Magneto-infrared Spectroscopy Research Articles

The discovery of three-dimensional Van Hove singularity

Arising from the extreme/saddle point in electronic bands, Van Hove singularity (VHS) manifests divergent density of states (DOS) and induces various new states of matter such as unconventional superconductivity. VHS is believed to exist in one and two dimensions, but rarely found in three dimension (3D). Here, we report the discovery of 3D VHS in a topological magnet EuCd2As2 by magneto-infrared spectroscopy. External magnetic fields effectively control the exchange interaction in EuCd2As2, and shift 3D Weyl bands continuously, leading to the modification of Fermi velocity and energy dispersion. Above the critical field, the 3D VHS forms and is evidenced by the abrupt emergence of inter-band transitions, which can be quantitatively described by the minimal model of Weyl semimetals. Three additional optical transitions are further predicted theoretically and verified in magneto-near-infrared spectra. Our results pave the way to exploring VHS in 3D systems and uncovering the coordination between electronic correlation and the topological phase.

Chirality selective magnon-phonon hybridization and magnon-induced chiral phonons in a layered zigzag antiferromagnet

Two-dimensional (2D) magnetic systems possess versatile magnetic order and can host tunable magnons carrying spin angular momenta. Recent advances show angular momentum can also be carried by lattice vibrations in the form of chiral phonons. However, the interplay between magnons and chiral phonons as well as the details of chiral phonon formation in a magnetic system are yet to be explored. Here, we report the observation of magnon-induced chiral phonons and chirality selective magnon-phonon hybridization in a layered zigzag antiferromagnet (AFM) FePSe3. With a combination of magneto-infrared and magneto-Raman spectroscopy, we observe chiral magnon polarons (chiMP), the new hybridized quasiparticles, at zero magnetic field. The hybridization gap reaches 0.25 meV and survives down to the quadrilayer limit. Via first principle calculations, we uncover a coherent coupling between AFM magnons and chiral phonons with parallel angular momenta, which arises from the underlying phonon and space group symmetries. This coupling lifts the chiral phonon degeneracy and gives rise to an unusual Raman circular polarization of the chiMP branches. The observation of coherent chiral spin-lattice excitations at zero magnetic field paves the way for angular momentum-based hybrid phononic and magnonic devices.

Topological Lifshitz transition and one-dimensional Weyl mode in HfTe5.

Landau band crossings typically stem from the intra-band evolution of electronic states in magnetic fields and enhance the interaction effect in their vicinity. Here in the extreme quantum limit of topological insulator HfTe5, we report the observation of a topological Lifshitz transition from inter-band Landau level crossings using magneto-infrared spectroscopy. By tracking the Landau level transitions, we demonstrate that band inversion drives the zeroth Landau bands to cross with each other after 4.5 T and forms a one-dimensional Weyl mode with the fundamental gap persistently closed. The unusual reduction of the zeroth Landau level transition activity suggests a topological Lifshitz transition at 21 T, which shifts the Weyl mode close to the Fermi level. As a result, a broad and asymmetric absorption feature emerges due to the Pauli blocking effect in one dimension, along with a distinctive negative magneto-resistivity. Our results provide a strategy for realizing one-dimensional Weyl quasiparticles in bulk crystals.

High-Field Magnetoelectric and Spin-Phonon Coupling in Multiferroic (NH4)2[FeCl5·(H2O)

We combine high field polarization, magneto-infrared spectroscopy, and lattice dynamics calculations with prior magnetization to explore the properties of (NH4)2[FeCl5·(H2O)]─a type II molecular multiferroic in which the mixing between charge, structure, and magnetism is controlled by intermolecular hydrogen and halogen bonds. Electric polarization is sensitive to the series of field-induced spin reorientations, increasing linearly with the field and reaching a maximum before collapsing to zero across the quasi-collinear to collinear-sinusoidal reorientation due to the restoration of inversion symmetry. Magnetoelectric coupling is on the order of 1.2 ps/m for the P∥c, H∥c configuration between 5 and 25 T at 1.5 K. In this range, the coupling takes place via an orbital hybridization mechanism. Other forms of mixing are active in (NH4)2[FeCl5·(H2O)] as well. Magneto-infrared spectroscopy reveals that all of the vibrational modes below 600 cm-1 are sensitive to the field-induced transition to the fully saturated magnetic state at 30 T. We analyze these local lattice distortions and use frequency shifts to extract spin-phonon coupling constants for the Fe-O stretch, Fe-OH2 rock, and NH4+ libration. Inspection also reveals subtle symmetry breaking of the ammonium counterions across the ferroelectric transition. The coexistence of such varied mixing processes in a platform with intermolecular hydrogen- and halogen-bonding opens the door to greater understanding of multiferroics and magnetoelectrics governed by through-space interactions.

Magneto-optic signatures in the gapped Dirac semimetal with mixed linear and parabolic dispersions of ZrTe5

In this paper, we give a systematic theoretical study on the Landau levels (LLs) and magneto-optical conductivity Re(σ αα ) in a gapped Dirac semimetal model with mixed linear and parabolic dispersions under a magnetic field, which was recently proposed by Jiang et al (2020 Phys. Rev. Lett. 125 046403) to explain the experimental magnetoinfrared spectroscopy in the three-dimensional ZrTe5 crystal. We find that the strong magnetic field can drive the LLs become noninverted and thus the strong topological insulator phase in ZrTe5 turns to be a trivial insulator. In the different magnetic field regions, the density of states and Re(σ αα ) can exhibit distinct signatures. Moreover, when the magnetic field is weak, a qualitative relation in Re(σ zz ) between the peaks at the saddle points is revealed as Re Re , which is in good agreement with the experiment.

Unraveling the Topological Phase of ZrTe_{5} via Magnetoinfrared Spectroscopy.

For materials near the phase boundary between weak and strong topological insulators (TIs), their band topology depends on the band alignment, with the inverted (normal) band corresponding to the strong (weak) TI phase. Here, taking the anisotropic transition-metal pentatelluride ZrTe_{5} as an example, we show that the band inversion manifests itself as a second extremum (band gap) in the layer stacking direction, which can be probed experimentally via magnetoinfrared spectroscopy. Specifically, we find that the band anisotropy of ZrTe_{5} features a slow dispersion in the layer stacking direction, along with an additional set of optical transitions from a band gap next to the Brillouin zone center. Our work identifies ZrTe_{5} as a strong TI at liquid helium temperature and provides a new perspective in determining band inversion in layered topological materials.

Electron-Hole Asymmetry of Surface States in Topological Insulator Sb2Te3 Thin Films Revealed by Magneto-Infrared Spectroscopy.

When surface states (SSs) form in topological insulators (TIs), they inherit the properties of bulk bands, including the electron-hole (e-h) asymmetry but with much more profound impacts. Here via combining magneto-infrared spectroscopy with theoretical analysis, we show that e-h asymmetry significantly modifies the SS electronic structures when interplaying with the quantum confinement effect. Compared with the case without e-h asymmetry, the SSs now bear not only a band asymmetry, such as that in the bulk, but also a shift of the Dirac point relative to the bulk bands and a reduction of the hybridization gap of up to 70%. Our results signify the importance of e-h asymmetry in the band engineering of TIs in the thin-film limit.

Valley and Zeeman Splittings in Multilayer Epitaxial Graphene Revealed by Circular Polarization Resolved Magneto-infrared Spectroscopy.

Circular-polarization-resolved magneto-infrared studies of multilayer epitaxial graphene (MEG) are performed using tunable quantum cascade lasers in high magnetic fields up to 17.5 T. Landau level (LL) transitions in the monolayer and bilayer graphene inclusions of MEG are resolved, and considerable electron-hole asymmetry is observed in the extracted electronic band structure. For monolayer graphene, a four-fold splitting of the n = 0 to n = 1 LL transition is evidenced and attributed to the lifting of the valley and spin degeneracy of the zeroth LL and the broken electron-hole symmetry. The magnetic field dependence of the splitting further reveals its possible mechanisms. The best fit to experimental data yields effective g-factors, gVS* = 6.7 and gZS* = 4.8, for the valley and Zeeman splittings, respectively.

Spin\u2013lattice and electron\u2013phonon coupling in 3d/5d hybrid Sr3NiIrO6

While 3d-containing materials display strong electron correlations, narrow band widths, and robust magnetism, 5d systems are recognized for strong spin–orbit coupling, increased hybridization, and more diffuse orbitals. Combining these properties leads to novel behavior. Sr3NiIrO6, for example, displays complex magnetism and ultra-high coercive fields—up to an incredible 55 T. Here, we combine infrared and optical spectroscopies with high-field magnetization and first-principles calculations to explore the fundamental excitations of the lattice and related coupling processes including spin–lattice and electron–phonon mechanisms. Magneto-infrared spectroscopy reveals spin–lattice coupling of three phonons that modulate the Ir environment to reduce the energy required to modify the spin arrangement. While these modes primarily affect exchange within the chains, analysis also uncovers important inter-chain motion. This provides a mechanism by which inter-chain interactions can occur in the developing model for ultra-high coercivity. At the same time, analysis of the on-site Ir4+ excitations reveals vibronic coupling and extremely large crystal field parameters that lead to a t2g-derived low-spin state for Ir. These findings highlight the spin–charge–lattice entanglement in Sr3NiIrO6 and suggest that similar interactions may take place in other 3d/5d hybrids.

Landau Quantization in Coupled Weyl Points: A Case Study of Semimetal NbP.

Weyl semimetal (WSM) is a newly discovered quantum phase of matter that exhibits topologically protected states characterized by two separated Weyl points with linear dispersion in all directions. Here, via combining theoretical analysis and magneto-infrared spectroscopy of an archetypal Weyl semimetal, niobium phosphide, we demonstrate that the coupling between Weyl points can significantly modify the electronic structure of a WSM and provide a new twist to the protected states. These findings suggest that the coupled Weyl points should be considered as the basis for analysis of realistic WSMs.

Phonon mode links ferroicities in multiferroic [(CH3)2NH2]Mn(HCOO)3

We combined magnetoinfrared spectroscopy, magnetization, and lattice dynamics calculations to explore the phase transitions in multiferroic $[{({\mathrm{CH}}_{3})}_{2}{\mathrm{NH}}_{2}]\mathrm{Mn}{(\mathrm{HCOO})}_{3}$. Both the 185 K ferroelectric transition and the magnetically driven transition to the fully saturated state at 15.3 T involve the formate bending mode, direct evidence of a common connection between the two types of ferroicities and for how lattice distortions promote high-temperature magnetoelectric coupling in this system.

Magnetic phase transitions and magnetoelastic coupling in S=12 Heisenberg antiferromagnets

We combined magneto-infrared spectroscopy and first-principles calculations to unravel the role of spin-phonon coupling in the vicinity of the magnetic field-driven phase transitions in two chemically similar $S=1/2$ Heisenberg antiferromagnets, ${\mathrm{CuF}}_{2}({\mathrm{H}}_{2}\mathrm{O}){}_{2}$(3-Clpy) and $[\mathrm{Cu}{(\mathrm{pyz})}_{2}({\mathrm{HF}}_{2})]{\mathrm{PF}}_{6}$. This comparison resolves questions about the conditions under which the lattice participates in magnetic-field-driven transitions and, at the same time, provides a way to predict how the lattice is likely to support microscopic spin rearrangements.

Direct Observation of Landau Level Resonance and Mass Generation in Dirac Semimetal Cd3As2 Thin Films.

Three-dimensional topological Dirac semimetals have hitherto stimulated unprecedented research interests as a new class of quantum materials. Breaking certain types of symmetries has been proposed to enable the manipulation of Dirac fermions, and that was soon realized by external modulations such as magnetic fields. However, an intrinsic manipulation of Dirac states, which is more efficient and desirable, remains a significant challenge. Here, we report a systematic study of quasi-particle dynamics and band evolution in Cd3As2 thin films with controlled chromium (Cr) doping by both magneto-infrared spectroscopy and electrical transport. We observe the √B relation of inter-Landau-level resonance in Cd3As2, an important signature of ultrarelativistic massless state inaccessible in previous optical experiments. A crossover from quantum to quasi-classical behavior makes it possible to directly probe the mass of Dirac fermions. Importantly, Cr doping allows for a Dirac mass acquisition and topological phase transition enabling a desired dynamic control of Dirac fermions. Corroborating with the density-functional theory calculations, we show that the mass generation can be explained by the explicit C4 rotation symmetry breaking and the resultant Dirac gap engineering through Cr substitution for Cd atoms. The manipulation of the system symmetry and Dirac mass in Cd3As2 thin films provides a tuning knob to explore the exotic states stemming from the parent phase of Dirac semimetals.

Probing the semiconductor to semimetal transition in InAs/GaSb double quantum wells by magneto-infrared spectroscopy

We perform a magnetoinfrared spectroscopy study of the semiconductor to semimetal transition of InAs/GaSb double quantum wells from the normal to the inverted state. We show that owing to the low carrier density of our samples, the magnetoabsorption spectra evolve from a single cyclotron resonance peak in the normal state to multiple absorption peaks in the inverted state with distinct magnetic field dependence. Using an eight-band Pidgeon-Brown model, we explain all the major absorption peaks observed in our experiment. We demonstrate that the semiconductor to semimetal transition can be realized by manipulating the quantum confinement, the strain, and the magnetic field. Our work paves the way for band engineering of optimal InAs/GaSb structures for realizing novel topological states as well as for device applications in the terahertz regime.

Magnetoinfrared Spectroscopy of Landau Levels and Zeeman Splitting of Three-Dimensional Massless Dirac Fermions in ZrTe(5).

We present a magnetoinfrared spectroscopy study on a newly identified three-dimensional (3D) Dirac semimetal ZrTe(5). We observe clear transitions between Landau levels and their further splitting under a magnetic field. Both the sequence of transitions and their field dependence follow quantitatively the relation expected for 3D massless Dirac fermions. The measurement also reveals an exceptionally low magnetic field needed to drive the compound into its quantum limit, demonstrating that ZrTe(5) is an extremely clean system and ideal platform for studying 3D Dirac fermions. The splitting of the Landau levels provides direct, bulk spectroscopic evidence that a relatively weak magnetic field can produce a sizable Zeeman effect on the 3D Dirac fermions, which lifts the spin degeneracy of Landau levels. Our analysis indicates that the compound evolves from a Dirac semimetal into a topological line-node semimetal under the current magnetic field configuration.

Publisher's Note: Landau level spectroscopy of surface states in the topological insulator Bi0.91Sb0.09via magneto-optics [Phys. Rev. B85, 195440 (2012)

We have performed broad-band zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi$_{0.91}$Sb$_{0.09}$. The zero-field results allow us to measure the value of the direct band gap between the conducting $L_a$ and valence $L_s$ bands. Under applied field in the Faraday geometry (\emph{k} $||$ \emph{H} $||$ C1), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence $\omega \propto \sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.

Landau level spectroscopy of surface states in the topological insulator Bi0.91Sb0.09via magneto-optics

We have performed broadband zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi${}_{0.91}$Sb${}_{0.09}$. The zero-field results allow us to measure the value of the direct band gap between the conducting ${L}_{a}$ and valence ${L}_{s}$ bands. Under applied field in the Faraday geometry ($k\ensuremath{\parallel}H\ensuremath{\parallel}\text{C1}$), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence $\ensuremath{\omega}\ensuremath{\propto}\sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.

Resonant subband Landau level coupling in symmetric quantum well

Subband structure and depolarization shifts in an ultrahigh mobility GaAs/Al0.24Ga0.76As quantum well are studied using magnetoinfrared spectroscopy via resonant subband Landau level coupling. Resonant couplings between the first and up to the fourth subbands are identified by well-separated antilevel-crossing split resonance, while the hy-lying subbands were identified by the cyclotron resonance linewidth broadening in the literature. In addition, a forbidden intersubband transition (first to third) has been observed. With the precise determination of the subband structure, we find that the depolarization shift can be well described by the semiclassical slab plasma model and the possible origins for the forbidden transition are discussed.

Magneto-infrared modes in InAs-AlSb-GaSb coupled quantum wells

We have studied a series of InAs/GaSb coupled quantum wells using magneto-infrared spectroscopy for high magnetic fields up to 33 T within temperatures ranging from 4 to 45 K in both Faraday and tilted field geometries. This type of coupled quantum wells consists of an electron layer in the InAs quantum well and a hole layer in the GaSb quantum well, forming the so-called two-dimensional electron-hole bilayer system. Unlike the samples studied in the past, the hybridization of the electron and hole subbands in our samples is largely reduced by having narrower wells and an AlSb barrier layer interposed between the InAs and the GaSb quantum wells, rendering them weakly hybridized. Previous studies have revealed multiple absorption modes near the electron cyclotron resonance of the InAs layer in moderately and strongly hybridized samples while only a single absorption mode was observed in the weakly hybridized samples. We have observed a pair of absorption modes occurring only at magnetic fields higher than 14 T, which exhibited several interesting phenomena. Among which we found two unique types of behavior that distinguish this work from the ones reported in the literature. This pair of modes is very robust against rising thermal excitations and increasing magnetic fields aligned parallel to the heterostructures. While the previous results were aptly explained by the antilevel crossing due to the hybridization of the electron and hole wave functions, i.e., conduction-valence Landau-level mixing, the unique features reported in this paper cannot be explained within the same concept. The unusual properties found in this study and their connection to the known models for InAs/GaSb heterostructures will be discussed; in addition, several alternative ideas will be proposed in this paper and it appears that a spontaneous phase separation can account for most of the observed features.

Magnetoelastic Coupling through the Antiferromagnet-to-Ferromagnet Transition of Quasi-Two-Dimensional[Cu(HF2)(pyz)2]BF4Using Infrared Spectroscopy

We investigated magnetoelastic coupling through the field-driven transition to the fully polarized magnetic state in quasi-two-dimensional [Cu(HF2)(pyz)2]BF4 by magnetoinfrared spectroscopy. This transition modifies out-of-plane ring distortion and bending vibrational modes of the pyrazine ligand. The extent of these distortions increases with the field, systematically tracking the low-temperature magnetization. These distortions weaken the antiferromagnetic spin exchange, a finding that provides important insight into magnetic transitions in other copper halides.