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We report the negative effective mass (density) metamaterials based on the electro-mechanical coupling exploiting plasma oscillations of a free electron gas. The negative mass appears as a result of the vibration of a metallic particle with a frequency of <i>ω</i>, which is close the frequency of the plasma oscillations of the electron gas <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>m</mi> <mn>2</mn> </msub> </mrow> </semantics> </math> </inline-formula> relative to the ionic lattice <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>m</mi> <mn>1</mn> </msub> </mrow> </semantics> </math> </inline-formula>. The plasma oscillations are represented with the elastic spring <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>k</mi> <mn>2</mn> </msub> <mo>=</mo> <msubsup> <mi>ω</mi> <mi>p</mi> <mn>2</mn> </msubsup> <msub> <mi>m</mi> <mn>2</mn> </msub> </mrow> </semantics> </math> </inline-formula>, where <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>ω</mi> <mi>p</mi> </msub> </mrow> </semantics> </math> </inline-formula> is the plasma frequency. Thus, the metallic particle vibrated with the external frequency <i>ω</i> is described by the effective mass <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>m</mi> <mrow> <mi>e</mi> <mi>f</mi> <mi>f</mi> </mrow> </msub> <mo>=</mo> <msub> <mi>m</mi> <mn>1</mn> </msub> <mo>+</mo> <mfrac> <mrow> <msub> <mi>m</mi> <mn>2</mn> </msub> <msubsup> <mi>ω</mi> <mi>p</mi> <mn>2</mn> </msubsup> </mrow> <mrow> <msubsup> <mi>ω</mi> <mi>p</mi> <mn>2</mn> </msubsup> <mo>−</mo> <msup> <mi>ω</mi> <mn>2</mn> </msup> </mrow> </mfrac> </mrow> </semantics> </math> </inline-formula>, which is negative when the frequency <inline-formula> <math display="inline"> <semantics> <mi>ω</mi> </semantics> </math> </inline-formula> approaches <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mi>ω</mi> <mi>p</mi> </msub> </mrow> </semantics> </math> </inline-formula> from above. The idea is exemplified with two conducting metals, namely Au and Li.