Search

Funding: This work was supported by the Alexander von Humboldt Foundation. A.W.R. acknowledges support by EPSRC Grant No. EP/P024564/1. Cryogenic scanning tunneling microscopy and spectroscopy (STM/STS) performed in a high vector magnetic field provide unique possibilities for imaging surface magnetic structures and anisotropic superconductivity and exploring spin physics in quantum materials with atomic precision. Here, we describe the design, construction, and performance of a low-temperature, ultra-high-vacuum (UHV) spectroscopic-imaging STM equipped with a vector magnet capable of applying a field of up to 3 T in any direction with respect to the sample surface. The STM head is housed in a fully bakeable UHV compatible cryogenic insert and is operational over variable temperatures ranging from ∼300 down to 1.5 K. The insert can be easily upgraded using our home-designed 3He refrigerator. In addition to layered compounds, which can be cleaved at a temperature of either ∼300, ∼77, or ∼4.2 K to expose an atomically flat surface, thin films can also be studied by directly transferring using a UHV suitcase from our oxide thin-film laboratory. Samples can be treated further with a heater and a liquid helium/nitrogen cooling stage on a three-axis manipulator. The STM tips can be treated in vacuo by e-beam bombardment and ion sputtering. We demonstrate the successful operation of the STM with varying the magnetic field direction. Our facility provides a way to study materials in which magnetic anisotropy is a key factor in determining the electronic properties such as in topological semimetals and superconductors. Postprint

Funding: A. W. R. and A. S. G. were supported by the Engineering and Physical Sciences Research Council (grant numbers EP/P024564/1 and EP/T011130/1 respectively). This work has been supported in part by the Alexander von Humboldt Foundation. The band topology of magnetic semimetals is of interest both from the fundamental science point of view and with respect to potential spintronics and memory applications. Unfortunately, only a handful of suitable topological semimetals with magnetic order have been discovered so far. One such family that hosts these characteristics is the antiperovskites, A3BO, a family of 3D Dirac semimetals. The A=Eu2+ compounds magnetically order with multiple phases as a function of applied magnetic field. Here, by combining band structure calculations with neutron diffraction and magnetic measurements, we establish the antiperovskite Eu3PbO as a new topological magnetic semimetal. This topological material exhibits a multitude of different topological phases with ordered Eu moments which can be easily controlled by an external magnetic field. The topological phase diagram of Eu3PbO includes an antiferromagnetic Dirac phase, as well as ferro- and ferrimagnetic phases with both Weyl points and nodal lines. For each of these phases, we determine the bulk band dispersions, the surface states, and the topological invariants by means of ab initio and tight-binding calculations. Our discovery of these topological phases introduces Eu3PbO as a new platform to study and manipulate the interplay of band topology, magnetism, and transport. Publisher PDF

One of the main developments in unconventional superconductivity in the past two decades has been the discovery that most unconventional superconductors form phase diagrams that also contain other strongly correlated states. Many systems of interest are therefore close to more than one instability, and tuning between the resultant ordered phases is the subject of intense research1. In recent years, uniaxial pressure applied using piezoelectric-based devices has been shown to be a particularly versatile new method of tuning, leading to experiments that have advanced our understanding of the fascinating unconventional superconductor Sr$_2$RuO$_4$. Here we map out its phase diagram using high-precision measurements of the elastocaloric effect in what we believe to be the first such study including both the normal and the superconducting states. We observe a strong entropy quench on entering the superconducting state, in excellent agreement with a model calculation for pairing at the Van Hove point, and obtain a quantitative estimate of the entropy change associated with entry to a magnetic state that is observed in proximity to the superconductivity. The phase diagram is intriguing both for its similarity to those seen in other families of unconventional superconductors and for extra features unique, so far, to Sr$_2$RuO$_4$., Comment: 34 pages, 23 figures, published version, manuscript and supplementary information in one document

One of the main developments in unconventional superconductivity in the past two decades has been the discovery that most unconventional superconductors form phase diagrams that also contain other strongly correlated states. Many systems of interest are therefore close to more than one instability, and tuning between the resultant ordered phases is the subject of intense research

In strongly correlated electron materials, the electronic, spin, and charge degrees of freedom are closely intertwined. This often leads to the stabilization of emergent orders that are highly sensitive to external physical stimuli promising opportunities for technological applications. In perovskite ruthenates, this sensitivity manifests in dramatic changes of the physical properties with subtle structural details of the RuO

C.M.Y., S.N.P., A.W.R., and P.W. acknowledge support from EPSRC through EP/S005005/1, and C.To. and A.W.R. through EP/P024564/1. C.M.Y. acknowledges additional support from a Shanghai talent program and funding through the Shanghai Pujiang Program (20PJ1408200). C.H. acknowledges support from the Austrian Science Fund (FWF), project no. P 32144-N36, and the VSC4 of the Vienna University of Technology. A key property of many quantum materials is that their ground state depends sensitively on small changes of an external tuning parameter, e.g., doping, magnetic field, or pressure, creating opportunities for potential technological applications. Here, we explore tuning of the ground state of the nonsuperconducting parent compound, Fe1+xTe, of the iron chalcogenides by uniaxial strain. Iron telluride exhibits a peculiar (π, 0) antiferromagnetic order unlike the (π, π) order observed in the Fe-pnictide superconductors. The (π, 0) order is accompanied by a significant monoclinic distortion. We explore tuning of the ground state by uniaxial strain combined with low-temperature scanning tunneling microscopy. We demonstrate that, indeed under strain, the surface of Fe1.1Te undergoes a transition to a (π, π)-charge-ordered state. Comparison with transport experiments on uniaxially strained samples shows that this is a surface phase, demonstrating the opportunities afforded by 2D correlated phases stabilized near surfaces and interfaces. Publisher PDF

A key property of many quantum materials is that their ground state depends sensitively on small changes of an external tuning parameter, e.g., doping, magnetic field, or pressure, creating opportunities for potential technological applications. Here, we explore tuning of the ground state of the nonsuperconducting parent compound, Fe1+xTe, of the iron chalcogenides by uniaxial strain. Iron telluride exhibits a peculiar (π, 0) antiferromagnetic order unlike the (π, π) order observed in the Fe-pnictide superconductors. The (π, 0) order is accompanied by a significant monoclinic distortion. We explore tuning of the ground state by uniaxial strain combined with low-temperature scanning tunneling microscopy. We demonstrate that, indeed under strain, the surface of Fe1.1Te undergoes a transition to a (π, π)-charge-ordered state. Comparison with transport experiments on uniaxially strained samples shows that this is a surface phase, demonstrating the opportunities afforded by 2D correlated phases stabilized near surfaces and interfaces.

This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (Grants No. 24224010, No. 15K13523, No. JP15H05852, No. JP15K21717,, No. 17H01140, No. 18H01162, and No. 17J05243), JSPS Core-to-Core Program (A) Advanced Research Networks, and the Alexander von Humboldt Foundation. S.S. acknowledges financial support by JSPS and the Materials Education program for the future leaders in Research, Industry, and Technology (MERIT). In Dirac semimetals, interband mixing has been known theoretically to give rise to a giant orbital diamagnetism when the Fermi level is close to the Dirac point. In Bi1−xSbx and other Dirac semimetals, an enhanced diamagnetism in the magnetic susceptibility χ has been observed and interpreted as a manifestation of such giant orbital diamagnetism. Experimentally proving their orbital origin, however, has remained challenging. The cubic antiperovskite Sr3PbO is a three-dimensional Dirac electron system and shows the giant diamagnetism in χ as in the other Dirac semimetals. 207Pb NMR measurements are conducted in this study to explore the microscopic origin of diamagnetism. From the analysis of the Knight shift K as a function of χ and the relaxation rate T1–1 for samples with different hole densities, the spin and the orbital components in K are successfully separated. The results establish that the enhanced diamagnetism in Sr3PbO originates from the orbital contribution of Dirac electrons, which is fully consistent with the theory of giant orbital diamagnetism. Publisher PDF

In strongly correlated electron materials, the electronic, spin, and charge degrees of freedom are closely intertwined. This often leads to the stabilization of emergent orders that are highly sensitive to external physical stimuli promising opportunities for technological applications. In perovskite ruthenates, this sensitivity manifests in dramatic changes of the physical properties with subtle structural details of the RuO$_6$ octahedra, stabilizing enigmatic correlated ground states, from a hotly debated superconducting state via electronic nematicity and metamagnetic quantum criticality to ferromagnetism. Here, it is demonstrated that the rotation of the RuO$_6$ octahedra in the surface layer of Sr$_2$RuO$_4$ generates new emergent orders not observed in the bulk material. Through atomic-scale spectroscopic characterization of the low-energy electronic states, four van Hove singularities are identified in the vicinity of the Fermi energy. The singularities can be directly linked to intertwined nematic and checkerboard charge order. Tuning of one of these van Hove singularities by magnetic field is demonstrated, suggesting that the surface layer undergoes a Lifshitz transition at a magnetic field of ~32T. The results establish the surface layer of Sr$_2$RuO$_4$ as an exciting 2D correlated electron system and highlight the opportunities for engineering the low-energy electronic states in these systems., Comment: 26 pages including supplementary material, replaced with published version

Novel topological phenomena are anticipated for three-dimensional (3D) Dirac electrons. The magnetotransport properties of cubic ${\mathrm{Sr}}_{3}\mathrm{PbO}$ antiperovskite, theoretically proposed to be a 3D massive Dirac electron system, are studied. The measurements of Shubnikov--de Haas oscillations and Hall resistivity indicate the presence of a low density ($\ensuremath{\sim}1\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$) of holes with an extremely small cyclotron mass of $0.01--0.06{m}_{e}$. The magnetoresistance $\mathrm{\ensuremath{\Delta}}{\ensuremath{\rho}}_{xx}(B)$ is linear in magnetic field $B$ with the magnitude independent of temperature. These results are fully consistent with the presence of 3D massive Dirac electrons in ${\mathrm{Sr}}_{3}\mathrm{PbO}$. The chemical flexibility of the antiperovskites and our findings in the family member ${\mathrm{Sr}}_{3}\mathrm{PbO}$ point to their potential as a model system in which to explore exotic topological phases.

The excitonic insulator (EI) is an intriguing phase of condensed excitons undergoing a Bose-Einstein-Condensation (BEC)-type transition. A prominent candidate has been identified in Ta2NiSe5. Ultrafast spectroscopy allows tracing the coherent response of the EI condensate directly in the time domain. Probing the collective electronic response we can identify fingerprints for the Higgs-amplitude equivalent mode of the condensate. In addition we find a peculiar coupling of the EI phase to a low frequency phonon mode. We will discuss the transient response on multiple energies scales ranging from the exciton dynamics to the coherent THz response of the gap. Publisher PDF

This work was supported by the Engineering and Physical Sciences Research Council, UK (grant EP/F044704/1) and the Max Planck Society.

By establishing magnetic order in a square lattice compound, we introduce the first magnetic “new fermion.”, Recent interest in topological semimetals has led to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry, and the introduction of magnetic order can influence these states by breaking time-reversal symmetry. We show that we can realize a rich variety of different topological semimetal states in a single material, CeSbTe. This compound can exhibit different types of magnetic order that can be accessed easily by applying a small field. Therefore, it allows for tuning the electronic structure and can drive it through a manifold of topologically distinct phases, such as the first nonsymmorphic magnetic topological phase with an eightfold band crossing at a high-symmetry point. Our experimental results are backed by a full magnetic group theory analysis and ab initio calculations. This discovery introduces a realistic and promising platform for studying the interplay of magnetism and topology. We also show that we can generally expand the numbers of space groups that allow for high-order band degeneracies by introducing antiferromagnetic order.

A wide variety of complex phases in quantum materials are driven by electron-electron interactions, which are enhanced through density of states peaks. A well known example occurs at van Hove singularities where the Fermi surface undergoes a topological transition. Here we show that higher order singularities, where multiple disconnected leaves of Fermi surface touch all at once, naturally occur at points of high symmetry in the Brillouin zone. Such multicritical singularities can lead to stronger divergences in the density of states than canonical van Hove singularities, and critically boost the formation of complex quantum phases via interactions. As a concrete example of the power of these Fermi surface topological transitions, we demonstrate how they can be used in the analysis of experimental data on Sr$_3$Ru$_2$O$_7$. Understanding the related mechanisms opens up new avenues in material design of complex quantum phases., Comment: 5 pages main paper + 10 pages supplemental material. Version accepted by Phys. Rev. Lett

This work was partly supported by JSPS KAKENHI Grants No. 24224010, No. 15H05852, and No. 17H01140.

The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ∼0.16 eV below TC comparable to the estimated exciton binding energy EB. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC–EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5.

In this work, we present a model of the surface states of nonsymmorphic semimetals. These are derived from surface mass terms that lift the high degeneracy imposed in the band structure by the nonsymmorphic bulk symmetries. Reflecting the reduced symmetry at the surface, the bulk bands are strongly modified. This leads to the creation of two-dimensional floating bands, which are distinct from Shockley states, quantum well states or topologically protected surface states. We focus on the layered semimetal ZrSiS to clarify the origin of its surface states. We demonstrate an excellent agreement between DFT calculations and ARPES measurements and present an effective four-band model in which similar surface bands appear. Finally, we emphasize the role of the surface chemical potential by comparing the surface density of states in samples with and without potassium coating. Our findings can be extended to related compounds and generalized to other crystals with nonsymmorphic symmetries., Comment: 8 pages, 5 figures

The excitonic insulator is an intriguing electronic phase of quasi-condensed excitons. A prominent candidate is the small bandgap semiconductor Ta2NiSe5, in which excitons are believed to undergo a BEC-like transition. But experimental evidence for the existence of a coherent condensate in this material is still missing. A direct fingerprint of such a state would be the observation of its collective modes, which are equivalent to the Higgs- and Goldstone-modes in superconductors. Here we report evidence for the existence of a coherent amplitude response in the excitonic insulator phase of Ta2NiSe5. Using non-linear excitations with short laser pulses we identify a phonon-coupled state of the condensate that can be understood as a coupling of its electronic Higgs-mode to a low frequency phonon. The Higgs-mode contribution substantiates the picture of an electronically driven phase transition and characterizes the transient order parameter of the excitonic insulator as a function of temperature and excitation density., Comment: 32 pages, 12 figures, 1 table

Mapping Out an Entropic Landscape Quantum critical points are continuous phase transitions occurring near absolute zero displaying interesting properties that may have valuable applications. Experimental thermodynamic information on quantum critical systems is sparse, partly because the better known systems are tuned using hydrostatic pressure, and thermodynamic measurements are difficult to perform under such conditions. Rost et al. (p. 1360 , published online 6 August 2009; see the Perspective by Fisk ) built and calibrated bespoke apparatus for a thermodynamic study of strontium ruthenate using a magnetic field as the tuning parameter. The specific heat and magnetocaloric measuremements were combined to map an “entropy landscape” of the quantum criticality and phase formation. This technique should offer a way to probe other materials to reveal unusual properties, such as novel metallic states and superconductivity.

The mechanism holding Cooper pairs together in iron-based superconductors is highly debated. Finding the fingerprint of the pairing mechanism would be a leap forward. Cooper pairing in the iron-based high-Tc superconductors1,2,3 is often conjectured to involve bosonic fluctuations. Among the candidates are antiferromagnetic spin fluctuations1,4,5 and d-orbital fluctuations amplified by phonons6,7. Any such electron–boson interaction should alter the electron’s ‘self-energy’, and then become detectable through consequent modifications in the energy dependence of the electron’s momentum and lifetime8,9,10. Here we introduce a novel theoretical/experimental approach aimed at uniquely identifying the relevant fluctuations of iron-based superconductors by measuring effects of their self-energy. We use innovative quasiparticle interference (QPI) imaging11 techniques in LiFeAs to reveal strongly momentum-space anisotropic self-energy signatures that are focused along the Fe–Fe (interband scattering) direction, where the spin fluctuations of LiFeAs are concentrated. These effects coincide in energy with perturbations to the density of states N(ω) usually associated with the Cooper pairing interaction. We show that all the measured phenomena comprise the predicted QPI ‘fingerprint’ of a self-energy due to antiferromagnetic spin fluctuations, thereby distinguishing them as the predominant electron–boson interaction.

We present measurement and analysis of field-dependent specific heat measurements on Sr 3 Ru 2 O 7 , showing that, at low temperatures, an incipient divergence is cut off by the formation of a new phase previously identified to show the transport properties of an electronic nematic. We discuss how to interpret a specific heat divergence in a system with van Hove singularities, and caution against a simple-minded comparison of experimentally determined power laws with the predictions of quantum critical theories.

We report on the electronic structure of the bilayer ruthenate Ca 3 Ru 2 O 7 using high quality single-crystals grown by a floating-zone method which have the lowest residual resistivity so far achieved. The quantum oscillation, specific heat, and angle-resolved photoemission-spectroscopy (ARPES) measurements performed on these crystals establish that Ca 3 Ru 2 O 7 is a quasi two-dimensional low-carrier metal with a density-wave instability.

The Cooper pairing mechanism of heavy-fermion superconductors, while long hypothesized as due to spin fluctuations, has not been determined. It is the momentum space (k-space) structure of the superconducting energy gap delta(k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure delta(k) for any heavy-fermion superconductor. Bogoliubov quasiparticle interference (QPI) imaging, a proven technique for measuring the energy gaps of high-Tc superconductors, has recently been proposed as a new method to measure delta(k) in heavy-fermion superconductors, specifically CeCoIn5. By implementing this method, we immediately detect a superconducting energy gap whose nodes are oriented along k||(+-1, +-1)pi/a0 directions. Moreover, we determine the complete k-space structure of the delta(k) of a heavy-fermion superconductor. For CeCoIn5, this novel information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that highest magnitude delta(k) opens on a high-k band so that gap nodes occur at quite unanticipated k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with dx2-y2 gap symmetry. The availability of such quantitative heavy band- and gap-structure data will be critical in identifying the microscopic mechanism of heavy fermion superconductivity in this material, and perhaps in general., Comment: 14 pages, 4 figures, supplementary information

We report a study of the magnetoresistivity of high purity Sr${}_{3}$Ru${}_{2}$O${}_{7}$, in the vicinity of its electronic nematic phase. By employing a triple-axis (9/1/1 T) vector magnet, we were able to precisely tune both the magnitude and direction of the in-plane component of the magnetic field (${H}_{\ensuremath{\parallel}}$). We report the dependence of the resistively determined anisotropy on ${H}_{\ensuremath{\parallel}}$ in the phase, as well as across the wider temperature-field region. Our measurements reveal a high-temperature anisotropy which mimics the behavior of fluctuations from the underlying quantum critical point, and suggest the existence of a more complicated phase diagram.

Uneven Gap Electron pairs that are responsible for the phenomenon of superconductivity can only be broken by investing a finite amount of energy, called the energy gap. The size of the gap may depend on the position on the Fermi surface; in cuprates, the gap completely disappears at certain points. What happens in the pnictide superconductors is still a subject of debate, not least because there appear to be differences between the different pnictide families. Allan et al. (p. 563 ) used scanning tunneling spectroscopy to study the compound LiFeAs. The gap was mapped on three of the five bands on which the Fermi surface resides and was found to be anisotropic in momentum space.

We present a brief review of the physical properties of Sr3Ru2O7, in which the approach to a magnetic-field-tuned quantum critical point is cut off by the formation of a novel phase with transport characteristics consistent with those of a nematic electronic liquid crystal. Our goal is to summarize the physics that led to that conclusion being drawn, describing the key experiments and discussing the theoretical approaches that have been adopted. Throughout the review we also attempt to highlight observations that are not yet understood, and to discuss the future challenges that will need to be addressed by both experiment and theory., 14 pages, 7 Figures, minor corrections and updated references, accepted for publication in Physica C

The low-temperature magnetic behavior of the double-layered ruthenate Sr${}_{3}$Ru${}_{2}$O${}_{7}$, as grown from a eutectic Sr${}_{2}$RuO${}_{4}$-Sr${}_{3}$Ru${}_{2}$O${}_{7}$ system, was investigated via zero- and transverse-field muon-spin rotation. The gradual increase of the muon relaxation rate observed below 2.5 K, even in the absence of applied magnetic fields, indicates the occurrence of a spontaneous breaking of time-reversal symmetry. The onset of the latter at a temperature above 1.5 K, the ${T}_{c}$ of the single phase Sr${}_{2}$RuO${}_{4}$, provides evidence about an unconventional superconducting state in the eutectic phase, which most likely takes place at the interface between the Sr${}_{2}$RuO${}_{4}$ and Sr${}_{3}$Ru${}_{2}$O${}_{7}$ domains, or even inside the Sr${}_{3}$Ru${}_{2}$O${}_{7}$ phase. We show that the superconducting state manifests a two-component behavior in the transverse-field response with change-over at about $T=2.5$ K and $T=1.5$ K. The comparison with zero-field $\ensuremath{\mu}$SR data in the Ru-Sr${}_{2}$RuO${}_{4}$ eutectic system rules out the possibility of spurious effects due to embedded Ru islands.

The behavior of matter near zero temperature continuous phase transitions, or "quantum critical points" is a central topic of study in condensed matter physics. In fermionic systems, fundamental questions remain unanswered: the nature of the quantum critical regime is unclear because of the apparent breakdown of the concept of the quasiparticle, a cornerstone of existing theories of strongly interacting metals. Even less is known experimentally about the formation of ordered phases from such a quantum critical "soup." Here, we report a study of the specific heat across the phase diagram of the model system Sr 3Ru 2O 7, which features an anomalous phase whose transport properties are consistent with those of an electronic nematic. We show that this phase, which exists at low temperatures in a narrow range of magnetic fields, forms directly from a quantum critical state, and contains more entropy than mean-field calculations predict. Our results suggest that this extra entropy is due to remnant degrees of freedom from the highly entropic state above T c . The associated quantum critical point, which is "concealed" by the nematic phase, separates two Fermi liquids, neither of which has an identifiable spontaneously broken symmetry, but which likely differ in the topology of their Fermi surfaces., Instituto de Física de Líquidos y Sistemas Biológicos

We report a detailed investigation of quantum oscillations in ${\text{Sr}}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$, observed inductively (the de Haas--van Alphen effect) and thermally (the magnetocaloric effect). Working at fields from 3 to 18 T allowed us to straddle the metamagnetic transition region and probe the low- and high-field Fermi liquids. The observed frequencies are strongly field dependent in the vicinity of the metamagnetic transition, and there is evidence for magnetic breakdown. We also present the results of a comprehensive rotation study. The most surprising result concerns the field dependence of the measured quasiparticle masses. Contrary to conclusions previously drawn by some of us as a result of a study performed with a much poorer signal-to-noise ratio, none of the five Fermi-surface branches for which we have good field-dependent data gives evidence for a strong-field dependence of the mass. The implications of these experimental findings are discussed.

The aim of this study is to investigate the entropy of Sr3Ru2O7 in the vicinity of the proposed quantum critical end point. Of particular interest are the entropic properties of the material in relation to the phase formation of the anomalous ‘electron nematic’ [1]. The phase diagram as established before this project was discussed in detail in Sect. 2.2.2. The most important aspects are summarized in the following, in order to be able to put the detailed results presented here in context with the wider phase diagram. For this purpose Fig. 2.15 from Chap. 2 is reproduced below. For more details please refer to Sect. 2.2.2.

The aim of the project presented in this thesis was the study of the entropic properties of Sr3Ru2O7 as a function of magnetic field. The sizable magnetic moment of this material as a function of field leads together with the anisotropy of the magnetisation to a significant torque at the metamagnetic phase transitions in the phase diagram of Sr3Ru2O7. Furthermore, in order to carry out the magnetocaloric measurements reported here, a high temperature stability and low noise thermometry under magnetic field is necessary. In order to optimise the experiment for these requirements a new setup was designed, built and characterised by the author. In this chapter first a general overview of the design will be given. An important component is the careful calibration of the thermometry as a function of temperature and magnetic field, which will be discussed in the second part of this chapter. Finally, characterisation measurements with Sr2RuO4 as well as Sr3Ru2O7 will be presented.

“My greatest concern was what to call it. I thought of calling it ‘information’, but the word was overly used, so I decided to call it ‘uncertainty’. When I discussed it with John von Neumann, he had a better idea. Von Neumann told me, ‘You should call it entropy, for two reasons. In the first place your uncertainty function has been used in statistical mechanics under that name, so it already has a name. In the second place, and more important, nobody knows what entropy really is, so in a debate you will always have the advantage.”’—Claude Shannon (1948) [1].

We report measurements of quantum oscillations detected in the putative nematic phase of ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$. Improvements in sample purity enabled the resolution of small amplitude de Haas--van Alphen (dHvA) oscillations between two first order metamagnetic transitions delimiting the phase. Two distinct frequencies were observed, whose amplitudes follow the normal Lifshitz-Kosevich profile. Variations of the dHvA frequencies are explained in terms of a chemical potential shift produced by reaching a peak in the density of states, and an anomalous field dependence of the oscillatory amplitude provides information on domains.

We study the effects of adding electrons to the conduction bands of the itinerant metamagnet Sr3 Ru2 O7 using substitution of La3+ onto the Sr2+ site. Small changes to the chemical potential have a large effect: adding only 0.03 electrons per Ru reduces the electronic specific-heat coefficient by 30%, but makes no corresponding change to the temperature (∼8 K) at which the electronic specific-heat coefficient shows its maximum. The observations are incompatible with a simple rigid-band shift, raising the possibility that a many-body resonance plays a key role in the physics of Sr3 Ru2 O7. © 2008 The American Physical Society.

De Haas-van Alphen oscillations were measured in lanthanum tritelluride (LaTe_3) to probe the partially gapped Fermi surface resulting from charge density wave (CDW) formation. Three distinct frequencies were observed, one of which can be correlated with a FS sheet that is unaltered by CDW formation. The other two frequencies arise from FS sheets that have been reconstructed in the CDW state., Comment: 8 pages

High-resolution angular resolved photoemission data reveal well-defined quasiparticle bands of unusually low weight, emerging in line with the metallic phase of Ca(3)Ru(2)O(7) below approximately 30 K . At the bulk structural phase transition temperature of 48 K, we find clear evidence for an electronic instability, gapping large parts of the underlying Fermi surface that appears to be nested. Metallic pockets are found to survive in the small, non-nested sections, constituting a low-temperature Fermi surface with 2 orders of magnitude smaller volume than in all other metallic ruthenates. The Fermi velocities and volumes of these pockets are in agreement with the results of complementary quantum oscillation measurements on the same crystal batches.

We describe transport measurements in single-crystal, high-purity Ca 3 Ru 2 O 7 . The observation of a large linear magnetoresistance together with low frequency quantum oscillations is shown to be consistent with a small volume Fermi surface incompletely gapped by density wave formation. This complements previous ARPES experiments. The quantum oscillations are more pronounced in the Hall signal than in the longitudinal resistivity. This unusual observation is also explained by the peculiar electronic structure in this material. We remark on the similarity between our observations in Ca 3 Ru 2 O 7 and the observations of quantum oscillations in the underdoped cuprate superconductors.