Low-energy Elementary Excitations Research Articles

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure.

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

Ubiquitous suppression of the nodal coherent spectral weight in Bi-based cuprates

High-temperature superconducting cuprates exhibit an intriguing phenomenology for the low-energy elementary excitations. In particular, an unconventional temperature dependence of the coherent spectral weight (CSW) has been observed in the superconducting phase by angle-resolved photoemission spectroscopy (ARPES), both at the antinode where the d-wave paring gap is maximum, as well as along the gapless nodal direction. Here, we combine equilibrium and time-resolved ARPES to track the temperature dependent meltdown of the nodal CSW in Bi-based cuprates with unprecedented sensitivity. We find the nodal suppression of CSW upon increasing temperature to be ubiquitous across single- and double-layer Bi cuprates, and uncorrelated to superconducting and pseudogap onset temperatures. We quantitatively model both the lineshape of the nodal spectral features and the anomalous suppression of CSW within the Fermi-Liquid framework, establishing the key role played by the normal state electrodynamics in the description of nodal quasiparticles in superconducting cuprates.

Nonequilibrium-DMFT based RIXS investigation of the two-orbital Hubbard model

Resonant inelastic X-ray scattering (RIXS) detects various types of high- and low-energy elementary excitations in correlated solids, and this tool will play an increasingly important role in investigations of time-dependent phenomena in photo-excited systems. While theoretical frameworks for the computation of equilibrium RIXS spectra are well established, the development of appropriate methods for nonequilibrium simulations is an active research field. Here, we apply a recently developed nonequilibrium dynamical mean field theory (DMFT) based approach to compute the RIXS response of photo-excited two-orbital Mott insulators. The results demonstrate the feasibility of multi-orbital nonequilibrium RIXS calculations and the sensitivity of the quasi-elastic fluorescence-like features and excitation peaks on the nonequilibrium population of the Hubbard bands.

Critical temperature and low-energy excitations in gapped spin systems with defects

We discuss theoretically the magnetically ordered phase induced by magnetic and nonmagnetic impurities in three-dimensional and quasi-low-dimensional systems with singlet ground states separated by a gap from excited triplet states. Using ideas of the percolation theory, we estimate the transition temperature $T_N(n)$ to the N\'eel phase at a small concentration $n$ of defects, derive the density of states of low-energy elementary excitations, and examine the contribution of these excitations to the specific heat and magnetization. Our expressions for $T_N(n)$ and for the specific heat describe well available experimental findings obtained in various appropriate systems: spin-$\frac12$ dimer materials, spin-ladder compounds, spin-Peierls and Haldane chain materials. However, our expression for $T_N(n)$ differs considerably from many of those proposed before.

Strain engineering of the charge and spin-orbital interactions in Sr2IrO4

In the high spin-orbit-coupled Sr2IrO4, the high sensitivity of the ground state to the details of the local lattice structure shows a large potential for the manipulation of the functional properties by inducing local lattice distortions. We use epitaxial strain to modify the Ir-O bond geometry in Sr2IrO4 and perform momentum-dependent resonant inelastic X-ray scattering (RIXS) at the metal and at the ligand sites to unveil the response of the low-energy elementary excitations. We observe that the pseudospin-wave dispersion for tensile-strained Sr2IrO4 films displays large softening along the [h,0] direction, while along the [h,h] direction it shows hardening. This evolution reveals a renormalization of the magnetic interactions caused by a strain-driven cross-over from anisotropic to isotropic interactions between the magnetic moments. Moreover, we detect dispersive electron-hole pair excitations which shift to lower (higher) energies upon compressive (tensile) strain, manifesting a reduction (increase) in the size of the charge gap. This behavior shows an intimate coupling between charge excitations and lattice distortions in Sr2IrO4, originating from the modified hopping elements between the t2g orbitals. Our work highlights the central role played by the lattice degrees of freedom in determining both the pseudospin and charge excitations of Sr2IrO4 and provides valuable information toward the control of the ground state of complex oxides in the presence of high spin-orbit coupling.

Giant suppression of phononic heat transport in a quantum magnet BiCu2PO6.

Thermal transport of quantum magnets has elucidated the nature of low energy elementary excitations and complex interplay between those excited states via strong scattering of thermal carriers. BiCu2PO6 is a unique frustrated spin-ladder compound exhibiting highly anisotropic spin excitations that contain both itinerant and localized dispersion characters along the b- and a-axes respectively. Here, we investigate thermal conductivity κ of BiCu2PO6 under high magnetic fields (H) of up to 30 tesla. A dip-feature in κ, located at ~15 K at zero-H along all crystallographic directions, moves gradually toward lower temperature (T) with increasing H, thus resulting in giant suppression by a factor of ~30 near the critical magnetic field of Hc ≅ 23.5 tesla. The giant H- and T-dependent suppression of κ can be explained by the combined result of resonant scattering of phononic heat carriers with magnetic energy levels and increased phonon scattering due to enhanced spin fluctuation at Hc, unequivocally revealing the existence of strong spin-phonon coupling. Moreover, we find an experimental indication that the remaining magnetic heat transport along the b-axis becomes almost gapless at the magnetic quantum critical point realized at Hc.

Possible observation of highly itinerant quantum magnetic monopoles in the frustrated pyrochlore Yb2Ti2O7.

The low-energy elementary excitations in frustrated quantum magnets have fascinated researchers for decades. In frustrated Ising magnets on a pyrochlore lattice possessing macroscopically degenerate spin-ice ground states, the excitations have been discussed in terms of classical magnetic monopoles, which do not contain quantum fluctuations. Here we report unusual behaviours of magneto-thermal conductivity in the disordered spin-liquid regime of pyrochlore Yb2Ti2O7, which hosts frustrated spin-ice correlations with large quantum fluctuations owing to pseudospin-1/2 of Yb ions. The analysis of the temperature and magnetic field dependencies shows the presence of gapped elementary excitations. We find that the gap energy is largely suppressed from that expected in classical monopoles. Moreover, these excitations propagate a long distance without being scattered, in contrast to the diffusive nature of classical monopoles. These results suggests the emergence of highly itinerant quantum magnetic monopole, which is a heavy quasiparticle that propagates coherently in three-dimensional spin liquids.

Spinon, soliton, and breather in the spin-12antiferromagnetic chain compoundKCuGaF6

Elementary excitations of the $S=\frac{1}{2}$ one-dimensional antiferromagnet ${\mathrm{KCuGaF}}_{6}$ were investigated by inelastic neutron scattering in zero and finite magnetic fields perpendicular to the $(1,1,0)$ plane combined with specific heat measurements. ${\mathrm{KCuGaF}}_{6}$ exhibits no long-range magnetic ordering down to 50 mK despite the large exchange interaction $J/{k}_{\mathrm{B}}=103$ K. At zero magnetic field, well-defined spinon excitations were observed. The energy of the des Cloizeaux and Pearson [J. des Cloizeaux and J. J. Pearson, Phys. Rev. 128, 2131 (1962)] mode of the spinon excitations is somewhat larger than that calculated with the above exchange constant. This discrepancy is mostly ascribed to the effective $XY$ anisotropy arising from the large Dzyaloshinsky-Moriya [T. Moriya, Phys. Rev. 120, 91 (1960)] interaction with an alternating $\mathbit{D}$ vector. ${\mathrm{KCuGaF}}_{6}$ in a magnetic field is represented by the quantum sine-Gordon model, for which low-energy elementary excitations are composed of solitons and antisolitons and their bound states called breathers. Unlike the theoretical prediction, it was found that the energy of a soliton is smaller than that of the first breather, although the energy of the first breather coincides with that observed in a previous electron spin resonance measurement.

Observation of structural relaxations in disordered solid media via spectral histories of single impurity molecules

This paper presents the technique and results of the investigation of the structural relaxation in disordered solid media (polyisobutylene, toluene, ortho-dichlorobenzene) at temperatures below the glass transition point via the spectra and fluorescence images of single impurity fluorescent molecules (terrylene, tetra-tert-butylterrylene). Application of the thermal-cycling method has made it possible to significantly extend the temperature range of investigation (from 4.5 K to the glass transition temperature). The changes observed in individual parameters of low-energy elementary excitations of the tunneling and vibrational types due to structural relaxation processes have been studied. It has been found that there is a spatial inhomogeneity in the distribution of the activation temperatures of local relaxation processes.

Observation of Elementary Excitations of Quantum Sine-Gordon Spin System KCuGaF6 Under High Magnetic Field

In magnetic field, KCuGaF6 can be mapped onto the quantum sine-Gordon model, for which low-energy elementary excitations are solitons, antisolitons and their bound states breathers. Using high-frequency, high-field ESR measurements, we observed these unusual magnetic excitations.

Matter fields near quantum critical point in [formula omitted]-dimensional [formula omitted] gauge theory

We study chiral phase transition and confinement of matter fields in ( 2 + 1 ) -dimensional U ( 1 ) gauge theory of massless Dirac fermions and scalar bosons. The vanishing scalar boson mass, r = 0 , defines a quantum critical point between the Higgs phase and the Coulomb phase. We consider only the critical point r = 0 and the Coulomb phase with r > 0 . The Dirac fermion acquires a dynamical mass when its flavor is less than certain critical value N f c , which depends quantitatively on the flavor N b and the scalar boson mass r. When N f < N f c , the matter fields carrying internal gauge charge are all confined if r ≠ 0 but are deconfined at the quantum critical point r = 0 . The system has distinct low-energy elementary excitations at the critical point r = 0 and in the Coulomb phase with r ≠ 0 . We calculate the specific heat and susceptibility of the system at r = 0 and r ≠ 0 , which can help to detect the quantum critical point and to judge whether dynamical fermion mass generation takes place.

Elementary excitations of a Bose-Einstein condensate in an effective magnetic field

We calculate the low-energy elementary excitations of a Bose-Einstein condensate in an effective magnetic field. The field is created by the interplay between light beams carrying orbital angular momentum and the trapped atoms [G. Juzeli\ifmmode \bar{u}\else \={u}\fi{}nas et al., Phys. Rev. A 71, 053614 (2005)]. We examine the role of the homogeneous magnetic field, familiar from studies of rotating condensates, and also investigate spectra for vector potentials with a more general radial dependence. We discuss the instabilities which arise and how these may be manifested.

Exact and Approximate Theoretical Techniques for Quantum Magnetism in Low Dimensions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Magnetic field effects on spin excitations in the spin-Peierls compoundCuGeO3

Neutron inelastic scattering measurements on an undoped single crystal of ${\mathrm{CuGeO}}_{3}$ and on a doped crystal ${\mathrm{CuGe}}_{0.997}{\mathrm{Si}}_{0.003}{\mathrm{O}}_{3}$ are performed in the presence of a magnetic field H up to 12 T. In the present work, a particular attention is given to the effect of H on the low-energy elementary excitations. This investigation is performed in the three different phases of the spin-Peierls system. In the dimerized D phase, the Zeeman splitting of the acoustic magnon branch is evaluated all along the dispersion branch $(0&lt;q&lt;~1/2).$ In the uniform U phase, it is established first that, at finite temperature $T\ensuremath{\ne}0,$ the low wave vector $(\stackrel{\ensuremath{\rightarrow}}{q}0)$ spinon modes are little affected by the thermal fluctuations $\ensuremath{\delta}{E}_{T}\ensuremath{\simeq}T.$ Second, the effect of H on the same low-$q$ mode results essentially in a transfer of the fluctuations towards the Zeeman-shifted lower boundary of the spinon continuum. Finally, our measurements on the doped compound establishes that an energy gap occurs in the elementary excitation spectrum of the incommensurate I phase.

Raman scattering in high Tc superconductors: phonons, electrons, and electron–phonon interaction

It was realized very soon after the discovery of high T c superconductors that Raman spectroscopy is an excellent technique for the investigation of low energy elementary excitations in these materials and also for their characterization. The primitive cell of high T c superconductors is usually centrosymmetric and contains a large number of atoms. This fact results in the existence of a large number of Raman-active (i.e., even) and also IR-active (odd) phonons. Although inelastic neutron scattering experiments have been performed, most of the experimental knowledge available for high T c superconductors originates from their Raman as well as IR spectra. Raman scattering can also be used to investigate low frequency electronic excitations. It was one of the first spectroscopic techniques that revealed the existence of a gap for electronic excitations in high T c superconductors. Soon after, it was realized that gaps observed in the electronic Raman spectra exhibit different features depending on the symmetry of the configuration used for the scattering experiments. Of particular interest is the behavior of the scattering efficiency when the Raman shift tends to zero. The electronic excitations responsible for the observed Raman gap couple noticeably to some of the Raman active phonons, a fact that results in changes in phonon self-energies and spectral strengths when crossing T c. Particularly strong effects that reveal a remarkably large electron–phonon interaction have been recently observed for Hg1234 and the isomorphic material (CuC) 1234. In this article these effects, and Raman scattering related to crystal field excitations of the rare earth constituents, are reviewed.

Low-energy elementary excitations of a trapped Bose-condensed gas

Bogolyubov-De Gennes equations for the excitations of a Bose condensate in the Thomas-Fermi regime in harmonic traps of any asymmetry and introduce a classification of eigenstates. In the case of cylindrical symmetry we emphasize the presence of an accidental degeneracy in the excitation spectrum at certain values of the projection of orbital angular momentum on the symmetry axis and discuss possible consequences of the degeneracy in the context of new signatures of Bose-Einstein condensation.

Low-energy elementary surface excitations of symmetric films of liquid4He atT=0 K

It is known that low-energy elementary excitations of symmetric films of liquid4He atT=0 K are characterized by a momentum ħq parallel to the surface and may be described by bound states. We have evaluated wave functions and energies of these states for both “best” short-ranged and optimal long-ranged correlations. Quantities of physical interest may be expressed in terms of these eigenstates and, in particular, for very small momenta (q<0.2 A−1) they are mainly determined by the contribution due to the lowest-lying one. We propose analytic expressions for the lowest-lying excitations and fluctuations in the long-wavelength limit. It is proved that in this limiting case, the excitation energy ħωLW(q) and the “averaged” static structure functionSLW(q) should go linearly to zero asq→0, whereas the “averaged” direct correlationX LW Dg (q) should diverge at the origin as 1/q. It is shown that numerical solutions exhibit the expected long-wavelength behavior provided that optimal correlations are used. All these results are displayed in a series of figures and are discussed in detail.