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arXiv2013

Pseudogap state from quantum criticality

Upon application of an external tuning parameter, a magnetic state can be driven to a normal metal state at zero temperature. This phenomenon is known as quantum criticality and leads to fascinating responses in thermodynamics and transport of the compound. In the standard picture, a single quantum critical point occurs at zero temperature, which results in a nontrivial critical behaviour in its vicinity. Here we show that in two dimensions the scenario is considerably more complex due to the enormous amount of quantum fluctuations. Instead of the single point separating the antiferromagnet from the normal metal, we have discovered a broad region between these two phases where the magnetic order is destroyed but certain areas of the Fermi surface are closed by a large gap. This gap reflects the formation of a novel quantum state characterised by a superposition of d-wave superconductivity and a quadrupole-density wave, id est a state in which an electron quadrupole density spatially oscillates with a period drastically different from the one of the original spin-density wave. At moderate temperatures both orders co-exist at short distances but thermal fluctuations destroy the long-range order. Below a critical temperature the fluctuations are less essential and superconductivity becomes stable. This new phenomenon may shed some light on the origin of the mysterious pseudogap state and of the high-temperature transition into the superconducting state in cuprates. Our results demonstrate that quantum phase transitions between antiferromagnets and normal metals in layered materials may be the proper playground for search of new high temperature superconductors.

cond-mat.str-el↗

arXiv2013

Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain

Assemblies of interacting quantum particles often surprise us with properties that are difficult to predict. One of the simplest quantum many-body systems is the spin 1/2 Heisenberg antiferromagnetic chain, a linear array of interacting magnetic moments. Its exact ground state is a macroscopic singlet entangling all spins in the chain. Its elementary excitations, called spinons, are fractional spin 1/2 quasiparticles; they are created and detected in pairs by neutron scattering. Theoretical predictions show that two-spinon states exhaust only 71% of the spectral weight while higher-order spinon states, yet to be experimentally located, are predicted to participate in the remaining. Here, by accurate absolute normalization of our inelastic neutron scattering data on a compound realizing the model, we account for the full spectral weight to within 99(8)%. Our data thus establish and quantify the existence of higher-order spinon states. The observation that within error bars, the entire weight is confined within the boundaries of the two-spinon continuum, and that the lineshape resembles a rescaled two-spinon one, allow us to develop a simple physical picture for understanding multi-spinon excitations.

cond-mat.str-el↗

arXiv2013

Visualizing Nodal Heavy Fermion Superconductivity in CeCoIn5

Understanding the origin of superconductivity in strongly correlated electron systems continues to be at the forefront of unsolved problems in all of physics. Among the heavy f-electron systems, CeCoIn5 is one of the most fascinating, as it shares many of the characteristics of correlated d-electron high-Tc cuprate and pnictide superconductors, including the competition between antiferromagnetism and superconductivity. While there has been evidence for unconventional pairing in this compound, high-resolution spectroscopic measurements of the superconducting state have been lacking. Previously, we have used high-resolution scanning tunneling microscopy techniques to visualize the emergence of heavy-fermion excitations in CeCoIn5 and demonstrate the composite nature of these excitations well above Tc. Here we extend these techniques to much lower temperatures to investigate how superconductivity develops within a strongly correlated band of composite excitations. We find the spectrum of heavy excitations to be strongly modified just prior to the onset of superconductivity by a suppression of the spectral weight near the Fermi energy, reminiscent of the pseudogap state in the cuprates. By measuring the response of superconductivity to various perturbations, through both quasiparticle interference and local pair-breaking experiments, we demonstrate the nodal d-wave character of superconducting pairing in CeCoIn5.

cond-mat.supr-con↗

arXiv2013

Ferromagnetic exchange, spin-orbit coupling and spiral magnetism at the LaAlO_3/SrTiO_3 interface

The electronic properties of the polar interface between insulating oxides is a subject of great current interest. An exciting new development is the observation of robust magnetism at the interface of two non-magnetic materials LaAlO_3 (LAO) and SrTiO_3 (STO). Here we present a microscopic theory for the formation and interaction of local moments, which depends on essential features of the LAO/STO interface. We show that correlation-induced moments arise due to interfacial splitting of orbital degeneracy. We find that gate-tunable Rashba spin-orbit coupling at the interface influences the exchange interaction mediated by conduction electrons. We predict that the zero-field ground state is a long-wavelength spiral and show that its evolution in an external field accounts semi-quantitatively for torque magnetometry data. Our theory describes qualitative aspects of the scanning SQUID measurements and makes several testable predictions for future experiments.

cond-mat.str-el↗

arXiv2013

An area law for entanglement from exponential decay of correlations

Area laws for entanglement in quantum many-body systems give useful information about their low-temperature behaviour and are tightly connected to the possibility of good numerical simulations. An intuition from quantum many-body physics suggests that an area law should hold whenever there is exponential decay of correlations in the system, a property found, for instance, in non-critical phases of matter. However, the existence of quantum data-hiding state--that is, states having very small correlations, yet a volume scaling of entanglement--was believed to be a serious obstruction to such an implication. Here we prove that notwithstanding the phenomenon of data hiding, one-dimensional quantum many-body states satisfying exponential decay of correlations always fulfil an area law. To obtain this result we combine several recent advances in quantum information theory, thus showing the usefulness of the field for addressing problems in other areas of physics.

quant-ph↗

arXiv2014

Far from equilibrium monopole dynamics in spin ice

Condensed matter in the low temperature limit reveals much exotic physics associated with unusual orders and excitations, examples ranging from helium superfluidity to magnetic monopoles in spin ice. The far-from-equilibrium physics of such low temperature states may be even more exotic, yet to access it in the laboratory remains a challenge. Here we demonstrate a simple and robust technique, the 'magnetothermal avalanche quench', and its use in the controlled creation of nonequilibrium populations of magnetic monopoles in spin ice at millikelvin temperatures. These populations are found to exhibit spontaneous dynamical effects that typify far-from-equilibrium systems, yet are captured by simple models. Our method thus opens the door to the study of far-from-equilibrium states in spin ice and other exotic magnets.

cond-mat.str-el↗

arXiv2014

Twisted magnetic patterns: Exploring the Dzyaloshinskii--Moriya vector

Magnetism - the spontaneous alignment of atomic moments in a material - is driven by quantum-mechanical `exchange' interactions which operate over atomic distances as a result of the fundamental symmetry of electrons. Currently, one of the most active fields of condensed matter physics involves the study of magnetic interactions that cause, or are caused by a twisting of nearby atoms. This can lead to the magnetoelectric effect that couples electric and magnetic properties, and is predicted to play a prominent role in future technology. Here, we discuss the complex relativistic interplay between magnetism and atomic crystal structure in a class of materials called `weak ferromagnets'. The sign of the underpinning Dzyaloshinskii--Moriya interaction has been determined for the first time, by using synchrotron radiation to study iron borate (FeBO3). We present a novel experimental technique based on interference between two x-ray scattering processes (one acts as a reference wave) which we combine with a second unusual approach of turning the atomic antiferromagnetic motif with a small magnetic field. We show that the experimental results provide a clear validation of state-of-the-art theoretical calculations. These experimental and theoretical approaches open up new possibilities for exploring, modelling and exploiting novel magnetic and magnetoelectric materials.

cond-mat.str-el↗

arXiv2013

Nematic order in iron superconductors - who is in the driver's seat?

Although the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. Nematic order breaks the discrete lattice rotational symmetry by making the $x$ and $y$ directions in the Fe plane non-equivalent. This can happen because of (i) a tetragonal to orthorhombic structural transition, (ii) a spontaneous breaking of an orbital symmetry, or (iii) a spontaneous development of an Ising-type spin-nematic order - a magnetic state that breaks rotational symmetry but preserves time-reversal symmetry. The Landau theory of phase transitions dictates that the development of one of these orders should immediately induce the other two, making the origin of nematicity a physics realization of a "chicken and egg problem". The three scenarios are, however, quite different from a microscopic perspective. While in the structural scenario lattice vibrations (phonons) play the dominant role, in the other two scenarios electronic correlations are responsible for the nematic order. In this review, we argue that experimental and theoretical evidence strongly points to the electronic rather than phononic mechanism, placing the nematic order in the class of correlation-driven electronic instabilities, like superconductivity and density-wave transitions. We discuss different microscopic models for nematicity in the iron pnictides, and link nematicity to other ordered states of the global phase diagram of these materials -- magnetism and superconductivity. In the magnetic model nematic order pre-empts stripe-type magnetic order, and the same interaction which favors nematicity also gives rise to an unconventional $s^{+-}$ superconductivity. In the charge/orbital model magnetism appears as a secondary effect of ferro-orbital order, and the interaction which favors nematicity gives rise to a conventional $s^{++}$ superconductivity.

cond-mat.str-el↗

arXiv2014

Bipartite magnetic parent phases in the iron-oxypnictide superconductor

High-temperature (high-$T_{\rm c}$) superconductivity appears as a consequence of the carrier-doping of an undoped parent compound exhibiting antiferromagnetic order; thereby, ground-state properties of the parent compound are closely relevant to the superconducting state. On the basis of the concept, a spin-fluctuation has been addressed as an origin of pairing of the superconducting electrons in cuprates. Whereas, there is growing interest in the pairing mechanism such as an unconventional spin-fluctuation or an advanced orbital-fluctuation due to the characteristic multi-orbital system in iron-pnictides. Here, we report the discovery of an antiferromagnetic order as well as a unique structural transition in electron-overdoped LaFeAsO$_{1-x}$H$_x$ ($x$ ~ 0.5), whereby another parent phase was uncovered, albeit heavily doped. The unprecedented two-dome superconducting phases observed in this material can be interpreted as a consequence of the carrier-doping starting from the original at $x\sim0$ and advanced at $x\sim0.5$ parent phases toward the intermediate region. The bipartite parent phases with distinct physical properties in the second magnetic phase provide us with an interesting example to illustrate the intimate interplay among the magnetic interaction, structural change and orbital degree of freedom in iron-pnictides.

cond-mat.str-el↗

arXiv2009

Quantum criticality in ferroelectrics

Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new effects, like the breakdown of the conventional Fermi-liquid theory of metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure or chemical or isotopic substitution. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the quantum fluctuations of the electrical polarization. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state as the difficulties found in metals due to a high density of gapless excitations on the Fermi surface are avoided. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO3 and KTaO3. We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2K in SrTiO3 and 3K in KTaO3 believed to arise from coupling to acoustic phonons. Looking ahead, we suggest that in ferroelectric materials supporting mobile charge carriers, quantum paraelectric fluctuations may mediate new effective electron-electron interactions giving rise to a number of possible states such as superconductivity.

cond-mat.str-el↗

arXiv2018

Discovery of a strain-stabilised smectic electronic order in LiFeAs

In many high temperature superconductors, small orthorhombic distortions of the lattice structure result in surprisingly large symmetry breaking of the electronic states and macroscopic properties, an effect often referred to as nematicity. To directly study the impact of symmetry-breaking lattice distortions on the electronic states, using low-temperature scanning tunnelling microscopy we image at the atomic scale the influence of strain-tuned lattice distortions on the correlated electronic states in the iron-based superconductor LiFeAs, a material which in its ground state is tetragonal, with four-fold ($C_4$) symmetry. Our experiments uncover a new strain-stabilised modulated phase which exhibits a smectic order in LiFeAs, an electronic state which not only breaks rotational symmetry but also reduces translational symmetry. We follow the evolution of the superconducting gap from the unstrained material with $C_4$ symmetry through the new nematic phase with two-fold ($C_2$) symmetry and charge-density-wave order to a state where superconductivity is completely suppressed.

cond-mat.str-el↗

arXiv2018

Discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet

Topological magnons are emergent quantum spin excitations featured by magnon bands crossing linearly at the points dubbed nodes, analogous to fermions in topological electronic systems. Experimental realization of topological magnons in three dimensions has not been reported so far. Here, by measuring spin excitations (magnons) of a three-dimensional antiferromagnet Cu$_{3}$TeO$_{6}$ with inelastic neutron scattering, we provide direct spectroscopic evidence for the coexistence of symmetry-protected Dirac and triply degenerate nodes, the latter involving three-component magnons beyond the Dirac-Weyl framework. Our theoretical calculations show that the observed topological magnon band structure can be well described by the linear-spin-wave theory based on a Hamiltonian dominated by the nearest-neighbour exchange interaction $J_1$. As such, we showcase Cu$_{3}$TeO$_{6}$ as an example system where Dirac and triply degenerate magnonic nodal excitations coexist, demonstrate an exotic topological state of matter, and provide a fresh ground to explore the topological properties in quantum materials.

cond-mat.str-el↗

arXiv2024

Mixed-valence state in the dilute-impurity regime of La-substituted SmB$_6$

Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of $f$-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the determination of the limiting cases for which MV emerges. Here we address this question for SmB$_6$, a prototypical MV system characterized by two nearly-degenerate Sm$^{2+}$ and Sm$^{3+}$ configurations. By combining angle resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence, $v_{Sm}$, in the Sm$_x$La$_{1-x}$B$_6$ series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at $x$$\,$=$\,$0.2, with $v_{Sm}$$\,$$\sim$$\,$2; surprisingly, by further reducing $x$, a re-entrant increase of $v_{Sm}$ develops, approaching the value of $v_{imp}$$\,$$\sim$$\,$2.35 in the dilute-impurity limit. Such observation departs from a monotonic evolution of $v_{Sm}$ across the whole series, as well as from the expectation of its convergence to an integer value for $x$$\,$$\rightarrow$$\,$0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the Sm$_x$La$_{1-x}$B$_6$ series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime.

cond-mat.str-el↗

arXiv2024

Electric-field-enhanced second-harmonic domain contrast and nonreciprocity in a van der Waals antiferromagnet

Imaging antiferromagnetic 180° domains with actively controlled visibility is vital for both fundamental science and sophisticated applications. While optical second-harmonic generation (SHG) is a well-known technique for distinguishing such domains in non-centrosymmetric antiferromagnets, a general material-based strategy to control domain contrast remains elusive. Using van der Waals antiferromagnet MnPS$_3$ as a proof of concept, we demonstrate the tuning of nonreciprocity-induced domain contrast in SHG through applying an in-plane electric field that transforms the magnetic point group to its unitary subgroup. The interference among intrinsic electric-dipole, magnetic-dipole, and field-induced electric-dipole transitions, each carrying distinct characters under space-inversion ($\mathcal{P}$) and time-reversal ($\mathcal{T}$) operations, enables large tuning of domain contrast and nonreciprocity in a broad spectral range. This strategy, generically applicable to systems characterized by $\mathcal{PT}$-symmetric magnetic groups with a polar unitary subgroup, offers a path to fast electrical modulation of nonlinear nonreciprocal photonic behaviors using antiferromagnets.

cond-mat.str-el↗

arXiv2017

Large positive correlation between the effective electron mass and the multipolar fluctuation in the heavy-fermion metal Ce$_{1-x}$La$_x$B$_6$

For the last few decades, researchers have been intrigued by multipolar ordering phenomena while looking for the related quantum criticality in the heavy-fermion Kondo system Ce$_{1-x}$La$_{x}$B$_6$. However, critical phenomena induced by substitution level ($x$), temperature ($T$), and magnetic field ($B$) are poorly understood despite a large collection of experimental results is available. In this work, we present $T$-$B$, $x$-$T$, and $x$-$B$ phase diagrams of Ce$_{1-x}$La$_x$B$_6$ ($\mathbf{B}\parallel[110]$). These are completed by analyzing heat capacity, magnetocaloric effect (MCE), and elastic neutron scattering. A drastic increase of the Sommerfeld coefficient $γ_0$, which is estimated from the heat capacity down to 0.05 K, is observed with increasing $x$. The precise $T$-$B$ phase diagram which includes an unforeseen high-entropy region is drawn by analyzing the MCE for the first time in Ce$_{1-x}$La$_x$B$_6$. The $x$-$B$ phase diagram, which supports the existence of a QCP at $x>0.75$, is obtained by the same analysis. A detailed interpretation of phase diagrams strongly indicates positive correlation between the fluctuating multipoles and the effective electron mass.

cond-mat.str-el↗

arXiv2018

Competing Coulomb and electron-phonon interactions in NbS$_2$

The interplay of Coulomb and electron-phonon interactions with thermal and quantum fluctuations facilitates rich phase diagrams in two-dimensional electron systems. Layered transition metal dichalcogenides hosting charge, excitonic, spin and superconducting order form an epitomic material class in this respect. Theoretical studies of materials like NbS$_2$ have focused on the electron-phonon coupling whereas the Coulomb interaction, particularly strong in the monolayer limit, remained essentially untouched. Here, we analyze the interplay of short- and long-range Coulomb as well as electron-phonon interactions in NbS$_2$ monolayers. The combination of these interactions causes electronic correlations that are fundamentally different to what would be expected from the interaction terms separately. The fully interacting electronic spectral function resembles the non-interacting band structure but with appreciable broadening. An unexpected coexistence of strong charge and spin fluctuations puts NbS$_2$ close to spin and charge order, suggesting monolayer NbS$_2$ as a platform for atomic scale engineering of electronic quantum phases.

cond-mat.str-el↗

arXiv2017

Even denominator fractional quantum Hall states at an isospin transition in monolayer graphene

Magnetic fields quench the kinetic energy of two dimensional electrons, confining them to highly degenerate Landau levels. In the absence of disorder, the ground state at partial Landau level filling is determined only by Coulomb interactions, leading to a variety of correlation-driven phenomena. Here, we realize a quantum Hall analog of the Neél-to-valence bond solid transition within the spin- and sublattice- degenerate monolayer graphene zero energy Landau level by experimentally controlling substrate-induced sublattice symmetry breaking. The transition is marked by unusual isospin transitions in odd denominator fractional quantum Hall states for filling factors $ν$ near charge neutrality, and the unexpected appearance of incompressible even denominator fractional quantum Hall states at $ν=\pm1/2$ and $\pm1/4$ associated with pairing between composite fermions on different carbon sublattices.

cond-mat.str-el↗

arXiv2017

Spin-relaxation time in materials with broken inversion symmetry and large spin-orbit coupling

We study the spin-relaxation time in materials where a large spin-orbit coupling (SOC) is present which breaks the spatial inversion symmetry. Such a spin-orbit coupling is realized in zincblende structures and heterostructures with a transversal electric field and the spin relaxation is usually described by the so-called D'yakonov-Perel' (DP) mechanism. We combine a Monte Carlo method and diagrammatic calculation based approaches in our study; the former tracks the time evolution of electron spins in a quasiparticle dynamics simulation in the presence of the built-in spin-orbit magnetic fields and the latter builds on the spin-diffusion propagator by Burkov and Balents [Burkov and Balents Phys. Rev. B. 69, 245312 (2004).]. Remarkably, we find a parameter free quantitative agreement between the two approaches and it also returns the conventional result of the DP mechanism in the appropriate limit. We discuss the full phase space of spin relaxation as a function of SOC strength, its distribution, and the magnitude of the momentum relaxation rate. This allows us to identify two novel spin-relaxation regimes; where spin relaxation is strongly non-exponential and the spin relaxation equals the momentum relaxation. A compelling analogy between the spin-relaxation theory and the NMR motional narrowing is highlighted.

cond-mat.str-el↗