We present a magneto-infrared 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 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 a direct and bulk spectroscopic evidence that a relatively weak magnetic field can produce a sizeable 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 current magnetic field configuration.PACS numbers: 71.55. Ak, 71.70.Di 3D topological Dirac/Weyl semimetals are new kinds of topological materials that possess linear band dispersion in the bulk along all three momentum directions [1][2][3][4][5][6][7]. Their low-energy quasiparticles are the condensed matter realization of Dirac and Weyl fermions in relativistic high energy physics [8,9]. These materials are expected to host many unusual phenomena [10][11][12], in particular the chiral and axial anomaly associated with Weyl fermions [3,[13][14][15]. It is well known that the Dirac nodes are protected by both timereversal and space inversion symmetry. Since magnetic field breaks the time-reversal symmetry, a Dirac node may be split into a pair of Weyl nodes along the magnetic field direction in the momentum space [16][17][18] or transformed into linenodes [17,19]. Therefore, a Dirac semimetal can be considered as a parent compound to realize other topological variant quantum states. However, past 3D Dirac semimetal materials (e.g. Cd 3 As 2 ) suffer from the problem of large residual carrier density which requires very high magnetic field (e.g. above 60 Tesla) to drive them to their quantum limit [20,21]. This makes it extremely difficult to explore the transformation from Dirac to Weyl or line-node semimetals. Up to now, there are no direct evidences of such transformations.ZrTe 5 appears to be a new topological 3D Dirac material that exhibits novel and interesting properties. The compound crystallizes in the layered orthorhombic crystal structure, with prismatic ZrTe 6 chains running along the crystallographic aaxis and linked along the c-axis via zigzag chains of Te atoms to form two-dimensional (2D) layers. Those layers stack along the b-axis. A recent ab initio calculation suggests that bulk ZrTe 5 locates close to the phase boundary between weak and strong topological insulators [22]. However, more recent transport and ARPES experiments identify it to be a 3D Dirac semimetal with only one Dirac node at the Γ point [23]. Interestingly, a chiral magnetic effect associated with the transform...

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Van der Waals heterostructures formed by assembling different two-dimensional atomic crystals into stacks can lead to many new phenomena and device functionalities. In particular, graphene/boron-nitride heterostructures have emerged as a very promising system for band engineering of graphene. However, the intrinsic value and origin of the bandgap in such heterostructures remain unresolved. Here we report the observation of an intrinsic bandgap in epitaxial graphene/boron-nitride heterostructures with zero crystallographic alignment angle. Magneto-optical spectroscopy provides a direct probe of the Landau level transitions in this system and reveals a bandgap of B38 meV (440 K). Moreover, the Landau level transitions are characterized by effective Fermi velocities with a critical dependence on specific transitions and magnetic field. These findings highlight the important role of manybody interactions in determining the fundamental properties of graphene heterostructures.

We report Fe Kβ x-ray emission spectroscopy study of local magnetic moments in various iron based superconductors in their paramagnetic phases. Local magnetic moments are found in all samples studied: PrFeAsO, Ba(Fe, Co)2As2, LiFeAs, Fe1+x(Te,Se), and A2Fe4Se5 (A=K, Rb, and Cs). The moment size varies significantly across different families. Specifically, all iron pnictides samples have local moments of about 1 µB/Fe, while FeTe and K2Fe4Se5 families have much larger local moments of ∼ 2µB /Fe, ∼ 3.3µB /Fe, respectively. In addition, we find that neither carrier doping nor temperature change affects the local moment size.