides include some of the highest conductivity metals ever discovered. Of these, PtCoO2, with a room temperature resistivity of 1.8 microOhmcm for in-plane transport, is the most conducting of all. The high conduction takes place in triangular lattice Pt layers, separated by layers of Co-O octahedra, and the electronic structure is determined by the interplay of the two types of layer. We present a detailed study of quantum oscillations in PtCoO2, at temperatures down to 35 mK and magnetic fields up to 30 T. As for PdCoO2 and PdRhO2, the Fermi surface consists of a single cylinder with mainly Pt character, and an effective mass close to the free electron value. Due to Fermi-surface warping, two close-lying high frequencies are observed. Additionally, a pronounced difference frequency appears. By analysing the detailed angular dependence of the quantum-oscillation frequencies, we establish the warping parameters of the Fermi surface. We compare these results to the predictions of first-principles electronic structure calculations including spin-orbit coupling on Pt and Co and on-site correlation U on Co, and hence demonstrate that electronic correlations in the Co-O layers play an important role in determining characteristic features of the electronic structure of PtCoO2.

cond-mat.str-el↗

arXiv2019

Metamagnetic texture in a polar antiferromagnet

The notion of a simple ordered state implies homogeneity. If the order is established by a broken symmetry, elementary Landau theory of phase transitions shows that only one symmetry mode describes this state. Precisely at points of phase coexistence domain states formed of large regions of different phases can be stabilized by long range interactions. In uniaxial antiferromagnets the so-called metamagnetism is an example of such a behavior, when an antiferromagnetic and field-induced spin-polarized paramagnetic/ferromagnetic state co-exist at a jump-like transition in the magnetic phase diagram. Here, combining experiment with theoretical analysis, we show that a different type of mixed state between antiferromagnetism and ferromagnetism can be created in certain acentric materials. In the small-angle neutron scattering experiments we observe a field-driven spin-state in the layered antiferromagnet Ca3Ru2O7, which is modulated on a scale between 8 and 20 nm and has both antiferromagnetic and ferromagnetic parts. We call this state a metamagnetic texture and explain its appearance by the chiral twisting effects of the asymmetric Dzyaloshinskii-Moriya (DM) exchange. The observation can be understood as an extraordinary coexistence, in one thermodynamic state, of spin orders belonging to different symmetries. Experimentally, the complex nature of this metamagnetic state is demonstrated by measurements of anomalies in electronic transport which reflect the spin-polarization in the metamagnetic texture, determination of the magnetic orbital moments, which supports the existence of strong spin-orbit effects, a pre-requisite for the mechanism of twisted magnetic states in this material.

cond-mat.str-el↗

arXiv2019

Normal state $^{17}$O NMR studies of Sr$_{2}$RuO$_{4}$ under uniaxial stress

The effects of uniaxial compressive stress on the normal state $^{17}$O nuclear magnetic resonance properties of the unconventional superconductor Sr$_{2}$RuO$_{4}$ are reported. The paramagnetic shifts of both planar and apical oxygen sites show pronounced anomalies near the nominal $\mathbf{a}$-axis strain $\varepsilon_{aa}\equiv\varepsilon_v$, that maximizes the superconducting transition temperature, $T_{c}$. The spin susceptibility weakly increases on lowering the temperature below $T$$\simeq$10 K, consistent with an enhanced density of states associated with passing the Fermi energy through a van Hove singularity. Although such a Lifshitz transition occurs in the $γ$ band, formed by the Ru $d_{xy}$ states hybridized with in-plane O $p_π$ orbitals, the large Hund's coupling renormalizes the uniform spin susceptibilty, which, in turn, affects the hyperfine fields of all nuclei. We estimate this \textquotedblleft Stoner\textquotedblright\ renormalization, $S,$ by combining the data with first-principles calculations and conclude that this is an important part of the strain effect, with implications for superconductivity.

cond-mat.str-el↗

arXiv2019

High-resolution photoemission on Sr2RuO4 reveals correlation-enhanced effective spin-orbit coupling and dominantly local self-energies

We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The self-energies for the $β$ and $γ$ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects, but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies, and provides strong evidence for an electronic origin of the observed non-linear frequency dependence of the self-energies, leading to `kinks' in the quasiparticle dispersion of Sr2RuO4.

cond-mat.str-el↗

arXiv2019

Perfect separation of intraband and interband excitations in PdCoO$_2$

The temperature dependence of the optical properties of the delafossite PdCoO$_2$ has been measured in the a-b planes over a wide frequency range. The optical conductivity due to the free-carrier (intraband) response falls well below the interband transitions, allowing the plasma frequency to be determined from the $f$-sum rule. Drude-Lorentz fits to the complex optical conductivity yield estimates for the free-carrier plasma frequency and scattering rate. The in-plane plasma frequency has also been calculated using density functional theory. The experimentally-determined and calculated values for the plasma frequencies are all in good agreement; however, at low temperature the optically-determined scattering rate is much larger than the estimate for the transport scattering rate, indicating a strong frequency-dependent renormalization of the optical scattering rate. In addition to the expected in-plane infrared-active modes, two very strong features are observed that are attributed to the coupling of the in-plane carriers to the out-of-plane longitudinal optic modes.

cond-mat.mtrl-sci↗

arXiv2019

Pronounced drop of $^{17}$O NMR Knight shift in superconducting state of Sr$_2$RuO$_4$

The superconducting state in the quasi-two-dimensional and strongly correlated Sr$_2$RuO$_4$ is uniquely held up as a solid state analog to superfluid $^3$He-$A$, with an odd-parity order parameter that also breaks time reversal symmetry, and for which the vector order parameter has the same direction in spin space for all electron momenta. The recent discovery that uniaxial pressure causes a steep rise and maximum in transition temperature ($T_c$) in strained samples motivated the study of $^{17}$O nuclear magnetic resonance (NMR) that we describe in this article. A reduction of Knight shifts $K$ was observed for all strain values and temperatures $T<T_c$, consistent with a drop in spin polarization in the superconducting state. In unstrained samples, our results are in contradiction with a body of previous NMR work, and with the most prominent previous proposals for the order parameter of Sr$_2$RuO$_4$. Possible alternative scenarios are discussed.

cond-mat.supr-con↗

arXiv2019

Direct Observation of a Uniaxial Stress-driven Lifshitz Transition in Sr$_{2}$RuO$_{4}$

Pressure represents a clean tuning parameter for traversing the complex phase diagrams of interacting electron systems and as such has proved of key importance in the study of quantum materials. Application of controlled uniaxial pressure has recently been shown to more than double the transition temperature of the unconventional superconductor Sr$_{2}$RuO$_{4}$ for example, leading to a pronounced peak in $T_\mathrm{c}$ vs. strain whose origin is still under active debate. Here, we develop a simple and compact method to apply large uniaxial pressures passively in restricted sample environments, and utilize this to study the evolution of the electronic structure of Sr$_{2}$RuO$_{4}$ using angle-resolved photoemission. We directly visualize how uniaxial stress drives a Lifshitz transition of the $γ$-band Fermi surface, pointing to the key role of strain-tuning its associated van Hove singularity to the Fermi level in mediating the peak in $T_\mathrm{c}$. Our measurements provide stringent constraints for theoretical models of the strain-tuned electronic structure evolution of Sr$_{2}$RuO$_{4}$. More generally, our novel experimental approach opens the door to future studies of strain-tuned phase transitions not only using photoemission, but also other experimental techniques where large pressure cells or piezoelectric-based devices may be difficult to implement.

cond-mat.str-el↗

arXiv2018

Improved Single-Crystal Growth of Sr$_2$RuO$_4$

High-quality single crystals are essentially needed for the investigation of the novel bulk properties of unconventional superconductors. The availability of such crystals grown by the floating-zone method has helped to unveil the unconventional superconductivity of the layered perovskite Sr$_2$RuO$_4$, which is considered as a strong candidate of a topological spin-triplet superconductor. Yet, recent progress of investigations urges further efforts to obtain ultimately high-quality crystalline samples. In this paper, we focus on the method of preparation of feed rods for the floating-zone melting and report on the improvements of the crystal growth. We present details of the improved methods used to obtain crystals with superconducting transition temperatures $T_\mathrm{c}$ that are consistently as high as 1.4 K, as well as the properties of these crystals.

cond-mat.supr-con↗

arXiv2018

Magnetic-field Induced Interconversion of Cooper Pairs and Density Wave States within Cuprate Composite Order

Recent studies establish that the cuprate pseudogap phase is susceptible at low temperatures to forming not only a $d$-symmetry superconducting (SC) state, but also a $d$-symmetry form factor (dFF) density wave (DW) state. The concurrent emergence of such distinct and unusual states from the pseudogap motivates theories that they are "intertwined" i.e derived from a quantum composite of dissimilar broken-symmetry orders. Some composite order theories predict that the balance between the different components can be altered, for example at superconducting vortex cores. Here, we introduce sublattice phase-resolved electronic structure imaging as a function of magnetic field and find robust dFF DW states induced at each vortex. They are predominantly unidirectional and co-oriented (nematic), exhibiting strong spatial-phase coherence. At each vortex we also detect the field-induced conversion of the SC to DW components and demonstrate that this occurs at precisely the eight momentum-space locations predicted in many composite order theories. These data provided direct microscopic evidence for the existence of composite order in the cuprates, and new indications of how the DW state becomes long-range ordered in high magnetic fields.

cond-mat.supr-con↗

arXiv2017

Itinerant ferromagnetism of the Pd-terminated polar surface of PdCoO$_2$

We study the electronic structure of the Pd-terminated surface of the non-magnetic delafossite oxide metal PdCoO$_2$. Combining angle-resolved photoemission spectroscopy and density-functional theory, we show how an electronic reconstruction driven by surface polarity mediates a Stoner-like magnetic instability towards itinerant surface ferromagnetism. Our results reveal how this leads to a rich multi-band surface electronic structure, and provide spectroscopic evidence for an intriguing sample-dependent coupling of the surface electrons to a bosonic mode which we attribute to electron-magnon interactions. Moreover, we find similar surface state dispersions in PdCrO$_2$, suggesting surface ferromagnetism persists in this sister compound despite its bulk antiferromagnetic order.

cond-mat.str-el↗

arXiv2017

Cascade of magnetic field induced Lifshitz transitions in the ferromagnetic Kondo lattice material YbNi4P2

A ferromagnetic quantum critical point is thought not to exist in two and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally but common probes such as ARPES and quantum oscillation measurements are lacking. Here, we studied the magnetic field dependence of transport and thermodynamic properties of YbNi4P2. The Kondo effect is continuously suppressed and additionally we identify nine Lifshitz transitions between 0.4 and 18 T. We analyze the transport coefficients in detail and identify the type of Lifshitz transitions as neck or void type to gain information on the Fermi surface of YbNi4P2. The large number of Lifshitz transitions observed within this small energy window is unprecedented and results from the particular flat renormalized band structure with strong 4f-electron character shaped by the Kondo lattice effect.

cond-mat.str-el↗

arXiv2017

Maximal Rashba-like spin splitting via kinetic energy-driven inversion symmetry breaking

Engineering and enhancing inversion symmetry breaking in solids is a major goal in condensed matter physics and materials science, as a route to advancing new physics and applications ranging from improved ferroelectrics for memory devices to materials hosting Majorana zero modes for quantum computing. Here, we uncover a new mechanism for realising a much larger energy scale of inversion symmetry breaking at surfaces and interfaces than is typically achieved. The key ingredient is a pronounced asymmetry of surface hopping energies, i.e. a kinetic energy-driven inversion symmetry breaking, whose energy scale is pinned at a significant fraction of the bandwidth. We show, from spin- and angle-resolved photoemission, how this enables surface states of 3d and 4d-based transition-metal oxides to surprisingly develop some of the largest Rashba-like spin splittings that are known. Our findings open new possibilities to produce spin textured states in oxides which exploit the full potential of the bare atomic spin-orbit coupling, raising exciting prospects for oxide spintronics. More generally, the core structural building blocks which enable this are common to numerous materials, providing the prospect of enhanced inversion symmetry breaking at judiciously-chosen surfaces of a plethora of compounds, and suggesting routes to interfacial control of inversion symmetry breaking in designer heterostructures.

cond-mat.str-el↗

arXiv2017

Single crystal growth, structure and electronic properties of metallic delafossite PdRhO_{2}

We report growth of single crystals of the non-magnetic metallic delafossite PdRhO_{2}, comparing the results from three different methods. Complete crystallographic data were obtained from single crystal X-ray diffraction, and electronic structure calculations made using the refined structural parameters. Focused-ion beam microstructuring was used to prepare a sample for measurements of the in- and out-of-plane electrical resistivity, and the large observed anisotropy is qualitatively consistent with the cylindrical Fermi surface predicted by the calculations.

cond-mat.mes-hall↗

arXiv2016

Emergent Weyl fermion excitations in TaP explored by 181-Ta quadrupole resonance

The $^{181}$Ta quadrupole resonance (NQR) technique has been utilized to investigate the microscopic magnetic properties of the Weyl semi-metal TaP. We found three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with $I$=7/2 nuclear spin. A quadrupole coupling constant, $ν_{\mathrm{Q}}$ =19.250 MHz, and an asymmetric parameter of the electric field gradient, $η$ = 0.423 were extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin lattice relaxation rate (1/$T_{\mathrm{1}}T$) has been measured for the $f_{\mathrm{2}}$-line ($\pm$5/2 $\leftrightarrow$ $\pm$3/2 transition). We found that there exists two regimes with quite different relaxation processes. Above $T\text{*}$ $\approx$ 30 K, a pronounced (1/$T_{\mathrm{1}}T$) $\propto$ $T^{2}$ behavior was found, which is attributed to the magnetic excitations at the Weyl nodes with temperature dependent orbital hyperfine coupling. Below $T\text{*}$, the relaxation is mainly governed by Korringa process with 1/$T_{\mathrm{1}}T$ = constant, accompanied by an additional $T^{-1/2}$ type dependence to fit our experimental data. We show that Ta-NQR is a novel probe for the bulk Weyl fermions and their excitations.

cond-mat.mtrl-sci↗

arXiv2016

Intermediate magnetisation state and competing orders in Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$

Among the frustrated magnetic materials, spin-ice stands out as a particularly interesting system. Residual entropy, freezing and glassiness, Kasteleyn transitions and fractionalisation of excitations in three dimensions all stem from a simple classical Hamiltonian. But is the usual spin-ice Hamiltonian a correct description of the experimental systems? Here we address this issue by measuring magnetic susceptibility in the two most studied spin-ice compounds, Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$, using a vector magnet. Using these results, and guided by a theoretical analysis of possible distortions to the pyrochlore lattice, we construct an effective Hamiltonian and explore it using Monte Carlo simulations. We show how this Hamiltonian reproduces the experimental results, including the formation of a phase of intermediate polarisation, and gives important information about the possible ground-state of real spin-ice systems. Our work suggests an unusual situation in which distortions might contribute to the preservation rather than relief of the effects of frustration.

cond-mat.stat-mech↗

arXiv2016

Strain Control of Fermiology and Many-Body Interactions in Two-Dimensional Ruthenates

Here we demonstrate how the Fermi surface topology and quantum many-body interactions can be manipulated via epitaxial strain in the spin-triplet superconductor Sr$_2$RuO$_4$ and its isoelectronic counterpart Ba$_2$RuO$_4$ using oxide molecular beam epitaxy (MBE), \emph{in situ} angle-resolved photoemission spectroscopy (ARPES), and transport measurements. Near the topological transition of the $γ$ Fermi surface sheet, we observe clear signatures of critical fluctuations, while the quasiparticle mass enhancement is found to increase rapidly and monotonically with increasing Ru-O bond distance. Our work demonstrates the possibilities for using epitaxial strain as a disorder-free means of manipulating emergent properties, many-body interactions, and potentially the superconductivity in correlated materials.

cond-mat.supr-con↗